Structure of a Muscle Cell

Muscle Tissue Traits

Excitation from a Motor (Efferent) Nerve

Contraction (Sliding Filament Theory)

Striations

Motor Unit Recruitment (Multiplication by Clustering):

Connective Tissue and Muscle Fiber Arrangement

Muscle Cell Structure and Function

NASM

ACSM

ProCert

AFAA

NCBTMB

CanFitPro

PTAGlobal

ISSA

REPS

REPSI

PACE

YOGA

WITS

CIMSPA

AUSPHYSIO

TPTA

NYPT

AOTA

This course describes the amazing structure and function of the skeletal <a id="muscle-cell" data-tip="1374" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle cell</a>.&#xA0;The human body is comprised of 4 cell types; <a id="epithelial-cell" data-tip="1384" target="_blank" href="/glossary-term/epithelial-cell" class="glossary">epithelial</a> cells (e.g. skin), <a id="nerve-cell" data-tip="1361" target="_blank" href="/glossary-term/nerve-cell" class="glossary">nerve</a> cells (e.g. sciatic <a id="nerve-cell" data-tip="1361" target="_blank" href="/glossary-term/nerve-cell" class="glossary">nerve</a>), <a id="connective-tissue-cell" data-tip="1368" target="_blank" href="/glossary-term/connective-tissue-cell" class="glossary">connective tissue</a> cells (e.g. ligaments, tendons, and fascia), and <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> cells (e.g. biceps brachii). There are even 3 distinct types of <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> tissue: 
<ul>
 <li>Smooth <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> tissue: involuntary and non-striated <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> tissue</li>
 <li>Cardiac <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> tissue: involuntary and striated <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> tissue</li>
 <li>Skeletal <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> tissue: voluntary and striated <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> tissue&#xA0;</li>
</ul>
Each cell type has components and attributes common to all cells, and unique modifications to components that give that cell type its unique characteristics. <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">Muscle</a> cells are specialized to maximize force production via &quot;contraction&quot;, and have a unique combination of <a id="responsiveness" data-tip="1313" target="_blank" href="/glossary-term/responsiveness" class="glossary">responsiveness</a>, <a id="excitability" data-tip="1310" target="_blank" href="/glossary-term/excitability" class="glossary">excitability</a>, <a id="conductivity" data-tip="1306" target="_blank" href="/glossary-term/conductivity" class="glossary">conductivity</a>, <a id="contractility" data-tip="1309" target="_blank" href="/glossary-term/contractility" class="glossary">contractility</a>, <a id="extensibility-2" data-tip="1308" target="_blank" href="/glossary-term/extensibility-2" class="glossary">extensibility</a>, and <a id="elasticity-2" data-tip="1307" target="_blank" href="/glossary-term/elasticity-2" class="glossary">elasticity</a>.&#xA0;&#xA0;
This course details cell classifications, <a id="muscle-cell" data-tip="1374" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle cell</a> <a id="organelle" data-tip="1352" target="_blank" href="/glossary-term/organelle" class="glossary">organelles</a>, <a id="muscle-cell" data-tip="1374" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle cell</a> function, action potentials, sliding filament theory (actin and myosin), excitation-contraction coupling, <a id="motor-unit" data-tip="1303" target="_blank" href="/glossary-term/motor-unit" class="glossary">motor unit</a> recruitment, striation, fascial layers, and <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> structure and organization. By the time you complete this course, you will understand why the interaction of actin and myosin, and the sliding filament theory help us understand why muscles are weaker in a lengthened position; or, how excitation-contraction coupling and the organization of motor units explain why you cannot target or shape one part of a <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a>. Movement professionals (personal trainers, fitness instructors, physical therapists, athletic trainers, massage therapists, chiropractors, occupational therapists, etc.) should consider this course to be foundational physiology content, which is essential for understanding future courses including acute variables, corrective exercise, strength training program design, physical rehabilitation, etc.
<h3>Additional Courses:</h3>
<ul>
 <li><a href="https://brookbushinstitute.com/courses/muscle-cell-structure-function">Muscle Cell Structure and Function</a></li>
 <li><a href="https://brookbushinstitute.com/courses/muscle-fiber-dysfunction-trigger-points">Muscle Fiber Dysfunction and Trigger Points</a></li>
</ul>
<img title="Motor unit by Electron Microscope: Note how you can see the neuromuscular junction and motor end plate, and if you look closely the striations in the muscle cells." src="https://www.datocms-assets.com/25800/1649647504-motor-unit.jpg" alt="Motor unit by electron microscope">
<h3>Snippet From This Course:</h3>
Understanding this process should bring feelings of sheer amazement at the intricacy of the human body. Consider that all of the steps detailed above (and summarized below) happen every time you need to generate force. In fact, consider that this sequence happens many times per second, at every <a id="muscle-cell" data-tip="1374" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle cell</a>, of every <a id="motor-unit" data-tip="1303" target="_blank" href="/glossary-term/motor-unit" class="glossary">motor unit</a>, of every <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> you recruit, at every joint, and every time you move or resist movement.
<ol>
 <li>Stimulus (intent, reflex, motor program, etc)</li>
 <li><a id="action-potential-2" data-tip="1240" target="_blank" href="/glossary-term/action-potential-2" class="glossary">Action potential</a> at motor neuron</li>
 <li>Conducted by axon to the synapse</li>
 <li>ACh <a id="release" data-tip="1253" target="_blank" href="/glossary-term/release" class="glossary">released</a> into the neuromuscular junction</li>
 <li>ACh binds to receptors on the motor endplate of the <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> cell</li>
 <li>Sodium-ion channels open on the motor endplate</li>
 <li>An <a id="action-potential-2" data-tip="1240" target="_blank" href="/glossary-term/action-potential-2" class="glossary">action potential</a> is propagated across the sarcolemma</li>
 <li><a id="action-potential-2" data-tip="1240" target="_blank" href="/glossary-term/action-potential-2" class="glossary">Action potential</a> conducts down T-tubules</li>
 <li>Opens voltage-regulated calcium channels</li>
 <li>Calcium is <a id="release" data-tip="1253" target="_blank" href="/glossary-term/release" class="glossary">released</a> and binds to troponin</li>
 <li>The change in the shape of the troponin-tropomyosin complex exposes binding sites</li>
 <li><a id="adenosine-triphosphate-atp" data-tip="1247" target="_blank" href="/glossary-term/adenosine-triphosphate-atp" class="glossary">ATP</a> binds to Myosin</li>
 <li>Hydrolyzed <a id="adenosine-triphosphate-atp" data-tip="1247" target="_blank" href="/glossary-term/adenosine-triphosphate-atp" class="glossary">ATP</a> becomes ADP and extends the myosin head</li>
 <li>The myosin head binds to an available binding site on troponin</li>
 <li><a id="release" data-tip="1249" target="_blank" href="/glossary-term/release" class="glossary">Release</a> of ADP results in flexing of the myosin head</li>
 <li>Flexing the myosin head pulls actin resulting in a <a id="power" data-tip="1399" target="_blank" href="/glossary-term/power" class="glossary">power</a> stroke</li>
 <li><a id="adenosine-triphosphate-atp" data-tip="1247" target="_blank" href="/glossary-term/adenosine-triphosphate-atp" class="glossary">ATP</a> binds to myosin <a id="release" data-tip="1250" target="_blank" href="/glossary-term/release" class="glossary">releasing</a> the head from troponin</li>
 <li>The process starts over at a binding site further down actin.</li>
 <li>This step is multiplied by the number of binding sites on each protein, the number of proteins in each <a id="sarcomere" data-tip="1323" target="_blank" href="/glossary-term/sarcomere" class="glossary">sarcomere</a>, the number of proteins in parallel in each myofibril, and the number of myofibrils in each <a id="muscle-cell" data-tip="1374" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle cell</a>.</li>
 <li>Force output is then multiplied again, and calibrated for function, by the number of cells in the <a id="motor-unit" data-tip="1303" target="_blank" href="/glossary-term/motor-unit" class="glossary">motor unit</a> recruited, and the number of motor units recruited.</li>
</ol>
Note: Even this step-by-step account is a simplified summary of this process. Understanding each step represents years of work from incredible minds. It is not necessary that you memorize this entire sequence today, (although, you may be able to write down most of these steps in your own words after studying this course). But, stop studying for a moment, and take the time to appreciate how amazing the <a id="muscle-cell" data-tip="1374" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle cell</a> really is. This knowledge is the first step in understanding the human movement system.&#xA0;

Course Description: Muscle Cell Structure and Function

The human body is comprised of 4 cell types (<a id="epithelial-cell" data-tip="1384" target="_blank" href="/glossary-term/epithelial-cell" class="glossary">epithelial</a> cells, <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> cells, <a id="nerve-cell" data-tip="1361" target="_blank" href="/glossary-term/nerve-cell" class="glossary">nerve</a> cells, and <a id="connective-tissue-cell" data-tip="1368" target="_blank" href="/glossary-term/connective-tissue-cell" class="glossary">connective tissue</a> cells). Each cell type has components that are found in all cells and unique modifications to components that give the cell type its unique attributes. These components may have special names <a id="specificity" data-tip="1224" target="_blank" href="/glossary-term/specificity" class="glossary">specific</a> to the cell type, for example, the &quot;cytoplasm&quot; in a <a id="muscle-cell" data-tip="1374" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle cell</a> is called &quot;sarcoplasm.&quot; <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> cells are specialized to maximize force production via &quot;contraction.&quot; In the table below, you can find a list of the <a id="muscle-cell" data-tip="1374" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle cell</a> <a id="organelle" data-tip="1352" target="_blank" href="/glossary-term/organelle" class="glossary">organelles</a> with their specialized names, the function of those <a id="organelle" data-tip="1352" target="_blank" href="/glossary-term/organelle" class="glossary">organelles</a>, and the equivalent of those <a id="organelle" data-tip="1352" target="_blank" href="/glossary-term/organelle" class="glossary">organelles</a> in a &quot;standard&quot; cell.
<h4> Etymology: </h4>
<ul>
 <li> <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">Muscle</a> (prefix - myo ) (n.) Late 14c., from Middle French <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">Muscle</a> &quot;muscle, sinew&quot; (14c.) and directly from Latin musculus &quot;a <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">Muscle</a>,&quot; literally &quot;little mouse,&quot; diminutive of mus &quot;mouse&quot;. So-called because the shape and movement of some muscles (notably biceps) were thought to resemble mice ( <a href="https://www.etymonline.com/word/mouse" target="_blank" rel="noopener"> Etymology Online </a> ).</li>
 <li> Sarco - before vowels sarc-, word-forming element meaning &quot;flesh, fleshy, of the flesh,&quot; from Latinized <a id="posture" data-tip="1660" target="_blank" href="/glossary-term/posture" class="glossary">form</a> of Greek sark-, combining <a id="posture" data-tip="1660" target="_blank" href="/glossary-term/posture" class="glossary">form</a> of sarx &quot;flesh&quot; ( <a href="https://www.etymonline.com/word/sarco-" target="_blank" rel="noopener"> Etymology Online </a> )</li>
</ul>
<a href="https://brookbush.s3.us-east-2.amazonaws.com/wordpress/wp-content/uploads/2017/09/Cell-Structure.png" target="_blank" rel="noopener"><img src="https://www.datocms-assets.com/25800/1649645730-cell-structure.png" title="Cell structure with cell components labeled" alt="Cell Structure"></a> By Kelvinsong - Own work, CC0, https://commons.wikimedia.org/w/index.php?curid=22952603
<h4> Components of a typical animal cell: </h4>
<ol>
 <li><a href="https://en.wikipedia.org/wiki/Nucleolus" target="_blank" rel="noopener"> Nucleolus </a></li>
 <li><a href="https://en.wikipedia.org/wiki/Cell_nucleus" target="_blank" rel="noopener"> Nucleus </a></li>
 <li><a href="https://en.wikipedia.org/wiki/Ribosome" target="_blank" rel="noopener"> Ribosome </a></li>
 <li><a href="https://en.wikipedia.org/wiki/Vesicle_%28biology_and_chemistry%29" target="_blank" rel="noopener"> Vesicle </a></li>
 <li><a href="https://en.wikipedia.org/wiki/Rough_endoplasmic_reticulum" target="_blank" rel="noopener"> Rough endoplasmic reticulum </a></li>
 <li><a href="https://en.wikipedia.org/wiki/Golgi_apparatus" target="_blank" rel="noopener"> Golgi apparatus </a></li>
 <li><a href="https://en.wikipedia.org/wiki/Cytoskeleton" target="_blank" rel="noopener"> Cytoskeleton </a></li>
 <li><a href="https://en.wikipedia.org/wiki/Smooth_endoplasmic_reticulum" target="_blank" rel="noopener"> Smooth endoplasmic reticulum </a></li>
 <li><a href="https://en.wikipedia.org/wiki/Mitochondrion" target="_blank" rel="noopener"> </a><a id="mitochondria" data-tip="1256" target="_blank" href="/glossary-term/mitochondria" class="glossary">Mitochondrion</a> </li>
 <li><a href="https://en.wikipedia.org/wiki/Vacuole" target="_blank" rel="noopener"> Vacuole </a></li>
 <li><a href="https://en.wikipedia.org/wiki/Cytosol" target="_blank" rel="noopener"> Cytosol </a></li>
 <li><a href="https://en.wikipedia.org/wiki/Lysosome" target="_blank" rel="noopener"> Lysosome </a></li>
 <li><a href="https://en.wikipedia.org/wiki/Centrosome" target="_blank" rel="noopener"> Centrosome </a></li>
 <li><a href="https://en.wikipedia.org/wiki/Cell_membrane" target="_blank" rel="noopener"> Cell membrane </a></li>
</ol>
<table>
 <tbody>
 <tr>
 <td> Name of Structure </td>
 <td> Function in Muscle Cell (1-8) </td>
 <td> Comparison to Other Cells (1-8) </td>
 </tr>
 <tr>
 <td> Nuclei </td>
 <td>
 <ul>
 <li>The nuclei contain all genetic material (DNA), allowing for replication, growth, and repair of cell structures.</li>
 <li><a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">Muscle</a> cells have multiple nuclei improving the cell&apos;s ability to repair and grow.</li>
 </ul>
 </td>
 <td style="text-align: left;">Most cells have a single nucleus ( listed as 1 above ); all nuclei carry a complete copy of the body&apos;s genome.</td>
 </tr>
 <tr>
 <td> Myosin and Actin </td>
 <td>
 <ul>
 <li>These protein filaments (myofilaments) are the contractile proteins responsible for contraction of a <a id="muscle-cell" data-tip="1374" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle cell</a> (5).</li>
 <li>Myosin is the thicker of the two structures with globular heads that attach to binding sites on actin.</li>
 <li>Actin is the thinner protein filament which myosin &quot;pulls on.&quot;</li>
 </ul>
 </td>
 <td>In non-muscle cells, actin and myosin are also important, but not well organized. They are involved in structural support ( listed as 7 above), nutrient transport, and cell replication (6). Muscle cells have specialized versions of these proteins in parallel, highly organized rows.</td>
 </tr>
 <tr>
 <td> Sarcoplasmic Reticulum </td>
 <td>
 <ul>
 <li>This <a id="organelle" data-tip="1350" target="_blank" href="/glossary-term/organelle" class="glossary">organelle</a> forms a network of reservoirs around the myofilaments, supplies the cell with nutrients, and <a id="release" data-tip="1252" target="_blank" href="/glossary-term/release" class="glossary">releases</a> calcium as part of the contraction process.</li>
 </ul>
 </td>
 <td>The sarcoplasmic reticulum is a modification of the smooth endoplasmic reticulum (ER) (listed above 8). The main function of the smooth ER is to make and distribute cellular products like hormones and lipids. The smooth ER also regulates and releases calcium ions in other cells, but not as a function of volitional contraction.</td>
 </tr>
 <tr>
 <td> Mitochondria </td>
 <td>
 <ul>
 <li>These <a id="organelle" data-tip="1352" target="_blank" href="/glossary-term/organelle" class="glossary">organelles</a> generate most of the supply of <a id="adenosine-triphosphate-atp" data-tip="1247" target="_blank" href="/glossary-term/adenosine-triphosphate-atp" class="glossary">ATP</a> by way of aerobic metabolism which breakdowns carbohydrates and fatty acids for fuel.</li>
 </ul>
 </td>
 <td>Mitochondria (listed as 9 above) are found in all cells; however, muscles have the highest number of mitochondria per cell.</td>
 </tr>
 <tr>
 <td> Sarcoplasm </td>
 <td>
 <ul>
 <li>This is the fluid within the cell in which various particles and <a id="organelle" data-tip="1352" target="_blank" href="/glossary-term/organelle" class="glossary">organelles</a> are suspended.</li>
 <li>This liquid is primarily comprised of water and glycogen.</li>
 </ul>
 </td>
 <td>This internal solution of non-muscle cells is referred to as the cytoplasm (listed above 8).</td>
 </tr>
 <tr>
 <td> Sarcolemma (Myolemma) </td>
 <td>
 <ul>
 <li>This is the cell&apos;s membrane, forming a barrier between extracellular and intracellular compartments.</li>
 <li>T-tubules penetrate the sarcolemma to allow for the transport of nutrients into and out of the cell.</li>
 </ul>
 </td>
 <td>This is referred to as the cell wall or plasma membrane (listed above as 14) in other cells of the body.</td>
 </tr>
 <tr>
 <td> Sarcomere </td>
 <td>
 <ul>
 <li>A segment of a <a id="muscle-cell" data-tip="1374" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle cell</a> repeated longitudinally, composed of parallel contractile filaments (actin and myosin) suspended between two &quot;Z-Disks&quot; (8).</li>
 <li>This is the smallest functional unit of a <a id="muscle-cell" data-tip="1374" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle cell</a>.</li>
 </ul>
 </td>
 <td>This is a unique feature of the highly organized muscle cell. This also gives muscle cells their striated appearance under a microscope.</td>
 </tr>
 <tr>
 <td> Titin </td>
 <td>
 <ul>
 <li>A specialized protein within a <a id="muscle-cell" data-tip="1374" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle cell</a> that extends from the &quot;Z-Disk&quot; to the middle of a <a id="sarcomere" data-tip="1323" target="_blank" href="/glossary-term/sarcomere" class="glossary">sarcomere</a> (<a id="m-line" data-tip="1316" target="_blank" href="/glossary-term/m-line" class="glossary">M-Line</a>).</li>
 <li>This protein aids in the alignment of actin and myosin and adds to the durability and <a id="elasticity-2" data-tip="1307" target="_blank" href="/glossary-term/elasticity-2" class="glossary">elasticity</a> (5) of a <a id="muscle-cell" data-tip="1374" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle cell</a>.</li>
 </ul>
 </td>
 <td>Although titin-like proteins have been found in smooth muscle and non-muscle cells, titin in skeletal muscle seems to be unique in its interaction with myosin.</td>
 </tr>
 <tr>
 <td> Microtubule </td>
 <td>
 <ul>
 <li>The microtubule in muscles cells is modified to aid in the <a id="stability" data-tip="1191" target="_blank" href="/glossary-term/stability" class="glossary">stability</a> and alignment of the highly organized arrangement of actin and myosin filaments.</li>
 </ul>
 </td>
 <td>Microtubules are components of all cells that aid in maintaining structure, intracellular transport, and movement of various organelles within the cell.</td>
 </tr>
 <tr>
 <td style="text-align: center;" colspan="3"> Cell organelles not mentioned above are the same or similar to those found in other cells. </td>
 </tr>
 <tr>
 <td style="text-align: center;"> Vesicles </td>
 <td style="text-align: center;"></td>
 <td style="text-align: left;">These structures aid in transporting materials through the cytoplasm (sarcoplasm in muscle cells), which may aid in metabolism, chemical reactions, and the storage of nutrients and enzymes (labeled as 4).</td>
 </tr>
 <tr>
 <td style="text-align: center;"> Rough Endoplasmic Reticulum </td>
 <td style="text-align: center;"></td>
 <td style="text-align: center;">An organelle responsible for producing various cell enzymes and proteins (labeled as 5).</td>
 </tr>
 <tr>
 <td style="text-align: center;"> Golgi Apparatus </td>
 <td style="text-align: center;"></td>
 <td style="text-align: center;">An organelle that receives proteins from the rough endoplasmic reticulum for modification. (labeled 6).</td>
 </tr>
 <tr>
 <td style="text-align: center;"> Centrosome </td>
 <td style="text-align: center;"></td>
 <td style="text-align: center;">Microtubule organizer that plays a key role in the replication of cells (mitosis) (labeled as 13).</td>
 </tr>
 </tbody>
</table>
<img src="https://www.datocms-assets.com/25800/1649645803-skeletal-muscle-fiber.png" title="A detailed image of skeletal muscle fibers" alt="An image of the skeletal muscle fibers">
Blausen.com staff (2014). &quot;Medical gallery of Blausen Medical 2014&quot;. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.

