Human Movement Science & Functional Anatomy of the:

Deltoids

by Brent Brookbush DPT, PT, COMT, MS, PES, CES, CSCS, ACSM H/FS

"Deltoid muscle top10" by Anatomography - en:Anatomography (setting page of this image). Licensed under CC BY-SA 2.1 jp via Commons - https://commons.wikimedia.org/wiki/File:Deltoid_muscle_top10.png#/media/File:Deltoid_muscle_top10.png

What's in a name:

The deltoid is commonly divided into 3 parts based on function, fiber arrangement, neural innervation, and separation at the proximal attachment. The 3 divisions are:

  • Anterior Deltoid
  • Middle Deltoid (There is no such thing as a "medial" deltoid)
  • Posterior Deltoid

  • anterior (adj.) 1610s, Latin, literally "former," comparative of ante "before" (see ante). Related: Anteriority. (Etymology Online)
  • mid (prep., adj.) Old English mid "with, in conjunction with, in company with, together with, among," from Proto-Germanic *medjaz (cognates: Old Norse miðr, Old Saxon middi, Old Frisian midde, Old High German mitti, Gothic midjis "mid, middle"), from PIE *medhyo- "middle" (see medial (adj.)). Now surviving in English only as a prefix (mid-air, midstream, etc.); as a preposition it often is a shortened form of amid (compare midshipman). (Etymology Online)
  • posterior (adj.) 1530s, "later," from Latin posterior "after, later, behind," comparative of posterus "coming after, subsequent," from post "after" (see post-). Meaning "situated behind" is from 1630s. (Etymology Online)

"Deltoideus posterior" by User:Mikael Häggström - Image:Gray409.png. Licensed under Public Domain via Commons - https://commons.wikimedia.org/wiki/File:Deltoideus_posterior.PNG#/media/File:Deltoideus_posterior.PNG

Attachments:

  • Anterior:

    • Origin: Anterior border, superior surface and lateral 1/3 of clavicle (11).
    • Insertion: Deltoid tuberosity of the humerus (11).

  • Middle:

    • Origin: Lateral margin and superior surface of acromion (11).
    • Insertion: Deltoid tuberosity of the humerus (11).

  • Posterior:

    • Origin: Inferior lip of the posterior border of the spine of the scapulae (11).
    • Insertion: Deltoid tuberosity of the humerus (11).

  • Despite having a common insertion, the 3 heads of the deltoids invest in the deltoid tuberosity via distinct intramuscular tendons (21).

Deltoid_muscle_top9

Relative Location:

The deltoid is commonly divided into 3 parts based on function, fiber arrangement, neural innervation, and separation at the proximal attachment. The 3 divisions are anterior, middle (note: there is no such thing as a "medial" deltoid) and posterior. All 3 divisions are superficial, deep only to the skin and a fairly well developed sheath of fascia, known simply as the deltoid fascia. The deltoid fascia continues to invest in the pectoral fascia anteriorly and the fascia covering the trapezius both medially and posteriorly (which is a continuation of the superficial layer of the deep cervical fascia). The origin of the deltoid, viewed from the top, creates a "