All cells have &quot;traits&quot;. Some of the characteristics that make a <a id="muscle-cell" data-tip="1374" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle cell</a> unique are <a id="responsiveness" data-tip="1313" target="_blank" href="/glossary-term/responsiveness" class="glossary">responsiveness</a>, <a id="conductivity" data-tip="1306" target="_blank" href="/glossary-term/conductivity" class="glossary">conductivity</a>, <a id="contractility" data-tip="1309" target="_blank" href="/glossary-term/contractility" class="glossary">contractility</a>, <a id="extensibility-2" data-tip="1308" target="_blank" href="/glossary-term/extensibility-2" class="glossary">extensibility</a>, and <a id="elasticity-2" data-tip="1307" target="_blank" href="/glossary-term/elasticity-2" class="glossary">elasticity</a>. These traits are explained in further detail below:
<ol>
 <li> Responsiveness: The ability of a cell to respond or react to stimuli. For example, a <a id="muscle-cell" data-tip="1374" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle cell</a> may respond to chemicals/hormones in the bloodstream by increasing the intracellular production of certain proteins.</li>
 <li> Excitability: The ability of a cell to change in membrane conductance as a response to stimulation. Although all cells are &quot;excitable&quot;, <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> cells are specialized to react to a change in membrane potential with contraction.</li>
 <li> Conductivity: Stimulation results in more than a local effect. For example, the stimulation of a motor endplate results in a change in cell membrane polarity that rapidly propagates along the entire length of the <a id="muscle-cell" data-tip="1374" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle cell</a> (origin to insertion).</li>
 <li> Contractility: <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">Muscle</a> cells are unique in the amount they can shorten when stimulated. This is due to the highly organized arrangement of myofilaments.</li>
 <li> Extensibility: This is the ability of a cell to be stretched or lengthened without damage. Most cells would rupture with even a modest stretch, but muscles can stretch up to 3 times their contracted length.</li>
 <li> Elasticity: Although this term is often confused with extensibility; <a id="elasticity-2" data-tip="1307" target="_blank" href="/glossary-term/elasticity-2" class="glossary">elasticity</a> refers to the ability of a cell to return to its resting length after being stretched.</li>
</ol>
Although all cells possess these traits to various degrees, the evolution of the cellular components of the <a id="muscle-cell" data-tip="1374" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle cell</a> has made them &quot;specialists&quot; in these traits. Take a moment to compare these traits to the description of the <a id="muscle-cell" data-tip="1374" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle cell</a> components above.

The next section will focus on changes that occur at the cellular level in a <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> due to excitation from a motor (efferent) <a id="nerve-cell" data-tip="1361" target="_blank" href="/glossary-term/nerve-cell" class="glossary">nerve</a>. That is, how an <a id="action-potential-2" data-tip="1240" target="_blank" href="/glossary-term/action-potential-2" class="glossary">action potential</a> goes from motor <a id="nerve-cell" data-tip="1360" target="_blank" href="/glossary-term/nerve-cell" class="glossary">neuron</a> to stimulating the sarcolemma, is conducted through the microtubules, to the myofibrils that are divided into sarcomere, where a cascade is stimulated that results in a change in the shape of the myofilaments, actin, and myosin, that results in contraction. (Note: the italicized words were discussed in the sections above).
<h4> Neural Activation: </h4>
The excitation of a <a id="muscle-cell" data-tip="1374" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle cell</a> starts at a motor <a id="nerve-cell" data-tip="1360" target="_blank" href="/glossary-term/nerve-cell" class="glossary">neuron</a> (found in the brain stem or spinal cord). An impulse is generated when the motor <a id="nerve-cell" data-tip="1360" target="_blank" href="/glossary-term/nerve-cell" class="glossary">neuron</a> is sufficiently stimulated to result in an &quot;action potential.&quot; The <a id="action-potential-2" data-tip="1240" target="_blank" href="/glossary-term/action-potential-2" class="glossary">action potential</a> is conducted along the length of an axon to the <a id="neuromuscular-junction" data-tip="1304" target="_blank" href="/glossary-term/neuromuscular-junction" class="glossary">neuromuscular junction</a>. In some cases that signal is conducted along an axon for nearly a meter, for example, from the spinal cord to the plantar interossei of the foot. Once the <a id="action-potential-2" data-tip="1240" target="_blank" href="/glossary-term/action-potential-2" class="glossary">action potential</a> reaches the synapse of the motor <a id="nerve-cell" data-tip="1361" target="_blank" href="/glossary-term/nerve-cell" class="glossary">nerve</a> it signals the <a id="release" data-tip="1249" target="_blank" href="/glossary-term/release" class="glossary">release</a> of the neurotransmitter, acetylcholine (ACh), which then leaks into the <a id="neuromuscular-junction" data-tip="1304" target="_blank" href="/glossary-term/neuromuscular-junction" class="glossary">neuromuscular junction</a>. This ACh binds to receptors on the <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> cell&apos;s &quot;motor endplate&quot; which open sodium ion channels that result in a change in polarity, an <a id="action-potential-2" data-tip="1240" target="_blank" href="/glossary-term/action-potential-2" class="glossary">action potential</a>, and the propagation of that <a id="action-potential-2" data-tip="1240" target="_blank" href="/glossary-term/action-potential-2" class="glossary">action potential</a> along the sarcolemma.
<ol>
 <li>Stimulus (intent, reflex, motor program, etc)</li>
 <li><a id="action-potential-2" data-tip="1240" target="_blank" href="/glossary-term/action-potential-2" class="glossary">Action potential</a> at motor neuron</li>
 <li>Conducted by axon to the synapse</li>
 <li>ACh <a id="release" data-tip="1253" target="_blank" href="/glossary-term/release" class="glossary">released</a> into the neuromuscular junction</li>
 <li>ACh binds to receptors on the motor endplate of the <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> cell</li>
 <li>Sodium-ion channels open</li>
 <li>An <a id="action-potential-2" data-tip="1240" target="_blank" href="/glossary-term/action-potential-2" class="glossary">action potential</a> is propagated across the sarcolemma</li>
</ol>
Note: Relaxation occurs due to the presence of the basal lamina in the synaptic cleft which contains an enzyme called acetylcholinesterase. This enzyme is responsible for the breakdown of ACh. The breakdown of ACh returns the cell to a relaxed state.
<a href="https://brookbush.s3.us-east-2.amazonaws.com/wordpress/wp-content/uploads/2017/10/Neuromuscular-Junction.png" target="_blank" rel="noopener"><img src="https://www.datocms-assets.com/25800/1649646018-neuromuscular-junction.png" title="Diagram of the neuromuscular junction with acetylcholine release" alt="Diagram of Neuromuscular Junction"></a> Elliejellybelly13 - Own work
 Muscles will contract or relax when they receive signals from the <a id="nervous-system" data-tip="1690" target="_blank" href="/glossary-term/nervous-system" class="glossary">nervous system</a>. The <a id="neuromuscular-junction" data-tip="1304" target="_blank" href="/glossary-term/neuromuscular-junction" class="glossary">neuromuscular junction</a> is the site of the signal exchange. The steps of this process in vertebrates occur as follows: (1) The <a id="action-potential-2" data-tip="1240" target="_blank" href="/glossary-term/action-potential-2" class="glossary">action potential</a> reaches the axon terminal. (2) Voltage-dependent calcium gates open, allowing calcium to enter the axon terminal. (3) Neurotransmitter vesicles fuse with the presynaptic membrane and acetylcholine (ACh) is <a id="release" data-tip="1253" target="_blank" href="/glossary-term/release" class="glossary">released</a> into the synaptic cleft via exocytosis. (4) ACh binds to postsynaptic receptors on the sarcolemma. (5) This binding causes ion channels to open and allows sodium ions to flow across the membrane into the <a id="muscle-cell" data-tip="1374" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle cell</a>. (6) The flow of sodium ions across the membrane into the <a id="muscle-cell" data-tip="1374" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle cell</a> generates an <a id="action-potential-2" data-tip="1240" target="_blank" href="/glossary-term/action-potential-2" class="glossary">action potential</a> that travels to the myofibril and results in <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> contraction. Labels: A: Motor <a id="nerve-cell" data-tip="1360" target="_blank" href="/glossary-term/nerve-cell" class="glossary">Neuron</a> Axon B: Axon Terminal C. Synaptic Cleft D. <a id="muscle-cell" data-tip="1374" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle cell</a> E. Part of a Myofibril
<h4> Excitation-Contraction Coupling: </h4>
The wave of the <a id="action-potential-2" data-tip="1240" target="_blank" href="/glossary-term/action-potential-2" class="glossary">action potential</a> is conducted across the sarcolemma, down the T tubules into the sarcoplasm. This change in polarity opens voltage-regulated gates linked to calcium channels (in the terminal cisternae of the sarcoplasmic reticulum). The <a id="release" data-tip="1253" target="_blank" href="/glossary-term/release" class="glossary">released</a> calcium binds to troponin (of the troponin-tropomyosin complex on actin filaments) and causes a shape change in the protein that makes binding sites available for myosin.
Note: Troponin is a protein that is bound to tropomyosin which wraps around actin (the thin filament). In this way, you could view troponin, tropomyosin, and actin as a single braided complex, that functionally acts as a single strand of proteins.
<ol>
 <li><a id="action-potential-2" data-tip="1240" target="_blank" href="/glossary-term/action-potential-2" class="glossary">Action potential</a> conducts down T-tubules</li>
 <li>Opens voltage-regulated calcium channels</li>
 <li>Calcium is <a id="release" data-tip="1253" target="_blank" href="/glossary-term/release" class="glossary">released</a> and binds to troponin</li>
 <li>The change in the shape of the troponin-tropomyosin complex exposes binding sites</li>
</ol>
<a href="https://brookbush.s3.us-east-2.amazonaws.com/wordpress/wp-content/uploads/2017/10/Myofilament.svg_.png" target="_blank" rel="noopener"><img src="https://www.datocms-assets.com/25800/1649646180-myofilament.png" title="Image of the actin, troponin, tropomyosin, and myosin relationship" alt="Actin, troponin, tropomyosin, and myosin in a muscle contraction"></a> Actin (Thin Filament), troponin, tropomyosin and myosin - H&#xE4;ggstr&#xF6;m, Mikael (2014). &quot;Medical gallery of Mikael H&#xE4;ggstr&#xF6;m 2014&quot;. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.008. ISSN 2002-4436.
<h4> Power Stroke: </h4>
For a &quot;power stroke&quot; to occur (the source of the force produced), myosin must be bound to an <a id="adenosine-triphosphate-atp" data-tip="1245" target="_blank" href="/glossary-term/adenosine-triphosphate-atp" class="glossary">adenosine triphosphate (</a><a id="adenosine-triphosphate-atp" data-tip="1247" target="_blank" href="/glossary-term/adenosine-triphosphate-atp" class="glossary">ATP</a>) molecule. The <a id="adenosine-triphosphate-atp" data-tip="1247" target="_blank" href="/glossary-term/adenosine-triphosphate-atp" class="glossary">ATP</a> is then &quot;hydrolyzed&quot; (broken down) to adenosine diphosphate (ADP) which &quot;cocks&quot; the myosin head into an extended position. If there is an active site available on troponin, the myosin head will bind to it, this is known as &quot;cross-bridging.&quot; The <a id="release" data-tip="1249" target="_blank" href="/glossary-term/release" class="glossary">release</a> of the ADP results in the &quot;flexing&quot; of the myosin head which pulls actin with it. This forceful pull on actin by myosin is known as the &quot;power stroke.&quot; The myosin head stays bound to troponin until more <a id="adenosine-triphosphate-atp" data-tip="1247" target="_blank" href="/glossary-term/adenosine-triphosphate-atp" class="glossary">ATP</a> is available to bind to the myosin head. The binding of &quot;new&quot; <a id="adenosine-triphosphate-atp" data-tip="1247" target="_blank" href="/glossary-term/adenosine-triphosphate-atp" class="glossary">ATP</a> &quot;releases&quot; the myosin head from troponin so it can start the process over on a binding site further down the actin filament.
<ol>
 <li><a id="adenosine-triphosphate-atp" data-tip="1247" target="_blank" href="/glossary-term/adenosine-triphosphate-atp" class="glossary">ATP</a> binds to Myosin</li>
 <li>Hydrolyzed <a id="adenosine-triphosphate-atp" data-tip="1247" target="_blank" href="/glossary-term/adenosine-triphosphate-atp" class="glossary">ATP</a> becomes ADP and extends the myosin head</li>
 <li>The myosin head binds to an available binding site on troponin</li>
 <li><a id="release" data-tip="1249" target="_blank" href="/glossary-term/release" class="glossary">Release</a> of ADP results in flexing of the myosin head</li>
 <li>Flexing the myosin head pulls actin resulting in a <a id="power" data-tip="1399" target="_blank" href="/glossary-term/power" class="glossary">power</a> stroke</li>
 <li><a id="adenosine-triphosphate-atp" data-tip="1247" target="_blank" href="/glossary-term/adenosine-triphosphate-atp" class="glossary">ATP</a> binds to myosin <a id="release" data-tip="1250" target="_blank" href="/glossary-term/release" class="glossary">releasing</a> the head from troponin</li>
 <li>The process starts over at a binding site further down actin.</li>
</ol>
<h4>Note:</h4>
<ul>
 <li>The <a id="specificity" data-tip="1224" target="_blank" href="/glossary-term/specificity" class="glossary">specific</a> number of myosin heads that attach to actin-binding sites (cross-bridging) at any one time dictates the amount of force that can be produced by the <a id="muscle-cell" data-tip="1374" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle cell</a> (2).</li>
 <li>This process occurs in a cyclical fashion, attaching, flexing, and detaching, to produce (concentric contraction), reduce (eccentric contraction), or maintain (isokinetic) force during movement.</li>
</ul>
<a href="https://brookbush.s3.us-east-2.amazonaws.com/wordpress/wp-content/uploads/2017/10/Sliding-Filament-Theory-2.png" target="_blank" rel="noopener"><img src="https://www.datocms-assets.com/25800/1649646250-sliding-filament-theory-2.png" title="Illustration of the molecular changes leading to a muscular contraction." alt="Molecular changes leading to contraction"></a> This illustration looks a bit complex, but if you look at each <a id="glide" data-tip="1815" target="_blank" href="/glossary-term/glide" class="glossary">slide</a> slowly you will note that it is just a visual representation of what we just reviewed. Sliding Filament Theory - Muskel-molekular.png: Hank van Helvete derivative work: GravityGilly

So far, we have covered the process from impulse (intent resulting in an <a id="action-potential-2" data-tip="1240" target="_blank" href="/glossary-term/action-potential-2" class="glossary">action potential</a>) to conformational (shape) changes in contractile proteins, resulting in a &quot;power stroke.&quot; Sliding filament theory (and the diagram below) explains the organization and structure of these contractile proteins. This aids in understanding how the <a id="power" data-tip="1399" target="_blank" href="/glossary-term/power" class="glossary">power</a> stroke process is multiplied 1,000,000&apos;s of times per <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> to generate force and facilitate function. It also explains why these cells are striated (striped) under a microscope.
In the diagram below, note the thick myosin filaments (horizontal and red) and the thin actin filaments (horizontal in blue). The bumps on the myosin proteins represent the myosin heads. This is the portion of myosin that binds to troponin (of the troponin-tropomyosin complex) embedded on the actin filaments. Note, there are multiple myosin heads ready to bind to each actin protein. The multiple heads on the myosin protein are due to the myosin chains in muscles being comprised of many myosin proteins, with their &quot;tails&quot; braided together, and their &quot;heads&quot; sticking out ready for a <a id="power" data-tip="1399" target="_blank" href="/glossary-term/power" class="glossary">power</a> stroke. The best analogy I have heard describing this arrangement is rowers in a rowboat. The legs of the rowers are in the boat like the tails of the myosin proteins, while their oars are sticking out ready to stroke the water, like the myosin heads. At either end of the myosin proteins, note the &quot;z-disks&quot; (vertical pink line). This represents a protein structure that borders and separates two adjacent sarcomeres. When the contraction is initiated, the synchronized &quot;power strokes&quot; (like synchronized rowers in a race) &quot;pull&quot; on actin, sliding the filaments across one another, and pulling the <a id="z-disk" data-tip="1335" target="_blank" href="/glossary-term/z-disk" class="glossary">z-disks</a> closer to one another. Now consider a 3-dimensional representation of this diagram (or at least a sagittal cross-section), each myosin protein is surrounded by approximately 6 actin, creating a hexagonal honeycomb-like arrangement. This honeycomb-like arrangement creates a grid of dozens or hundreds of contractile proteins, running parallel to one another in each myofibril (tube of contractile proteins), with many opportunities for cross-bridging on each myofilament. Then consider how many microscopic sarcomeres must be connected in series to span the full length of a <a id="muscle-cell" data-tip="1377" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle fiber</a>, and how many parallel tubes of the <a id="sarcomere" data-tip="1323" target="_blank" href="/glossary-term/sarcomere" class="glossary">sarcomere</a> (myofibril) are then clustered into <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> cells. <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> cells are then clustered into fascicles (fibers), finally fascicles into <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> bellies. It is not hard to imagine how a small, seemingly insignificant force generated by a single protein, is multiplied millions of times, to generate significant force every time you contract a <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a>. The reason <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> cells are so unique in their ability to contract and produce force is this repeated pattern of parallel proteins throughout the entirety of a <a id="muscle-cell" data-tip="1374" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle cell</a> (and <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> as a whole) ensures that the force produced is along one axis. Like rowers in a boat (<a id="power" data-tip="1399" target="_blank" href="/glossary-term/power" class="glossary">power</a> stroke), all stroking the water in the same direction (parallel filaments), with a million other boats stroking in the same direction (e.g. <a id="sarcomere" data-tip="1323" target="_blank" href="/glossary-term/sarcomere" class="glossary">sarcomere</a>, myofibril, <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> cells, fascicles), all tied to a single rope (e.g. tendon), all pulling the same object (e.g bone), with the same goal (e.g. move or stabilize a joint).
<h4> Multiplication by Organization: </h4>
<ul>
 <li>Each myosin filament has multiple heads capable of producing a <a id="power" data-tip="1399" target="_blank" href="/glossary-term/power" class="glossary">power</a> stroke.</li>
 <li>Approximately 6 actin, with multiple binding sites, surround each strand of myosin filament.</li>
 <li>There are many parallel myosin and actin per myofibril (tubes of contractile proteins in a honeycomb-like arrangement).</li>
 <li>There are many sarcomeres in series per myofibril along the length of a <a id="muscle-cell" data-tip="1374" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle cell</a>.</li>
 <li><a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">Muscle</a> cells (myocytes) are comprised of many myofibrils There are many <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">Muscle</a> cells per fascicle.</li>
 <li>Muscles are generally comprised of many fascicles.</li>
</ul>
<img src="https://www.datocms-assets.com/25800/1649646390-sarcomere.png" title="Diagram of a sarcomere, used to describe the sliding filament theory" alt="A sarcomere including the actin and myosin filaments">
 Diagram of a <a id="sarcomere" data-tip="1323" target="_blank" href="/glossary-term/sarcomere" class="glossary">sarcomere</a> (Sliding Filament Theory) By David Richfield (User:Slashme)When using this image in external works, it may be cited as follows:Richfield, David (2014). &quot;Medical gallery of David Richfield&quot;. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.009. ISSN 2002-4436. - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=2264027

The electron microscope image below (and others like it) is what led researchers to our current understanding of the organization of contractile proteins in a <a id="muscle-cell" data-tip="1374" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle cell</a>. Take a second to consider how what we have discussed up to this point is represented below; consider how the length of a <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> may influence the number of cross-bridges available, how the structure of a <a id="muscle-cell" data-tip="1374" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle cell</a> may limit the capacity to shorten, how stretching may be limited by protein length, and then consider how those traits affect motion, exercise selection, rehab, and performance.
<ul>
 <li> <a id="sarcomere" data-tip="1323" target="_blank" href="/glossary-term/sarcomere" class="glossary">Sarcomere</a> - A segment of a myofibril (<a id="muscle-cell" data-tip="1374" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle cell</a>) from one <a id="z-disk" data-tip="1332" target="_blank" href="/glossary-term/z-disk" class="glossary">Z disk</a> to the next. This is the smallest functional unit of a <a id="muscle-cell" data-tip="1374" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle cell</a>. A <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> shortens because all of the individual sarcomeres shorten, pulling <a id="z-disk" data-tip="1333" target="_blank" href="/glossary-term/z-disk" class="glossary">Z disks</a> closer to one another.</li>
 <li> <a id="z-disk" data-tip="1332" target="_blank" href="/glossary-term/z-disk" class="glossary">Z disk</a> (line) - Refers to a dark line between adjacent I-bands under a microscope. This area is the border, or walls of the &quot;sarcomere&quot;, and is comprised of connectin protein that acts as an anchor for the actin filaments.</li>
 <li> <a id="m-line" data-tip="1316" target="_blank" href="/glossary-term/m-line" class="glossary">M-line</a> (not labeled below but visible down the middle of the <a id="sarcomere" data-tip="1323" target="_blank" href="/glossary-term/sarcomere" class="glossary">sarcomere</a>)- Refers to a dark line through the middle of a <a id="sarcomere" data-tip="1323" target="_blank" href="/glossary-term/sarcomere" class="glossary">sarcomere</a>, bisecting the two halves between <a id="z-disk" data-tip="1333" target="_blank" href="/glossary-term/z-disk" class="glossary">Z disks</a> when viewed under a microscope. The M-band is composed of proteins myomesin, C-protein, the <a id="m-line" data-tip="1316" target="_blank" href="/glossary-term/m-line" class="glossary">M-line</a> portion, as well as other cross-links between the thick filament system (myosin).</li>
 <li> <a id="h-band" data-tip="1324" target="_blank" href="/glossary-term/h-band" class="glossary">H-Band</a> - Refers to the central zone of a <a id="sarcomere" data-tip="1323" target="_blank" href="/glossary-term/sarcomere" class="glossary">sarcomere</a> that looks lighter under a microscope. This area of the <a id="sarcomere" data-tip="1323" target="_blank" href="/glossary-term/sarcomere" class="glossary">sarcomere</a> is a lighter portion of the <a id="a-band" data-tip="1345" target="_blank" href="/glossary-term/a-band" class="glossary">A-band</a> where there is no overlap between the thinner filaments (actin) and the thicker filaments (myosin).</li>
 <li> <a id="i-band" data-tip="1337" target="_blank" href="/glossary-term/i-band" class="glossary">I-Band</a> - Refers to the part of the <a id="muscle-cell" data-tip="1374" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle cell</a> that looks lighter under a microscope. This area of the cell is lighter due to the parallel arrangement of thinner filaments (actin), and no overlap with the thicker filaments (myosin).</li>
 <li> <a id="a-band" data-tip="1345" target="_blank" href="/glossary-term/a-band" class="glossary">A-Band</a> - Refers to the part of the <a id="muscle-cell" data-tip="1374" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle cell</a> that looks darker under a microscope. This area of the cell is darker due to the parallel arrangement of thicker filaments (myosin) and overlap with the thinner filaments (actin).</li>
</ul>
<h4> Examples of how the Sliding Filament Theory Influence our Understanding of Motion: </h4>
<ul>
 <li>The number of cross-bridges, which is correlated with the amount of overlap of actin and myosin (<a id="a-band" data-tip="1345" target="_blank" href="/glossary-term/a-band" class="glossary">A-band</a>), is correlated with the amount of force a <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> can produce. Changes in <a id="sarcomere" data-tip="1323" target="_blank" href="/glossary-term/sarcomere" class="glossary">sarcomere</a> length (<a id="z-disk" data-tip="1332" target="_blank" href="/glossary-term/z-disk" class="glossary">Z disk</a> to <a id="z-disk" data-tip="1757" target="_blank" href="/glossary-term/z-disk" class="glossary">Z-disk</a>), both shorter and longer, result in a decrease in the number of cross-bridges. This results in a relationship between length and <a id="tension" data-tip="1229" target="_blank" href="/glossary-term/tension" class="glossary">tension</a> discussed below under &quot;Length/Tension Relationship.&quot;</li>
 <li>There are limits to the capacity of a <a id="muscle-cell" data-tip="1374" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle cell</a> to shorten and lengthen and produce force at the extreme ends of these ranges. This is sometimes referred to as active insufficiency (too short) and passive insufficiency (too long).</li>
 <li>A loss of the total number of sarcomeres (as is found in long-term casting and contractures) will reduce strength, <a id="extensibility-2" data-tip="1308" target="_blank" href="/glossary-term/extensibility-2" class="glossary">extensibility</a>, and function.</li>
</ul>
<img alt="Sarcomere under electron microscope matched to diagram of zones. Author User: Sameerb in English WP" title="Sarcomere under electron microscope matched to diagram of zones. Author User: Sameerb in English WP" src="https://www.datocms-assets.com/25800/1632846077-sarcomere-2.gif">
 <a id="sarcomere" data-tip="1323" target="_blank" href="/glossary-term/sarcomere" class="glossary">Sarcomere</a> under electron microscope matched to a diagram of zones. Author User: Sameerb in English WP

Striations (Why are muscle cells striped under a microscope?)

The information above is presented as one motor <a id="nerve-cell" data-tip="1360" target="_blank" href="/glossary-term/nerve-cell" class="glossary">neuron</a> stimulating a single <a id="muscle-cell" data-tip="1374" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle cell</a>, but there is an additional layer of organization that increases the efficiency of the neuromuscular system and allows for the control of force output. When an <a id="action-potential-2" data-tip="1240" target="_blank" href="/glossary-term/action-potential-2" class="glossary">action potential</a> reaches the end of an axon it spreads out over many terminal branches, and each branch stimulates a different <a id="muscle-cell" data-tip="1374" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle cell</a>. This results in all <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> cells at the end of these terminal branches contracting simultaneously from the impulse of a single motor <a id="nerve-cell" data-tip="1360" target="_blank" href="/glossary-term/nerve-cell" class="glossary">neuron</a>. In essence, a motor <a id="nerve-cell" data-tip="1360" target="_blank" href="/glossary-term/nerve-cell" class="glossary">neuron</a> and all of the <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> cells it innervates function as a unit. A motor <a id="nerve-cell" data-tip="1360" target="_blank" href="/glossary-term/nerve-cell" class="glossary">neuron</a>, axon (<a id="nerve-cell" data-tip="1361" target="_blank" href="/glossary-term/nerve-cell" class="glossary">nerve</a> fiber), terminal branches, and all of the <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> cells (<a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> fibers) it innervates are known as a motor unit. The <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> cells of a single <a id="motor-unit" data-tip="1303" target="_blank" href="/glossary-term/motor-unit" class="glossary">motor unit</a> are generally dispersed throughout a <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a>, resulting in a weak diffuse contraction of the <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a>, rather than a strong local twitch. This simultaneous excitation of a unit of <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> cells improves the efficiency of the neuromuscular system.
<ol>
 <li>Single motor <a id="nerve-cell" data-tip="1360" target="_blank" href="/glossary-term/nerve-cell" class="glossary">neuron</a> and axon</li>
 <li>Many terminal branches</li>
 <li>All innervating a different <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> cell</li>
 <li>Stimulating a motor unit</li>
</ol>
<h4> Basics of Human Movement Systems (Connective Tissue, 3 Rules of Muscles, Intro to Motor Unit Recruitment) </h4>
<iframe src="https://player.vimeo.com/video/97592416" frameborder="0" allowfullscreen="allowfullscreen">
 </iframe>
<h4> Motor Unit Recruitment and Strength: </h4>
When a <a id="muscle-cell" data-tip="1374" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle cell</a> is stimulated the cell is stimulated completely, and contraction is carried out to the best of the cell&apos;s ability. Regardless of the strength of the impulse or the intensity of the external stimulus, an <a id="action-potential-2" data-tip="1240" target="_blank" href="/glossary-term/action-potential-2" class="glossary">action potential</a> is either initiated or not, and that <a id="action-potential-2" data-tip="1240" target="_blank" href="/glossary-term/action-potential-2" class="glossary">action potential</a> results in contraction of the stimulated <a id="muscle-cell" data-tip="1374" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle cell</a>, or it does not. It is analogous to a simple light switch and lamp. You cannot move the switch to the halfway position to turn on half the bulb or turn the switch on completely, but adjust the bulb to &quot;half brightness.&quot; Motor neurons and <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> cells are either &quot;on&quot; or &quot;off&quot;, there is no &quot;dimmer switch.&quot; This is known as the all-or-none principle - a <a id="nerve-cell" data-tip="1361" target="_blank" href="/glossary-term/nerve-cell" class="glossary">nerve</a> or <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> cell&apos;s response to a stimulus is independent of the strength of the stimulus. If that stimulus exceeds the <a id="excitation-threshold" data-tip="1239" target="_blank" href="/glossary-term/excitation-threshold" class="glossary">threshold potential</a> (threshold of an <a id="action-potential-2" data-tip="1240" target="_blank" href="/glossary-term/action-potential-2" class="glossary">action potential</a>) the <a id="nerve-cell" data-tip="1361" target="_blank" href="/glossary-term/nerve-cell" class="glossary">nerve</a> and/or <a id="muscle-cell" data-tip="1377" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle fiber</a> will respond completely; if it does not exceed <a id="excitation-threshold" data-tip="1239" target="_blank" href="/glossary-term/excitation-threshold" class="glossary">threshold potential</a> there is no response at all. This begs the question of how we control the force output of any <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a>. How do we pick up a pencil and not impart the same force we would impart on a 30lb dumbbell? The answer is motor unit recruitment. Although each <a id="motor-unit" data-tip="1303" target="_blank" href="/glossary-term/motor-unit" class="glossary">motor unit</a> contracts completely when sufficiently stimulated, motor units may include as few as 3 - 6 <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> fibers as found in the eye or 1000 or more as found in the larger muscles of the body like that gastrocnemius. Smaller motor units are well-suited for fine motor skills, whereas larger motor units are better for larger crude outputs of force. Larger motor units are attached to larger motor nerves that are harder to stimulate. The size and number of motor units recruited determine the force output of any <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a>. To continue the lamp and light switch analogy, although we do not have a dimmer switch, we have multiple lamps of varied brightness and a switch for each. If we need a little light, we can easily flip small switches to tiny lamps with more exacting control of brightness. If we need more light, we have to flip large switches for large diffuse amounts of light.
Motor units are not only an efficient means of recruiting <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> cells, this type of organization also allows motor units to work &quot;in-shifts&quot;. When tasks that require multiple repetitions, or a force must be held for a long duration, fatiguing motor units may be replaced by previously unrecruited motor units, who can then be replaced by more unrecruited motor units, who may then be replaced by previously recruited motor units who have now had sufficient time to rest (replenish <a id="adenosine-triphosphate-atp" data-tip="1247" target="_blank" href="/glossary-term/adenosine-triphosphate-atp" class="glossary">ATP</a>), etc., in a coordinated and sequential fashion.
<h4> Force Output (Strength) is Determined By: </h4>
<ol>
 <li>The number of motor units recruited</li>
 <li>The size of the motor units recruited</li>
</ol>
<h4> Endurance is Determined By: </h4>
<ol>
 <li>Available ATP</li>
 <li>The number and coordination of motor units working &quot;in-shifts&quot;</li>
</ol>
Note: In later lessons, we will discuss how <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> cells adapt to exercise to enhance strength, endurance, and <a id="power" data-tip="1399" target="_blank" href="/glossary-term/power" class="glossary">power</a>.
<a href="https://brookbush.s3.us-east-2.amazonaws.com/wordpress/wp-content/uploads/2017/10/Motor-Unit.jpg" target="_blank" rel="noopener"><img src="https://www.datocms-assets.com/25800/1649647504-motor-unit.jpg" title="Neuromuscular junction, motor endplates, striated cells." alt="Motor unit by electron microscope"></a>
<a id="motor-unit" data-tip="1303" target="_blank" href="/glossary-term/motor-unit" class="glossary">Motor Unit</a> by Electron Microscope: Note how you can see the <a id="neuromuscular-junction" data-tip="1304" target="_blank" href="/glossary-term/neuromuscular-junction" class="glossary">neuromuscular junction</a> and motor end plate and if you look closely the striations in the <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> cells.
<h4> Why is this information important: </h4>
First, it is important to be aware of <a id="muscle-cell" data-tip="1374" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle cell</a> physiology to inform the decision-making process during interventions for rehab, fitness, or performance. For example, the knowledge that available <a id="adenosine-triphosphate-atp" data-tip="1247" target="_blank" href="/glossary-term/adenosine-triphosphate-atp" class="glossary">ATP</a> is necessary for continued contraction may influence our understanding of research on energy systems and acute variables (reps, load, tempo). The physiology of contraction may also aid in understanding how creatine phosphate supplementation plays a role in augmenting training. Last, it may aid in understanding pathological processes like myasthenia gravis, muscular dystrophy, or understanding how Botox injections work by disrupting the <a id="neuromuscular-junction" data-tip="1304" target="_blank" href="/glossary-term/neuromuscular-junction" class="glossary">neuromuscular junction</a>.
Second, knowledge of physiology becomes a misinformation filter. Too many myths, misguided recommendations, and physiologically impossible promises are peddled to the professional and consumer alike. Having a strong foundation of physiology may aid in deciding which recommendations have the potential to work and which do not. For example, there is no place in the steps above that suggests this process can be &quot;confused&quot; to enhance adaptation. This process does not depend on which exercise or technique is chosen; it is the only way a <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> will contract. Another example, consider how an <a id="action-potential-2" data-tip="1240" target="_blank" href="/glossary-term/action-potential-2" class="glossary">action potential</a> is propagated along the entire sarcolemma of a cell. This results in a complete contraction of that cell from origin to insertion - it is not possible to contract, be tight, or target the <a id="distal" data-tip="1563" target="_blank" href="/glossary-term/distal" class="glossary">distal</a> or <a id="proximal" data-tip="1564" target="_blank" href="/glossary-term/proximal" class="glossary">proximal</a> portions of a <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a>.
Understanding this process should bring feelings of sheer amazement at the intricacy of the human body. Consider that all of the steps detailed above (and summarized below) happen every time you need to generate force. In fact, consider that this sequence happens many times per second, at every <a id="muscle-cell" data-tip="1374" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle cell</a>, of every <a id="motor-unit" data-tip="1303" target="_blank" href="/glossary-term/motor-unit" class="glossary">motor unit</a>, of every <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> you recruit, at every joint, and every time you move or resist movement.
<ol>
 <li>Stimulus (intent, reflex, motor program, etc)</li>
 <li><a id="action-potential-2" data-tip="1240" target="_blank" href="/glossary-term/action-potential-2" class="glossary">Action potential</a> at motor neuron</li>
 <li>Conducted by axon to the synapse</li>
 <li>ACh <a id="release" data-tip="1253" target="_blank" href="/glossary-term/release" class="glossary">released</a> into the neuromuscular junction</li>
 <li>ACh binds to receptors on the motor endplate of the <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> cell</li>
 <li>Sodium-ion channels open on the motor endplate</li>
 <li>An <a id="action-potential-2" data-tip="1240" target="_blank" href="/glossary-term/action-potential-2" class="glossary">action potential</a> is propagated across the sarcolemma</li>
 <li><a id="action-potential-2" data-tip="1240" target="_blank" href="/glossary-term/action-potential-2" class="glossary">Action potential</a> conducts down T-tubules</li>
 <li>Opens voltage-regulated calcium channels</li>
 <li>Calcium is <a id="release" data-tip="1253" target="_blank" href="/glossary-term/release" class="glossary">released</a> and binds to troponin</li>
 <li>The change in the shape of the troponin-tropomyosin complex exposes binding sites</li>
 <li><a id="adenosine-triphosphate-atp" data-tip="1247" target="_blank" href="/glossary-term/adenosine-triphosphate-atp" class="glossary">ATP</a> binds to Myosin</li>
 <li>Hydrolyzed <a id="adenosine-triphosphate-atp" data-tip="1247" target="_blank" href="/glossary-term/adenosine-triphosphate-atp" class="glossary">ATP</a> becomes ADP and extends the myosin head</li>
 <li>The myosin head binds to an available binding site on troponin</li>
 <li><a id="release" data-tip="1249" target="_blank" href="/glossary-term/release" class="glossary">Release</a> of ADP results in flexing of the myosin head</li>
 <li>Flexing the myosin head pulls actin resulting in a <a id="power" data-tip="1399" target="_blank" href="/glossary-term/power" class="glossary">power</a> stroke</li>
 <li><a id="adenosine-triphosphate-atp" data-tip="1247" target="_blank" href="/glossary-term/adenosine-triphosphate-atp" class="glossary">ATP</a> binds to myosin <a id="release" data-tip="1250" target="_blank" href="/glossary-term/release" class="glossary">releasing</a> the head from troponin</li>
 <li>The process starts over at a binding site further down actin.</li>
 <li>This step is multiplied by the number of binding sites on each protein, the number of proteins in each <a id="sarcomere" data-tip="1323" target="_blank" href="/glossary-term/sarcomere" class="glossary">sarcomere</a>, the number of proteins in parallel in each myofibril, and the number of myofibrils in each <a id="muscle-cell" data-tip="1374" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle cell</a>.</li>
 <li>Force output is then multiplied again, and calibrated for function, by the number of cells in the <a id="motor-unit" data-tip="1303" target="_blank" href="/glossary-term/motor-unit" class="glossary">motor unit</a> recruited, and the number of motor units recruited.</li>
</ol>
Note: Even this step-by-step account is a simplified summary of this process. Understanding each step represents years of work from incredible minds. It is not necessary that you memorize this entire sequence today, (although, you may be able to write down most of these steps in your own words after studying this course). But, stop studying for a moment, and take the time to appreciate how amazing the <a id="muscle-cell" data-tip="1374" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle cell</a> really is. This knowledge is the first step in understanding the human movement system.&#xA0;

Motor Unit Recruitment (Multiplication by Clustering)

Skeletal <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> is surrounded by layers of <a id="connective-tissue-cell" data-tip="1368" target="_blank" href="/glossary-term/connective-tissue-cell" class="glossary">connective tissue</a>, traditionally subdivided into 3 &quot;levels.&quot; These <a id="connective-tissue-cell" data-tip="1368" target="_blank" href="/glossary-term/connective-tissue-cell" class="glossary">connective tissue</a> layers add to the strength and integrity of a <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a>, as well as play a key role in transmitting the force generated by muscular contraction to the bones they are attached to. For example, by forming a tendon that attaches <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> to bone (a connective tissues structure).
<ul>
 <li>Epimysium: Layer of dense <a id="connective-tissue-cell" data-tip="1368" target="_blank" href="/glossary-term/connective-tissue-cell" class="glossary">connective tissue</a> encasing the entire <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a>. (The epimysium is often what is implied by the word &quot;fascia&quot; as these are sheaths of fascia covering a <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a>.)</li>
 <li>Perimysium: Layer of dense <a id="connective-tissue-cell" data-tip="1368" target="_blank" href="/glossary-term/connective-tissue-cell" class="glossary">connective tissue</a> surrounding each fascicle (bundle of <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> fibers).</li>
 <li>Endomysium: A thin layer of areolar (loose) <a id="connective-tissue-cell" data-tip="1368" target="_blank" href="/glossary-term/connective-tissue-cell" class="glossary">connective tissue</a> around each individual <a id="muscle-cell" data-tip="1374" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle cell</a> (<a id="muscle-cell" data-tip="1375" target="_blank" href="/glossary-term/muscle-cell" class="glossary">myocyte</a>).</li>
</ul>
The epimysium either fuses with the periosteum of bone directly (e.g. the origin of the <a href="https://brookbushinstitute.com/article/subscapularis" target="_blank" rel="noopener">subscapularis </a> ) or continues to fuse and <a id="posture" data-tip="1660" target="_blank" href="/glossary-term/posture" class="glossary">form</a> into tendons that fuse with the periosteum (e.g. the insertion of the <a href="https://brookbushinstitute.com/article/gastrocnemius-plantaris" target="_blank" rel="noopener">gastrocnemius</a> via the Achilles tendon).
<a href="https://brookbush.s3.us-east-2.amazonaws.com/wordpress/wp-content/uploads/2017/10/Layers-of-Fascia.jpe" target="_blank" rel="noopener"><img src="https://www.datocms-assets.com/25800/1649647617-layers-of-fascia.jpeg" title="The layers of fascia in a muscle fiber, including the epimysium, perimysium, and ectomysium" alt="Layers of fascia in a muscle fiber"></a> Layers of fascia. (Note the column projecting from the bottom picture is the myofibril, each cell contains these columns of myofilaments, there are not individually wrapped in <a id="connective-tissue-cell" data-tip="1368" target="_blank" href="/glossary-term/connective-tissue-cell" class="glossary">connective tissue</a>)
<h2>Length-Tension Relationship:</h2>
The amount of overlap between actin and myosin filaments (<a id="a-band" data-tip="1345" target="_blank" href="/glossary-term/a-band" class="glossary">A-band</a>) is correlated with the number of cross-bridges that can <a id="posture" data-tip="1660" target="_blank" href="/glossary-term/posture" class="glossary">form</a> and the amount of force that can be generated. As a <a id="muscle-cell" data-tip="1374" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle cell</a> is elongated the amount of overlap decreases, decreasing the amount of force that can be produced. Further, the available distance myosin can <a id="glide" data-tip="1598" target="_blank" href="/glossary-term/glide" class="glossary">glide</a> over actin before abutting <a id="z-disk" data-tip="1335" target="_blank" href="/glossary-term/z-disk" class="glossary">z-disks</a> also has an impact on the amount of force that can be produced. That is, a <a id="muscle-cell" data-tip="1374" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle cell</a> will produce less force when approaching a maximally shortened length. This results in an &quot;upside-down U&quot; shaped relationship between length and <a id="tension" data-tip="1229" target="_blank" href="/glossary-term/tension" class="glossary">tension</a>, with most muscles producing the most force close to their resting length.
<a href="https://brookbush.s3.us-east-2.amazonaws.com/wordpress/wp-content/uploads/2017/11/Length-tension.jpg" target="_blank" rel="noopener"><img src="https://www.datocms-assets.com/25800/1649647710-length-tension.jpg" title="The &lt;a id=" lengthtension-relationship" data-tip="1685" target="_blank" href="/glossary-term/lengthtension-relationship" class="glossary">Length/Tension</a> relationship: Note as length of a <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> gets longer or shorter the tension decreases&quot; alt=&quot;Length tension graph &quot;&gt; <a id="lengthtension-relationship" data-tip="1685" target="_blank" href="/glossary-term/lengthtension-relationship" class="glossary">Length/Tension</a> Relationship: Note, the &quot;active&quot; portion of this graph is explained by the interaction between actin and myosin described above, the passive relationship is as if the <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> was being stretched and resistance to <a id="extensibility-2" data-tip="1308" target="_blank" href="/glossary-term/extensibility-2" class="glossary">extensibility</a> is being imparted by proteins like titin and the <a id="connective-tissue-cell" data-tip="1368" target="_blank" href="/glossary-term/connective-tissue-cell" class="glossary">connective tissue</a> layers. By Mokele, via Wikimedia Commons Sliding Filaments, Length and Strength: 
<ul>
 <li>Middle length <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> = <a id="optimal" data-tip="1571" target="_blank" href="/glossary-term/optimal" class="glossary">optimal</a> overlap between actin/myosin and <a id="optimal" data-tip="1571" target="_blank" href="/glossary-term/optimal" class="glossary">optimal</a> force production</li>
 <li>Long <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> = little overlap between actin/myosin and less force production</li>
 <li>Short <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> = less ability to contract further and less force production</li>
</ul>
Note: In later lessons, we will discuss how the central <a id="nervous-system" data-tip="1690" target="_blank" href="/glossary-term/nervous-system" class="glossary">nervous system</a> aids in the control of resting <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> length via gamma motoneurons, <a id="muscle-cell" data-tip="1781" target="_blank" href="/glossary-term/muscle-cell" class="glossary">muscle</a> spindles, and <a id="tone" data-tip="1639" target="_blank" href="/glossary-term/tone" class="glossary">tone</a>. The intent of this system may be to maintain muscles close to <a id="optimal" data-tip="1571" target="_blank" href="/glossary-term/optimal" class="glossary">optimal</a> length to ensure <a id="optimal" data-tip="1571" target="_blank" href="/glossary-term/optimal" class="glossary">optimal</a> force production. In the video below, the relationship between <a id="lengthtension-relationship" data-tip="1685" target="_blank" href="/glossary-term/lengthtension-relationship" class="glossary">length/tension</a>, <a id="posture" data-tip="1658" target="_blank" href="/glossary-term/posture" class="glossary">posture</a>, performance, and injury is highlighted.
<iframe src="https://player.vimeo.com/video/76675908" frameborder="0" allowfullscreen="allowfullscreen">


 </iframe>

<ol>
 <li>
 Schoenfield, B. (2016). Science &amp; Development of Muscle Hypertrophy. Human Kinetics; Champaign, Il.
 </li>
 <li>
 Haff, G.G., Triplett, N.T. (2016). Essentials of Strength and Conditioning Training. (4th Ed.). Human Kinetics; Champaign, Il.
 </li>
 <li>
 Tiidus, P. (2008). Skeletal Muscle Damage and Repair. Human Kinetics; Champaign, Il.
 </li>
 <li>
 Saladin, K. (2012). Anatomy &amp; Physiology: The unity of form and function. (6th ed.). New York: McGraw-Hill.
 </li>
 <li>
 Cooper, G.M. (2000). The Cell: A Molecular Approach. (2nd ed.). Massachusetts: Sinauer Associates.
 </li>
 <li>
 Lodish, H., et al. (2000). Molecular Cell Biology. (4th ed.). New York: W.H. Freeman.
 </li>
 <li>
 Grosberg, A., et al. (2011). Self-organization of muscle cell structure and function. PLoSOne. http://doi.org/10.1371/journal.pcbi.1001088.
 </li>
 <li>
 Bodriau, S., Vincent, M., Cote, C.H., &amp; Rogers, P.A. (1993). Cytoskeletal structure of skeletal muscle: identification of an intricate exosarcomeric microtubule lattice in slow-and fast-twitch muscle fibers.
 </li>
</ol>

Bibliography

© 2017 Brent Brookbush


 Questions, comments, and criticisms are welcomed and encouraged

Copyright

A-Band - Refers to the part of the muscle cell that looks darker under a microscope.  This area of the cell is darker due to parallel arrangement of thicker filaments (myosin) and overlap with the thinner filaments (actin).
<ul>
 	<li>As a muscle contracts, the A-band is where actin and myosin interact.</li>
</ul>
<ol>
 	<li>Saladin, K. (2014). Anatomy &amp; physiology: The unity of form and function. McGraw-Hill Higher Education, New York. ISBN: 9780073403717</li>
</ol>

Accuracy - the proportion of individuals correctly identified by the test results.
<ul>
 	<li>For example, a special test with perfect accuracy would imply a test has 100% specificity, 100% sensitivity, 100% positive predictive value and 100% negative predictive values.</li>
</ul>

Action Potential - A rapid change in voltage, resulting in a wave of excitation, that may result in stimulation of a nerve or muscle cell.
<ul>
 	<li>In order for an action potential to be initiated, the resting membrane voltage must exceed the excitation threshold, which then results in a cascade of depolarization and the "wave of excitation." Any stimulus below that threshold will not result in an action potential, and a stimulus greater than the threshold will not increase a single action potential, but may result in additional action potentials.  In this way, the action potential functions more like an on/off switch (all or none), and less like a dimmer switch.</li>
</ul>

A single-joint movement pattern (most often) designed to load a specific under-active muscle(s), while minimizing the contribution of over-active synergists via specific cues/joint motions.
<ul>
 	<li style="text-align: left;">Activation techniques/exercises are performed with the intent of addressing a muscle exhibiting a reduction in activity (tone) as a result of, or contributing to postural dysfunction.</li>
 	<li style="text-align: left;">Activation techniques/exercises cannot be multi-joint movement patterns due to relative flexibility, altered reciprocal inhibition, and synergistic dominance resulting in compensation patterns.</li>
</ul>
Example, <a href="https://brentbrookbush.com/articles/corrective-exercise-articles/activation/gluteus-medius-activation-progression/" target="_blank">gluteus medius activation</a> involves resisted hip abduction while minimizing the contribution of the TFL via hip extension.

Active stretching is a term used to describe a stretch that involves an active contraction of opposing musculature to lengthen a muscle to its end range. Generally these stretches are held for 2-5 seconds and repeated for 8-15 repetitions.

Adenosine triphosphatase (ATPase): is the enzyme responsible for catalyzing (initiating and accelerating) the decomposition of ATP to ADP.
<ul>
 	<li>This dephosphorylation (breakdown) reaction releases energy for processes like muscular contraction.</li>
</ul>

Adenosine triphosphate (ATP): The molecule that functions as a universal energy-transfer molecule, providing most of the energy associated with muscular contraction and human movement. ATP is composed of an adenine group, a ribose group, and three molecules of inorganic phosphates.
<ul>
 	<li>Energy is provided when hydrolysis results in the cleaving of a phophate group; resulting in adenosine diphosphate, an inorganigic phosphate group, and energy.</li>
</ul>

Algometry - measuring the amount of pressure to result in sensitivity or pain, using an algometer.
<ul>
 	<li>Generally, research uses algometry to establish a minimum pain pressure threshold (PPT), in which a lower PPT implies more sensitivity and pain, and a higher PPT implies less sensitivity and pain. PPT is a common outcome measure in trigger point research.</li>
</ul>
<a href="https://brentbrookbush.com/wp-content/uploads/2019/07/Algometer.jpg"><img class="aligncenter wp-image-140177" src="https://brentbrookbush.com/wp-content/uploads/2019/07/Algometer.jpg" alt="Image of an Algometer" width="500" height="333" /></a>

All-or-none principle - a nerve or muscle cell's response to a stimulus is independent of the strength of the stimulus. If that stimulus exceeds the threshold potential (threshold of an action potential), the nerve or muscle fiber will respond completely; otherwise, there is no response.

Example, when a motor unit in the biceps brachii is stimulated it contracts the same way whether a 10 lb or 50 lb dumbbell is lifted.  It is actually the number of muscle cells and the size of the motor unit that determines force output.

&nbsp;

The transition period between eccentric load and concentric contraction during a repetition of an exercise.  This term is most often used in reference to power/plyometric exercises, in which a shorter amortization phase is believed to be beneficial.  The shorter amortization phase may better use the stored energy from the elastic deformation of connective tissue and enhance synchronization of myotatic (stretch) reflex with maximum voluntary contraction.

Example, the athlete was cued to spend as little time as possible in the bottom of their squat stance (the amortization phase) during a set of box jumps.

Muscles that oppose the prime mover and synergists for a given joint action.

That is, all the muscles that can perform the opposing joint action.

Example: The triceps and anconeus are antagonists during elbow flexion.

An anatomical direction; toward the front

Example, the face is on the anterior aspect of the head

Appendicular Skeleton - referring to the bones the bones of the arms and legs, as well as the bones that support them. This includes the pelvis, clavicle and scapula. There are approximately 126 bones in the appendicular skeleton.
<ul>
 	<li>Etymology - Appendicular, as in appendage, "that which is appended to something as a proper part," 1640s, from <a class="crossreference" href="https://www.etymonline.com/word/append?ref=etymonline_crossreference">append</a> + <a class="crossreference" href="https://www.etymonline.com/word/-age?ref=etymonline_crossreference">-age</a>. - <a href="https://www.etymonline.com/word/appendage" target="_blank" rel="noopener">Etymology Online</a></li>
</ul>
"Appendicular skeleton" is a label referring to a set of bones whose function is distinct from the bones of the "<a href="https://brentbrookbush.com/glossary/axial-skeleton/" target="_blank" rel="noopener">axial skeleton</a>".

A reflexive decrease in muscle activity due to joint dyskinesis.
For example, a reduction in deep cervical flexor activity as a result of cervical spine dyskinesis.

Arthrokinematic Motion - Small amplitude motions between bone surfaces at a joint.  The arthrokinematic motions are roll, glide (slide or translation), spin, compression and distraction (traction). Arthrokinematic motions accompany osteokinematic motions to maintain joint congruence.
<ul>
 	<li>Example, arthrokinematic motion of the talus includes posterior glide on the tibia during ankle dorsiflexion.</li>
</ul>
Synonyms include: arthrokinematics, passive accessory motion, joint play and component motion

&nbsp;

Assessment - is the act of assessing, i.e. determining the importance, size, or value of -

In most cases, the results of an assessment are compared to normative data. If the individual exhibits values that are within normal parameters, it may be assumed that the aspect of health or motion assessed is not contributing to the patient's complaint, symptoms or movement impairment. If the results fall outside of normal parameters, it may indicate that the aspect of health or motion assessed is contributing to, or being affected by the patient/client's complaint, symptoms, or movement impairment. In some cases, the results of an assessment are compared to an "ideal model." In this case, deviations from the ideal are noted as potential issues. For example, deviations noted in the <a title="Introduction to the Overhead Squat Assessment" href="https://brentbrookbush.com/online-courses/articles/assessment/introduction-overhead-squat-assessment/" target="_blank">Overhead Squat Assessment</a> are compared to an "ideal model" of posture and motion.

Assessments used by human movement professionals can be divided into three broad categories:
<ul>
 	<li>"Clear" the Patient/Client for Intervention - These are tests, assessments and evaluations used to determine if a patient/client's issue may improve given the assessing professional's scope, skills, abilities and willingness to treat that individual.
<ul>
 	<li>For example, many of the special tests used in orthopedic medicine assist in determining the level of pathology or likelihood of a particular diagnosis. Certain issues and diagnoses are beyond the scope of the human movement professional, and are better treated by diagnostic professionals in the medical community (physicians, podiatrists, surgeons, etc.). A Calf Squeeze (Thompson) Test is one example, in which a positive sign may indicate a rupture of the Achilles tendon that may be better treated via surgical intervention.</li>
</ul>
</li>
 	<li>Highlight Contraindications - Tests, assessments and evaluations used to stratify risk or preclude a professional from addressing certain tissues, motions or using a particular technique.
<ul>
 	<li>For example, the Vertebral Artery Test (VBI) is often used to determine if high velocity thrust mobilizations (manipulations) are safe for a patient with cervical dysfunction.</li>
</ul>
</li>
 	<li>Refine Exercise/Intervention Selection: Tests, assessments and evaluations used to assist the professional in determining which techniques, modalities and exercises will best address a patient/client's complaints or desired goals.
<ul>
 	<li>Most movement assessments fall into this category. For example, a positive <a href="https://brentbrookbush.com/vimeo_videos/elys-test-rectus-femoris-flexibility-assessment/" target="_blank">Ely's Test</a> may indicate a need to release and/or lengthen the <a href="https://brentbrookbush.com/articles/anatomy-articles/muscular-anatomy/rectus-femoris/" target="_blank">rectus femoris</a>.</li>
</ul>
</li>
</ul>
Note: An assessment, or assessment result, may fall into more than one category. For example, although goniometry is an assessment most often used to Refine Exercise/Intervention Selection, if a range of motion is significantly altered, with an abnormal end-feel, and/or causes the patient or client/pain, the individual may need to be referred to a physician for further testing and Clearance to resume rehab activities.  Further, that same patient may return to the human movement professional with a list of Contraindicated Activities from the physician. - "When in doubt, refer out!"

Axial Skeleton - referring to the bones of the head, neck, trunk and spinal column (does not include pelvis). There are approximately 80 bones in the axial skeleton.
<ul>
 	<li>Etymology - Axial, as in "axis", referring to "imaginary motionless straight line around which a body (such as the Earth) rotates," from Latin axis "axle, pivot, axis of the earth or sky" - <a href="https://www.etymonline.com/word/axis" target="_blank" rel="noopener">Etymology Online</a></li>
</ul>
"Axial skeleton" is a label referring to a set of bones whose function is distinct from the bones of the "<a href="https://brentbrookbush.com/glossary/appendicular-skeleton/" target="_blank" rel="noopener">appendicular skeleton</a>".

Balance - The ability to maintain a body’s center of mass over its base of support. Balance can be static or dynamic.
<ul>
 	<li>For example, maintaining balance is harder on one leg because the center of mass must stay over a much smaller base of support.</li>
</ul>

A ball and socket joint (also called a spheroid joint or spheroidal joint) is a synovial joint in which the rounded or spherical end of one bone fits into a cup-like depression of another bone. The distal bone is capable of motion around an indefinite number of axes, which have one common center. Examples include the hip and shoulder (glenohumeral joint)

Base of support (BoS) - refers to the area beneath an object or person, and the area within the perimeter created by every point of contact that the object or person makes with the supporting surface. This may include the <a href="https://brentbrookbush.com/articles/anatomy-articles/muscular-anatomy/gluteus-maximus/" target="_blank" rel="noopener">glutes</a> of someone sitting on a chair, or the back of someone leaning against a wall.
<ul>
 	<li>For example, during a single leg balance the BoS is the area of the foot in contact with the surface; however, when a person is standing on two feet, the BoS is the area of both feet and the surface between them.</li>
</ul>

Pertaining to both sides

Example, bilateral rows are performed with both arms at the same time.

Case-control Study - a particular form of retrospective study that samples a matched group of people who have, and do not have the outcome being studied. A case-control study is a quasi-experimental design, often used in medicine to aid in determining potential contributing factors (cannot determine causality). In this study type exposure is the dependent variable and outcome is independent.
<ul>
 	<li>Example, in this retrospective case-control, medical records were investigated for trends correlated with cervical pain (1).
<ol>
 	<li>Mäkela, M., Heliövaara, M., Sievers, K., Impivaara, O., Knekt, P., &amp; Aromaa, A. (1991). Prevalence, determinants, and consequences of chronic neck pain in Finland. American journal of epidemiology, 134(11), 1356-1367.</li>
</ol>
</li>
</ul>

An anatomical direction; toward the tail, relative to humans this term is often used to refer to "in the direction of the tale (toward the feet)".

Example, to release the trapezius muscle the therapist used a force directed caudally.

Center of mass (CoM) - an object's mean position of mass; that is, a point that is perfectly surrounded by an equal amount mass in all directions.

Center of gravity (CoG) - point where gravity appears to act. Since the human body is so small compared to the earth, we can assume gravity acts uniformly on the body, and that CoG and CoM are the same.
<ul>
 	<li>Example, the CoM of the human body while lying down or standing straight up is close to our navel. However, it is possible that during a resistance exercise, the combination of the body's weight and the weight of the resistance will place the combined CoM outside the body. For example, during the mid-portion of a <a href="https://brentbrookbush.com/vimeo_videos/back-squat/" target="_blank" rel="noopener">back squat</a> the CoM would be several anterior to the naval.</li>
</ul>

An anatomical direction; toward the head (synonym: cranial)

Example, hip pain was felt with any force directed cephalad.

Cohort Study - a particular form of longitudinal study that samples a cohort (a group of people who share a defining characteristic), performing a cross-section at intervals through-out a period of time. A cohort study is a quasi-experimental design often used in medicine to aid in determining etiology or a cause-and-effect relationship. In this study type exposure is the independent variable and outcome is dependent.
<ul>
 	<li>Example, the study by Cholewicki et al. (1) investigated Yale Varsity Athletes (the cohort) with 2 and 3 year follow up (longitudinal design) and found that trunk muscle response time to off-loading was predictive of future low back injury.</li>
</ul>
<ol>
 	<li><a href="https://brentbrookbush.com/articles/research-corner/core/increased-trunk-muscle-latency-may-cause-rather-than-result-from-low-back-pain/" target="_blank" rel="noopener noreferrer">Cholewicki, J., Silfies, S., Shah, R., Greene, H., Reeves, N. Alvi, K., Goldberg, B. (2005). Delayed trunk muscle reflex responses increase the risk of low back injuries. Spine. 30(23), 2614-2620</a></li>
</ol>

Comparable Sign: A combination of pain, stiffness and/or spasm during examination that is comparable to the patients symptoms.
<ul>
 	<li>One common error made during evaluation with special tests is attention given to any discomfort or pain, rather than attempting to identify the pain/discomfort related to the patient's complaint (concordant/comparable sign). Many special tests purposefully test tissue limits and are at least mildly uncomfortable in every individual. Basing your assessment on positive tests that resulted in "concordant/comparable signs" will improve your chances of arriving at the correct diagnosis.</li>
</ul>

An alternative movement pattern that has been adopted to compensate for dysfunction/impairment in the human movement system.
Example: A lack of calf extensibility may limit dorsiflexion, leading to the knees bowing inward (functional valgus) and excessive forward lean of the torso during a squat.

An arthrokinematic motion - the approximation of joint surfaces; that is, a force that directs one bone surface into another resulting in a reduction in intra-articular volume. Although widely thought of as a "destructive force", it is likely best to consider compression as a naturally occurring, stabilizing force. Any arthrokinematic force in excess (spin, glide, roll, traction) may be viewed as potentially deleterious.

For example, during a squat the femoral head is compressed into the acetabulum by muscular forces, bodyweight and any additional external load.

Concave (Concavity) - Having an outline or surface curved like the interior surface of a bowl or sphere - to cave-in or curve inward.
<ul>
 	<li>Example, the internal surfaces of the pelvis is concave, holding some of the bodies viscera.</li>
</ul>

Concordant Sign - The patient's specific pain, symptom or complaint. 
<ul>
 	<li>One common error made during evaluation with special tests is attention given to any discomfort or pain, rather than attempting to identify the pain/discomfort related to the patient's complaint (concordant/comparable sign). Many special tests purposefully test tissue limits and are at least mildly uncomfortable in every individual. Basing your assessment on positive tests that resulted in "concordant/comparable signs" will improve your chances of arriving at the correct diagnosis.</li>
</ul>

Conductivity: Stimulation results in more than a local effect (1).
<ul>
 	<li>For example, in a muscle cell the stimulation of a motor endplate results in a change in cell membrane polarity that rapidly propagates (is conducted) along the entire length of the muscle cell, stimulating all sacromere.</li>
</ul>
<ol>
 	<li>Saladin, K. (2012). Anatomy &amp; Physiology: The unity of form and function. (6th ed.). New York: McGraw-Hill.</li>
</ol>

A condyloid joint (also called condylar, bicondylar, ellipsoid, or ellipsoidal) is an ovoid articular surface, or condyle, that is received into an elliptical cavity.  These joints permit movement in two planes.  Examples include the knee, metacarpophalangeal joints and metatarsophalangeal joints.

Connective Tissue Cell - One of the four basic types of animal tissue, consisting mostly fibers and ground substance.  These cells are specialized to binds organs together, holds them in place, protect them from external forces, and may fill space in the body's cavities.

Example, the body's muscles are enclosed in durable connective tissue sheets known as the "epimysium", "perimysium", and "endomysium," which and in support and transmitting force.
<ol>
 	<li>Saladin, K. (2014). Anatomy &amp; physiology: The unity of form and function. McGraw-Hill Higher Education, New York. ISBN: 9780073403717</li>
</ol>

Contractility: The ability to shorten (1).
<ul>
 	<li>Contractility: Muscle cells are unique in the amount they can shorten when stimulated; this is this largerly due to the highly organized and unique arrangement of myofilaments (actin and myosin).</li>
</ul>
<ol>
 	<li>Saladin, K. (2012). Anatomy &amp; Physiology: The unity of form and function. (6th ed.). New York: McGraw-Hill.</li>
</ol>

Pertaining to the opposite side

Example, the external oblique muscle performs contralateral rotation; rotation to the opposite side.

Convex (Convexity) - Having an outline or surface curved like the exterior of a circle or sphere - to bulge or curve outward.
<ul>
 	<li>Example, the top of your head is shaped by the convex exterior surface of your skull.</li>
</ul>

Correlational Research - Non-experimental <a href="https://www.questionpro.com/blog/what-is-research/">research</a> method in which the statistical relationship between two variables is investigated.
<ul>
 	<li>In this research by Kwon et al., a correlation was noted between a decrease in extensibility during a hamstring length test (goniometry) and current knee pain (1).</li>
</ul>
<ol>
 	<li><a href="https://brentbrookbush.com/articles/research-corner/knee/correlation-patellofemoral-pain-syndrome-pfps-hamstring-activity/" target="_blank" rel="noopener">Kwon O, Yun M, and Lee W. (2014). Correlation between intrinsic patellofemoral pain syndrome in young adults and lower extremity biomechanics. J. Phys. Ther. Sci. 26: 961-964</a></li>
</ol>

Counter-nutation - posterior rotation of sacrum relative to ilium, and/or anterior rotation of ilium relative to sacrum.
<ul>
 	<li>The pelvic floor muscles are examples of muscle that may counter-nutate the sacrum.</li>
</ul>
[caption id="attachment_126335" align="aligncenter" width="400"]<a href="https://brentbrookbush.com/wp-content/uploads/2018/10/Nutation.png"><img class=" wp-image-126335" src="https://brentbrookbush.com/wp-content/uploads/2018/10/Nutation.png" alt="Nutation and Counter-nutation of the Sacroiliac Joint" width="400" height="967" /></a> Nutation and Counter-nutation of the Sacroiliac Joint[/caption]

&nbsp;

Crossover (Study) Design - A type of longitudinal, repeated measures quasi-experimental design in which each group receives both treatments, but at different times. It is named "crossover" due to the patients "crossing-over" to the other treatment or sham group after a specified period of time.
<ul>
 	<li>This type of study has the advantage of every individual serving as their own control, which may reduce the influence of confounding variables.</li>
 	<li>Unfortunately, the design itself introduces the chances that anyone receiving the experimental variable in the first period will continue to be effected in the second period.</li>
</ul>
In a study by Senna et al., a cross-over design was used to investigate the difference load made on inter-set rest intervals (1).
<ol>
 	<li><a href="https://brentbrookbush.com/articles/research-corner/strength-training-research-corner/heavy-vs-light-load-single-joint-exercise-performance-with-different-rest-periods/" target="_blank" rel="noopener">Senna, G.W., Rodrigues, B.M., Sandy, Scudese, Bianco, A, &amp; Dantas, E.H.M. (2017). Heavy vs light load single-joint exercise performance with different intervals. Journal of Human Kinetics, 58, 197-206. </a></li>
</ol>

Cross-sectional Study - a research design that involves gathering data from a population or representative subset at a specific point in time.
<ul>
 	<li>Example, the study by Karimi-Mobarake et al.(1) investigated the prevalence of genu varum and genu valgum among school children 7-11 years old in Iran.</li>
</ul>
<ol>
 	<li><a href="https://brentbrookbush.com/articles/research-corner/knee/low-prevalence-of-genu-varum-and-valgum-among-elementary-school-children/" target="_blank" rel="noopener noreferrer">Karimi-Mobarake, M., Kashefipour, A., Yousfnejad, Z. The prevalence of genu varum and genu valgum in primary school children in Iran 2003-2004. (2005) Journal of Medical Science 5(1). 52-54</a></li>
</ol>

Cytokine - A broad and loose category of small proteins that act through receptors, and are especially important to immune function (inflammation).
<ul>
 	<li>Cytokines include substances such as interferon, interleukin, and growth factors, which are secreted by certain cells of the immune system and have an effect on other cells. Their definite distinction from hormones is a topic of continued research.</li>
</ul>
&nbsp;

Dependent variable - in research, this is the variable that is changed by the independent variable, most often this is the "outcome" of the study.
<ul>
 	<li>Example, in the study by Cleland et al. (1), the independent variable is "thoracic manipulation," and the dependent variable is "lower trapezius muscle strength."</li>
</ul>
<ol>
 	<li><a href="https://brentbrookbush.com/articles/research-corner/short-term-effects-lower-thoracic-manipulation-lower-trapezius-muscle-strength/" target="_blank" rel="noopener noreferrer">Cleland J, Selleck B, Stowell T, Brown L, Alberini S, St. Cyr H, Caron T. Short-term Effects of Thoracic Manipulation on Lower Trapezius Muscle Strength.  Journal of Manual &amp; Manipulative Therapy. 2004; 12(2): 82-90</a></li>
</ol>

An anatomical direction; further from the trunk or center of the body

Example, the foot is at the distal end of the leg.

An arthrokinematic motion - a force applied to a body part that may result in the separation of joint surfaces, a reduction in intra-articular pressure, and/or an increase in intra-articlar space.

For example, to hang from one's hands (i.g. in preparation for a pull-up) results in a traction force on the glenohumeral joint (shoulder).

An anatomical term of location; back or upper surface

Example, shoe laces may compress tissues on the dorsal surface of the foot

The "drawing-in" maneuver is a common cue used during exercise and rehabilitation programs. A light contraction of the <a href="https://brentbrookbush.com/articles/muscular-anatomy/transverse-abdominis/" target="_blank" rel="noopener">transverse abominis</a> (and potentially the entire <a href="https://brentbrookbush.com/articles/corrective-exercise-articles/core-subsystems/intrinsic-stabilization-subsystem/" target="_blank" rel="noopener">intrinsic stabilization subsystem</a>) achieved by gently pulling the lower abdominal region away from one's waist band. This should have minimal effect on breathing, and should not be confused with a maximal contraction of the <a href="https://brentbrookbush.com/articles/muscular-anatomy/transverse-abdominis/" target="_blank" rel="noopener">transverse abdominis</a> resulting in a "vacuum pose".
<ul>
 	<li>Research often refers to this maneuver as the "abdominal drawing-in maneuver"; abbreviated ADIM.</li>
</ul>
Development and introduction of this cue into practice is based on the pivotal research and resulting text:
<ol>
 	<li>Carolyn Richardson, Paul Hodges, Julie Hides. Therapeutic Exercise for Lumbo Pelvic Stabilization – A Motor Control Approach for the Treatment and Prevention of Low Back Pain: 2nd Edition (c) Elsevier Limited, 2004</li>
</ol>

Faulty or abnormal movement; an alteration of normal kinematics.

For example, joint stiffness or a reduction in arthrokinematics motion that results in limited osteokinematic range of motion - inadequate inferior glide of the femoral head resulting in limited abduction of the hip.

The seeping into, or abnormal collection of fluid in a body cavity.

For example, knee effusion describes swelling within the knee joint.

Note: the word "effusion" is used differently in medicine than it is used in reference to the movement (seeping out) of liquids and gases, for example in chemistry.

&nbsp;

A trait of body tissues that allows a tissue to return to its resting length or state after deformation or stretch.
Example:  Muscle is elastic tissue; many muscles will return to their resting length after being stretched to 150% of that length without any appreciable change in tissue quality.

Elasticity: Elasticity refers to the ability of a cell to return to its resting length after being stretched/lengthened.
<ul>
 	<li>Note: this term is often confused with extensibility. Elasticity is not the ability to stretch; it is the ability to return to normal length after being stretched. Muscle tissue, connective tissue and even nerve tissue have varying levels of elasticity.</li>
</ul>
<ol>
 	<li>Saladin, K. (2012). Anatomy &amp; Physiology: The unity of form and function. (6th ed.). New York: McGraw-Hill.</li>
</ol>

Epithelial cell - One of the four basic cell types, found lining the body's cavities, blood vessels, organs and constitutes most gland tissue.  These cells are specialized to aid with absorption, secretion, or act as a barrier from foreign objects.
<ol>
 	<li>Saladin, K. S., &amp; Miller, L. (1998). Anatomy &amp; physiology. New York (NY): WCB/McGraw-Hill.</li>
</ol>

Equilibrium - a state in which opposing forces or influences are balanced.
<ul>
 	<li>For example, to maintain a neutral trunk while pushing or pulling a weight in one hand, your core musculature must balance the force by creating an equal amount of force in the opposite direction.</li>
</ul>

The study of causation, or origination. The word is commonly used in the medical professions, where it may refer to the study of why things occur, or the reason behind why things act a certain way.

For example, the etiology of low back pain is a complex subject, with multiple contributing factors and various structures implicated.

<div class="gs_citr" tabindex="0">Evidence-based medicine (practice) is the integration of best research evidence with clinical expertise and patient(client) values (2)."</div>
<ul>
 	<li class="gs_citr" tabindex="0">...the conscientious, explicit and judicious use of current best evidence in making decisions about the care of individual patients (clients).  It involves the integration of at least (a) clinical expertise/expert opinion, (b) external scientific evidence, and (c) client/patient/caregiver perspectives to provide high-quality services reflecting the interests, values, needs, and choices of the individuals we serve (1).</li>
</ul>
&nbsp;
<ol>
 	<li id="gs_cit1" class="gs_citr" tabindex="0">Sackett, D. L., Rosenberg, W. M., Gray, J. M., Haynes, R. B., &amp; Richardson, W. S. (1996). Evidence based medicine: what it is and what it isn't. Bmj, 312(7023), 71-72.</li>
 	<li class="gs_citr" tabindex="0">Sackett D et al. Evidence-Based Medicine: How to Practice and Teach EBM, 2nd edition. Churchill Livingstone, Edinburgh, 2000, p.1</li>
</ol>

Excitability: The ability of a cell to change in membrane conductance as a response to stimulation.

<ul>
 	<li>For example, muscle cells are specialized to react to a change in membrane potential with contraction, where as nerve cells are specialized to propagate a change in membrane potential along there axons and communicate that message to other tissues via neurotransmitters.
</li>
</ul>
<ol>
 	<li>Saladin, K. (2012). Anatomy &amp; Physiology: The unity of form and function. (6th ed.). New York: McGraw-Hill.</li>
</ol>

Excitation threshold: The level that a depolarization must reach for an action potential to occur.
<ul>
 	<li>The excitation threshold is a contributing factor to the "all-or-none" response of muscle and nerves cells to a stimulus.</li>
</ul>

Exercise Progression - Modification of an exercise with the intent of promoting adaptation by increasing demand. This can be accomplished by increasing reps, load, tempo, range of motion, complexity, and/or challenge to an individual's stability.
<ul>
 	<li>Because load, reps, tempo and complexity are often explicitly noted, the term "exercise progression" is most often used throughout Brookbush Institute (BI) content to refer to an exercise, or series of exercises that make it harder to maintain stability with good form.
<ul>
 	<li>Sample Exercise Progression: <a href="https://brentbrookbush.com/vimeo_videos/kneeling-chop-pattern/" target="_blank" rel="noopener">Kneeling Chop</a> to <a href="https://brentbrookbush.com/vimeo_videos/static-standing-chop-pattern/" target="_blank" rel="noopener">Standing Chop</a> to <a href="https://brentbrookbush.com/vimeo_videos/single-leg-chop-pattern/" target="_blank" rel="noopener">Single-leg Chop</a></li>
</ul>
</li>
</ul>

Exercise regression - The opposite of progression; that is, modification of an exercise with the intent of reducing demand. This can be accomplished by decreasing reps, load, tempo, range of motion, complexity, and/or challenge to an individual's stability. Generally, regressions are used to modify an exercise to the client's current level of ability, with the intent of improving form, motor learning and retention.
<ul>
 	<li>Because load, reps, tempo and complexity are often explicitly noted, the term "exercise regression" is most often used throughout Brookbush Institute (BI) content to refer to an exercise or series of exercises that make it easier to maintain stability with good form.
<ul>
 	<li>Sample Exercise Regression: <a href="https://brentbrookbush.com/vimeo_videos/single-leg-chop-pattern/" target="_blank" rel="noopener">Single-leg Chop</a> to <a href="https://brentbrookbush.com/vimeo_videos/static-standing-chop-pattern/" target="_blank" rel="noopener">Standing Chop</a> to <a href="https://brentbrookbush.com/vimeo_videos/kneeling-chop-pattern/" target="_blank" rel="noopener">Kneeling Chop</a></li>
</ul>
</li>
</ul>

Experimental Research - Studies in which the researchers introduce the variable to be studied, with the intent of observing the outcome. This is different than observational research which observes the effect of a variable already present. Many texts also assert that experimental research should include randomization and ideally a control group, as is seen in Randomized Control Trials. Studies in which the variable is introduced by the researchers, but participants are not randomized may be considered Quasi-experimental research.
<ul>
 	<li>In this study by Esformes, the experimental variable was squat depth and the outcome measures observed were various performance measures related to power (1).</li>
</ul>
<ol>
 	<li><a href="https://brookbushinstitute.com/article/back-squat-depth-lower-body-post-activation-potentiation/" target="_blank" rel="noopener">Esformes, J., &amp; Bampouras, T. (2013). Effect of back squat depth on lower-body postactivation potentiation. Journal of Strength and Conditioning Research, 27(11), 2997-3000.</a></li>
</ol>
&nbsp;

Extensibility: The ability of a cell to be stretched or lengthened without damage (1).
<ul>
 	<li>For example, most cells would rupture with even a modest stretch, but muscles can stretch up to 3 times their contracted length.</li>
</ul>
<ol>
 	<li>Saladin, K. (2012). Anatomy &amp; Physiology: The unity of form and function. (6th ed.). New York: McGraw-Hill.</li>
</ol>

False Negative - the proportion of negatives in a group that were assessed incorrectly (assessed as negative when the patient was actually positive)
<ul>
 	<li>Test Results: Although most tests/assessments result in either a positive or negative, the accuracy of a test is dependent on the number  of positives and negatives that are correct. Therefore, when the accuracy of a test is determined, their are 4 possible results, because a positive or negative result could be right (true) or wrong (false).</li>
</ul>

False Positive - the proportion of positives in a group that were assessed incorrectly (assessed as positive when the patient was actually negative)
<ul>
 	<li>Test Results: Although most tests/assessments result in either a positive or negative, the accuracy of a test is dependent on the number  of positives and negatives that are correct. Therefore, when the accuracy of a test is determined, their are 4 possible results, because a positive or negative result could be right (true) or wrong (false).</li>
</ul>

The function of fascia is to transmit force, provide support, and protect tissues. Fascia can effect motion by:
Transmitting force from one or more muscles (example: the thoracolumbar fascia)
Restricting motion in cases of adaptive shortening and/or adhesion to proximal structures (example: iliotibial band in cases of lower-leg dysfunction)
Elastic recoil after stretching can contribute to force production (example: plyometric exercise)

Note: There is evidence that fascia may house more receptors related to human movement than other tissues. In this way fascia may act as a motherboard for the nervous system

The fascial system does not contract or develop trigger points, and its capacity to adapt to stretching and or strengthening exercise is limited, especially when compared to the adaptive capacity of muscle tissue.

Fibromyalgia - A loosely defined diagnosis that is associated with a combination of widespread pain in combination with 2) tenderness at 11 or more of the 18 specific tender point sites. Consideration may also be given to the presence of psychosocial stressors and inflammatory disease in the patients history. Newer research suggests that fibromyalgia may be at least in part due to central sensitization of myofascial pain, often associated with trigger points (1 - 4).
<ol>
 	<li>Wolfe, F. (2018). Fibromyalgia Syndrome and Fibromyalgia Syndrome Criteria: Problems in Definition and Validity. Fibromyalgia Syndrome and Widespread Pain: From Construction to Relevant Recognition.</li>
 	<li>Wolfe, F., Smythe, H. A., Yunus, M. B., Bennett, R. M., Bombardier, C., Goldenberg, D. L., ... &amp; Fam, A. G. (1990). The American College of Rheumatology 1990 criteria for the classification of fibromyalgia. Arthritis &amp; Rheumatism: Official Journal of the American College of Rheumatology, 33(2), 160-172.</li>
 	<li>Alonso-Blanco, C., Fernandez-de-las-Penas, C., Morales-Cabezas, M., Zarco-Moreno, P., Ge, H. Y., &amp; Florez-García, M. (2011). Multiple active myofascial trigger points reproduce the overall spontaneous pain pattern in women with fibromyalgia and are related to widespread mechanical hypersensitivity. The Clinical journal of pain, 27(5), 405-413.</li>
 	<li>Staud, R., Nagel, S., Robinson, M. E., &amp; Price, D. D. (2009). Enhanced central pain processing of fibromyalgia patients is maintained by muscle afferent input: a randomized, double-blind, placebo-controlled study. PAIN®, 145(1-2), 96-104.</li>
</ol>
&nbsp;

Muscles that act to reduce or prevent movement at proximal joints to improve force transmission.
Example: The muscles of the core are important fixators; reducing motion of the spine/trunk to enhance force transmission between the extremities and environment.

An increase in joint stability via compressive forces resulting from muscular contraction and increased tension in fascial structures (1,2).
<ul>
 	<li>This term is most often used in reference to core intrinsic stabilizers, the thoracolumbar fascia and the sacroiliac joint; however, force closure and form closure have been mentioned in the literature referring to other joints.</li>
 	<li>Related term: Form closure</li>
</ul>
<ol>
 	<li>Andry Vleeming, Vert Mooney, Rob Stoeckart. Movement, Stability &amp; Lumbopelvic Pain: <a class="glossaryLink " title="Glossary: Integration" href="https://brentbrookbush.com/glossary/integration/" target="_blank" rel="noopener noreferrer" data-cmtooltip="&lt;div class=glossaryItemTitle&gt;Integration&lt;/div&gt;&lt;div class=glossaryItemBody&gt;Making a whole of parts.&lt;br /&gt;&lt;br /&gt;Example, The Brookbush Institute uses an integrative approach to explaining and addressing human movement.&lt;/div&gt;">Integration</a> of Research and Therapy. (c) 2007 Elsevier Limited</li>
 	<li id="gs_cit1" class="gs_citr" tabindex="0">Vleeming, A., Pool-Goudzwaard, A. L., Stoeckart, R., van Wingerden, J. P., &amp; Snijders, C. J. (1995). The Posterior Layer of the Thoracolumbar Fascia| Its Function in Load Transfer From Spine to Legs. Spine, 20(7), 753-758.</li>
</ol>

The relative contribution of joint shape and structure to the stability of a joint (1,2).
<ul>
 	<li>This term is most often used in reference to the "rough" articulating surfaces of the sacroiliac joint and the wedge shape of the sacrum; however, force closure and form closure have been mentioned in the literature referring to other joints.</li>
 	<li>Related term: Force closure</li>
</ul>
<ol>
 	<li>Andry Vleeming, Vert Mooney, Rob Stoeckart. Movement, Stability &amp; Lumbopelvic Pain: <a class="glossaryLink " title="Glossary: Integration" href="https://brentbrookbush.com/glossary/integration/" target="_blank" rel="noopener noreferrer" data-cmtooltip="&lt;div class=glossaryItemTitle&gt;Integration&lt;/div&gt;&lt;div class=glossaryItemBody&gt;Making a whole of parts.&lt;br /&gt;&lt;br /&gt;Example, The Brookbush Institute uses an integrative approach to explaining and addressing human movement.&lt;/div&gt;">Integration</a> of Research and Therapy. (c) 2007 Elsevier Limited</li>
 	<li id="gs_cit1" class="gs_citr" tabindex="0">Vleeming, A., Pool-Goudzwaard, A. L., Stoeckart, R., van Wingerden, J. P., &amp; Snijders, C. J. (1995). The Posterior Layer of the Thoracolumbar Fascia| Its Function in Load Transfer From Spine to Legs. Spine, 20(7), 753-758.</li>
</ol>

An arthrokinematic motion - linear motion across a joint surface parallel to the plane of the adjacent joint surface, just as your posterior moves relative to a "slide" in a park.

For example, the glide of facet joint in the spine during spinal flexion and extension, or the posterior glide of the humeral head that must accompany anterior roll to maintain congruence with glenoid fossa during shoulder horizontal adduction.

A plane joint (also called an arthrodial joint, gliding joint or plane articulation) is a synovial joint which allows only gliding movement in the plane of the articular surfaces. The opposed surfaces of the bones are flat or almost flat, with movement generally limited by tight capsules and ligaments. Plane joints are numerous, are most often small, and allow very little motion.  Examples include the carpal joints of the wrist, the tarsal joints of the ankle, and the facet joints of the spine.

"...refers to the measurement of angles, in particular the measurement of angles created at human joints by the bones of the body (1).

&nbsp;
<ol>
 	<li>Cynthia C. Norkin, D. Joyce White. Measurement of Joint Motion: A Guide to Goniometry 3rd Edition. Copyright (C) 2003 by F.A. Davis Company</li>
</ol>

Ground reaction force (GRF) - The force exerted by the ground on the body in contact with it.

For example, when a runner's foot strikes the ground with a force equal to the mass of the individual times their acceleration due to gravity, momentum and muscular exertion, the ground exerts an equal and opposite force on the runner's foot.
<ul>
 	<li>GRF is often measured in research studies using force plates as a means of quantitatively measuring the force generated by an individual.</li>
</ul>

H-Band - Refers to the central zone of a sarcomere that looks lighter under a microscope.  This area of the sarcomere is a lighter portion of the A-band where there is no overlap between the thinner filaments (actin), and the thicker filaments (myosin).
<ul>
 	<li>Example, when a muscle contracts, actin and myosin cross-bridging occurs in the region of the A-band until the H-band is reached where no overlap occurs.</li>
</ul>
<ol>
 	<li>Saladin, K. (2014). Anatomy &amp; physiology: The unity of form and function. McGraw-Hill Higher Education, New York. ISBN: 9780073403717</li>
</ol>

A hinge joint (also called a ginglymus) is a synovial joint in which the articular surfaces fit one another in a way that only permits motion in one plane. Examples include the elbow, the knee (unless considered a condyloid joint) and the interphalangeal joints.

Holism (holistic) (biological holism) - (from Greek holos "all, whole, entire") is the idea that systems and their properties should be viewed as wholes, not just as a collection of parts.
<ul>
 	<li>Summarized by Aristotle in his "Metaphysics": "The whole is more than the sum of its parts". However, the term "holism" was introduced into language by the South African statesman Jan Smuts as recently as 1926. (Smuts J. C. 1987 [1926]. Holism and evolution. Cape Town: N &amp; S Press.)</li>
</ul>
Example: The Brookbush Institute promotes a holistic approach to human movement science, in which the muscular, fascial, articular and neural systems are not viewed as separable and all proximal segments are considered.

Hormone: Chemical messengers produced by the endocrine system, transported via tissue fluids such as blood, to stimulate cells to "act."
<ul>
 	<li>Example: Growth hormone is secreted by the pituitary gland in the brain; stimulating growth, cell reproduction and cell regeneration in certain types of cells</li>
</ul>
The English physician E. H. Starling discovered in collaboration with the physiologist W. M. Bayliss secretin, the first hormone, in 1902. (1).
<ol>
 	<li>Henriksen, J. H., &amp; de Muckadell, O. S. (2000). Secretin, its discovery, and the introduction of the hormone concept. Scandinavian journal of clinical and laboratory investigation, 60(6), 463-472.</li>
</ol>

A range of motion achieved by the patient/client that is more than normal.
<ul>
 	<li>Note: Both hyper-mobility and hypo-mobility may adversely affect motion and may lead to deleterious effects on joints and soft tissue over time. There is no evidence to suggest that more or less flexibility than normal is beneficial.</li>
</ul>
Example: If an individual can place their forehead on their knees they are likely hypermobile at one, if not several joints

Example 2: 110° of shoulder external rotation would be considered hyper-mobile.

Hyperplasia is an increase in organic tissue size due to an adaptive increase in the number of cells per organ.
<ul>
 	<li>Although muscle hyperplasia has been noted in some animals, human muscle tissue does not seem to have this adaptive ability.</li>
</ul>

A condition of excessive tone of the skeletal muscles; increased resistance of muscle to passive stretching.  May be related to increased neural drive, synergistic dominance, trigger points, reflexive over-activity, and/or muscle length.
Example:  Individuals who exhibit ankle eversion and abduction during an overhead squat assessment likely have hypertonic fibularis (peroneal) muscles.

Muscular hypertrophy is an increase in muscle mass/size due to increase in the volume and cross-sectional area (CSA) of individual muscle cells. The increase in cross-sectional area can be attributed to an an increase in contractile proteins and structural elements (myofibrillar hypertophy), as well as an increase in glycogen storage and elements related to metabolism (sarcoplasmic hypertophy). Hypertrophy is an adaptation of both cardiac and skeletal muscle fibers in response to progressive increases in workload.
<ul>
 	<li>Note: Hypertrophy is not hyperplasia. Hyperplasia is an increase in muscle size due to an adaptive increase in the number of muscle fibers/cells per muscle. This type of adaptation does not occur in human muscle tissue.</li>
</ul>

A range of motion achieved by a patient/client that is less than normal.
<ul>
 	<li>Note: Both hyper-mobility and hypo-mobility may adversely affect motion and may lead to deleterious effects on joints and soft tissue over time. There is no evidence to suggest that more or less flexibility than normal is beneficial.</li>
</ul>
Example: If the end range of shoulder external rotation is reached and measured as 75° during goniometric assessment, the client/patient is exhibiting hypomobility of the glenohumeral joint (shoulder).

Hypothesis - a proposed explanation for a phenomenon; "scientific hypotheses" are generally constructed in a way that is testable or falsifiable.
<ul>
 	<li>Example: The hypothesis, "excessive knee valgus may be correlated with future knee pain and injury," has been the subject of several studies published over the last decade.</li>
</ul>

Diminished tone of skeletal muscles; diminished resistance of muscles to passive stretching.   May be related to inhibition, reciprocal inhibition, arthrokinematics inhibition and/or  increased resting length of muscle.
Example:  If your knees cave (hip adduction) during a squat or leg-press it is likely that your gluteus medius is hypotonic.

I-Band - Refers to the part of the muscle cell that looks lighter under a microscope.  This area of the cell is lighter due to parallel arrangement of thinner filaments (actin), and no overlap with the thicker filaments (myosin).
<ul>
 	<li>As a muscle contracts, the I-bands will decrease in width and move closer to one another.</li>
</ul>
<ol>
 	<li>Saladin, K. (2014). Anatomy &amp; physiology: The unity of form and function. McGraw-Hill Higher Education, New York. ISBN: 9780073403717</li>
</ol>

Independent variable - in research, this is the variable or phenomenon being manipulated in the study.
<ul>
 	<li>Example, in the study by Ghanbari et al. (1), the independent variable is "knee joint mobilization", the dependent variable is "quadriceps muscle strength."</li>
</ul>
<ol>
 	<li><a href="https://brentbrookbush.com/articles/research-corner/effect-knee-joint-mobilization-quadriceps-muscle-strength/" target="_blank" rel="noopener noreferrer">Ghanbari A. and Kamalgharibi S. (2013). Effect of knee joint mobilization on quadriceps muscle strength. International Journal of Health and Rehabilitation Sciences. 2(4): 186-191</a></li>
</ol>

An anatomical direction; toward the bottom (below)

Example, the mouth is inferior to the eyes

A reduction in neural drive, muscular activity, and or force output due to altered neuromuscular reflex.  Alterations in neuromuscular reflex may occur due to increases in muscle length (hypothesis: increased excitation threshold), altered reciprocal inhibition in response to increased antagonist tone, and or alterations in joint motion (arthrokinematics inhibition).

Making a whole of parts.

Example, The Brookbush Institute uses an integrative approach to explaining and addressing human movement.

Intermuscular Coordination - Optimal timing and motor unit recruitment of agonists, antagonists, synergists, neutralizers, stabilizers and fixators.

Example: Role of Muscles during Hip Extension
<ul>
 	<li><a class="glossaryLink " style="box-sizing: border-box; font-family: roboto; color: #000000 !important; font-size: 15px; outline: none; background: transparent; border-bottom: 1px dotted #000000 !important; text-decoration: none;" title="Glossary: Prime Mover" href="https://brentbrookbush.com/glossary/prime-mover/" target="_blank" rel="noopener" data-cmtooltip="&lt;div class=glossaryItemTitle&gt;Prime Mover&lt;/div&gt;The muscle most responsible for a joint action under load -&lt;br /&gt;&lt;br /&gt;That is, the muscle that can produce the most force for a given joint action. Often this term is used synonymously with the term agonist. The prime mover as defined above, may or may not be the most active muscle for a joint action during activities of daily living.&lt;br /&gt;&lt;br /&gt;Example: the gluteus maximus is the most powerful hip extensor and therefore the prime mover; however, the hamstrings are likely the most active hip extensors during low level activities like walking or standing.">Prime Mover</a>:  <a href="https://player.vimeo.com/external/82855344.hd.mp4?s=54c3e7d98fb9d4e0af8d282c85ba9b3e">Gluteus maximus</a></li>
 	<li><a class="glossaryLink " style="box-sizing: border-box; font-family: roboto; color: #000000 !important; font-size: 15px; outline: none; background: transparent; border-bottom: 1px dotted #000000 !important; text-decoration: none;" title="Glossary: Synergists" href="https://brentbrookbush.com/glossary/synergists/" target="_blank" rel="noopener" data-cmtooltip="&lt;div class=glossaryItemTitle&gt;Synergists&lt;/div&gt;Muscles that assist the prime mover in performing a joint action.&lt;br /&gt;&lt;br /&gt;That is, all muscles that can perform a joint action that are not the prime mover, are termed synergists.&lt;br /&gt;Example: The rectus abdominis is the prime mover of spinal flexion. All other muscles that can perform spinal flexion are synergists including – external obliques, internal obliques and psoas.">Synergists</a>: <a href="https://brentbrookbush.com/articles/muscular-anatomy/biceps-femoris/">Biceps femoris (long head)</a>, <a href="https://brentbrookbush.com/articles/muscular-anatomy/semitendinosus-and-semimembranosus/">semitendinosus, semimembranosus</a>, <a href="https://brentbrookbush.com/articles/muscular-anatomy/adductors/">posterior head of adductor magnus</a></li>
 	<li><a class="glossaryLink " style="box-sizing: border-box; font-family: roboto; color: #000000 !important; font-size: 15px; outline: none; background: transparent; border-bottom: 1px dotted #000000 !important; text-decoration: none;" title="Glossary: Antagonist" href="https://brentbrookbush.com/glossary/antagonists/" target="_blank" rel="noopener" data-cmtooltip="&lt;div class=glossaryItemTitle&gt;Antagonist&lt;/div&gt;Muscles that oppose the prime mover and synergists for a given joint action.&lt;br /&gt;&lt;br /&gt;That is, all the muscles that can perform the opposing joint action.&lt;br /&gt;&lt;br /&gt;Example: The triceps and anconeus are antagonists during elbow flexion.">Antagonists</a>: <a href="https://brentbrookbush.com/articles/muscular-anatomy/psoas/">Psoas</a>, <a href="https://brentbrookbush.com/articles/muscular-anatomy/iliacus/">iliacus</a>, <a href="https://brentbrookbush.com/articles/muscular-anatomy/tensor-fascia-latae-tfl/">tensor fascia latae</a> (TFL),<a href="https://brentbrookbush.com/articles/muscular-anatomy/rectus-femoris/"> rectus femoris</a>, <a href="https://brentbrookbush.com/articles/muscular-anatomy/adductors/">anterior adductors (especially pectineus)</a>, <a href="https://brentbrookbush.com/articles/muscular-anatomy/sartorius/">sartorius</a></li>
 	<li><a class="glossaryLink " style="box-sizing: border-box; font-family: roboto; color: #000000 !important; font-size: 15px; outline: none; background: transparent; border-bottom: 1px dotted #000000 !important; text-decoration: none;" title="Glossary: Neutralizer" href="https://brentbrookbush.com/glossary/neutralizers/" target="_blank" rel="noopener" data-cmtooltip="&lt;div class=glossaryItemTitle&gt;Neutralizer&lt;/div&gt;Muscles that oppose unwanted joint motions created by the prime mover and/or synergists, and/or muscles that prevent unwanted ancillary motion.&lt;br /&gt;Example: The teres minor and infraspinatus neutralize the internal rotation force created by the pectoralis major during horizontal adduction of the shoulder.">Neutralizers</a>: <a href="https://brentbrookbush.com/articles/muscular-anatomy/gluteus-minimus/">Gluteus minimus</a> and <a href="https://brentbrookbush.com/articles/muscular-anatomy/gluteus-medius/">anterior fibers of gluteus medius</a> neutralize external rotation force of <a href="https://brentbrookbush.com/articles/muscular-anatomy/gluteus-maximus/">gluteus maximus</a></li>
 	<li><a class="glossaryLink " style="box-sizing: border-box; font-family: roboto; color: #000000 !important; font-size: 15px; outline: none; background: transparent; border-bottom: 1px dotted #000000 !important; text-decoration: none;" title="Glossary: Stabilizer" href="https://brentbrookbush.com/glossary/stabilizers/" target="_blank" rel="noopener" data-cmtooltip="&lt;div class=glossaryItemTitle&gt;Stabilizer&lt;/div&gt;Muscles whose primary role is to improve arthrokinematics by maintaining optimal alignment of joint surfaces.&lt;br /&gt;&lt;br /&gt;Most often these muscles are the most intrinsic muscles of a joint, have a larger percentage of Type I muscle fibers and are rich in proprioceptors.&lt;br /&gt;Example: The muscles of the rotator cuff act as stabilizers for most shoulder motions.">Stabilizers</a>: <a href="https://brentbrookbush.com/articles/muscular-anatomy/deep-rotators-of-the-hip/">Deep rotators of hip</a></li>
 	<li><a class="glossaryLink " style="box-sizing: border-box; font-family: roboto; color: #000000 !important; font-size: 15px; outline: none; background: transparent; border-bottom: 1px dotted #000000 !important; text-decoration: none;" title="Glossary: Fixator" href="https://brentbrookbush.com/glossary/fixators/" target="_blank" rel="noopener" data-cmtooltip="&lt;div class=glossaryItemTitle&gt;Fixator&lt;/div&gt;Muscles that act to reduce or prevent movement at proximal joints to improve force transmission.&lt;br /&gt;Example: The muscles of the core are important fixators; reducing motion of the spine/trunk to enhance force transmission between the extremities and environment.">Fixators</a>: <a href="https://brentbrookbush.com/articles/core-subsystems/intrinsic-stabilization-subsystem/">Intrinsic stabilization subsystem</a>, <a href="https://brentbrookbush.com/articles/muscular-anatomy/rectus-abdominis/">rectus abdominis</a>, <a href="https://brentbrookbush.com/articles/muscular-anatomy/internal-obliques/">internal</a> and <a href="https://brentbrookbush.com/articles/muscular-anatomy/external-obliques/">external obliques</a>, <a href="https://brentbrookbush.com/articles/muscular-anatomy/quadratus-lumborum/">quadratus lumborum</a>, <a href="https://brentbrookbush.com/articles/muscular-anatomy/erector-spinae/">erector spinae</a></li>
</ul>
&nbsp;

Example from: <a href="https://brentbrookbush.com/articles/anatomy-articles/introduction-to-functional-anatomy/kinesiology-of-the-hip/" target="_blank" rel="noopener">Introduction to Functional Anatomy: Kinesiology of the Hip</a>

Intramuscular coordination refers to the coordinated recruitment of motor units within a single muscle. This concept is used to explain increases in maximal strength and power; however, the coordinated and sequential recruitment of motor units is also essential to strength endurance and smooth coordinated motion.  Related terminology may include; motor unit recruitment, preferential recruitment, rate coding, sequence, etc..

Example: It is hypothesized that the "shaking" noted during a novice exercisers attempt at an activity like the <a href="https://brentbrookbush.com/vimeo_videos/ball-crunch-and-progressions/" target="_blank">stability ball crunch</a> may be due to relatively poor intramuscular coordination. Rather than smooth, sequential firing of rectus abdominis motor units, groups of motor units turn on and off in an uncoordinated manner.

Pertaining to the same side

Example, the quadratus lumborum performs ipsilateral flexion; lateral flexion toward the same side as the muscle.

An isoform, or variant, is a member of a set of similar proteins, that originate from the same gene, or gene family.
<ul>
 	<li>Example, the variations in myosin heavy chain proteins (MHC I, MHC IIA, MHC IIB/X) that contribute to the characteristic differences between muscle fiber types.</li>
</ul>

“The Prime Assumption”
Everything crossing a joint will influence joint motion during every joint action.

A concept that has expanded over the past several decades to describe the integration of 4 body systems (muscle, joints, fascia and nerves), as well as, the coordinated function of various body segments (e.g. hip, knee and ankle) to produce motion. Kinetic referring to motion, and chain being an analogy for the interdependent link between systems.

Example, the posterior kinetic chain, refers to the integrated function of nerves, muscles, fascia and joints on the posterior side of the body.

The concept of the kinetic chain was originally adapted from the work of a mechanical engineer named Franz Reuleaux (Kinematics of Machinery (1875)) by Dr. Arthur Steindler in 1955 (Steindler A: Kinesiology of the Human Body. Springfield, IL, Charles C. Thomas Co.. 1955).

Latent Trigger Point - A myofascial trigger point that is not painful at rest, only painful when palpated, and may or may not exhibit a characteristic referral pain pattern when palpated.  Latent trigger points do result in an acute point of sensitivity, in a taut band of muscle, and may restrict range of motion (1).

Note: Based on the clinical experience, the Brookbush Institute suggests that latent trigger points (sometimes refereed to as Tender Points) are more common than active trigger points. Further, they are likely addressed more often than trigger points; being the intended target of many soft-tissue mobility techniques.
<ol>
 	<li>David G. Simons, Janet Travell, Lois S. Simons, Travell &amp; Simmons’ Myofascial Pain and Dysfunction, The Trigger Point Manual, Volume 1. Upper Half of Body: Second Edition,© 1999 Williams and Wilkens</li>
</ol>

An anatomical direction; further from the midline

Example, the anterior deltoid is lateral to the pectoralis major

Length/Tension Relationship - The amount of force generated by a muscle is dependent on sarcomere length.
<ul>
 	<li>The ability of sarcomere to maximally produce force occurs when sarcomere length results in optimal overlap between actin and myosin. Stretching a sarcomere decreases overlap between actin and myosin filaments and reduces force production (1-3). When length is less than optimal, actin from opposite ends overlap and interfere with cross-bridging, and myosin filament are compressed as they contact the the Z-disk also reducing force (4)</li>
</ul>
<ol>
 	<li>Gordon, A. M., Huxley, A. F., &amp; Julian, F. J. (1966). The variation in isometric tension with sarcomere length in vertebrate muscle fibres. The Journal of physiology, 184(1), 170-192.</li>
 	<li>Edman, K. A. P. (1966). The relation between sarcomere length and active tension in isolated semitendinosus fibres of the frog. The Journal of Physiology, 183(2), 407-417.</li>
 	<li>Lieber, R. L., Loren, G. J., &amp; Friden, J. (1994). In vivo measurement of human wrist extensor muscle sarcomere length changes. Journal of neurophysiology, 71(3), 874-881.</li>
 	<li>Lieber, R. L. (2002). Skeletal muscle structure, function, and plasticity. Lippincott Williams &amp; Wilkins.</li>
</ol>
&nbsp;

Levels of Evidence - Relative to evidence-based practice, levels of evidence are an attempt to describe the strength of the results measured in a clinical trial or research study.  Note: levels of evidence do not supersede the critical evaluation of an experienced professional, and should be viewed as guidelines - a meta-analysis may be poorly constructed (Level IA), and a comparative study (Level III) may employ new technologies and a strong methodology.


The Levels of Evidence used by the Brookbush Institute, as described by Shekelle et al.(1):
<ul>
 	<li class="p1">IA - Evidence from meta-analysis of randomized controlled trials</li>
 	<li class="p1">IB - Evidence from at least one randomized controlled trial</li>
 	<li class="p1">IIA - Evidence from at least one controlled study without randomization</li>
 	<li class="p1">IIB - Evidence from at least one other type of quasi-experimental study</li>
 	<li class="p1">III - Evidence from non-experimental descriptive studies, such as comparative studies, correlation studies, and case-control studies</li>
 	<li class="p1">IV - Evidence from expert committee reports or opinions or clinical experience of respected authorities, or both</li>
</ul>
<ol>
 	<li>Shekelle, P. G., Woolf, S. H., Eccles, M., &amp; Grimshaw, J. (1999). Developing clinical guidelines. Western Journal of Medicine, 170(6), 348.</li>
</ol>

Likelihood Ratios - Likilihood ratios are used for assessing the value of performing a diagnostic test. They use sensitivity and specificity to determine whether a test result usefully changes the practitioner's chance (probability) of reaching the correct assessment. Calculation of likelihood ratios is based on Baye's theorem.
<ul>
 	<li>Positive Likelihood Ratio (LR+) - The ratio of a positive test result in people with the pathology to a positive test result in people without the pathology.</li>
 	<li>Negative Likelihood Ratio (LR-) - The ratio of a negative test result in people with the pathology to a negative test result in people without the pathology.</li>
</ul>

Longitudinal Study - a research design that involves observing the same variables, several times over a period of time.
<ul>
 	<li>Example, the study by Hewett et al. (1) is a type of longitudinal study called a "cohort study." This study investigated whether excessive knee valgus during landing was a predictor of knee injury risk in adolescent females over the course of a season.</li>
</ul>
<ol>
 	<li><a href="https://brentbrookbush.com/articles/research-corner/knee/research-review-increased-valgus-during-landing-predicts-acl-injury-in-adolescent-female-athletes/" target="_blank" rel="noopener noreferrer">Hewett, T. E., Myer, G. D., Ford, K. R., Heidt, R. S., Colosimo, A. J., McLean, S. G., &amp; Succop, P. (2005). Biomechanical measures of neuromuscular control and valgus loading of the knee predict anterior cruciate ligament injury risk in female athletes A prospective study. The American journal of sports medicine, 33(4), 492-501</a></li>
</ol>

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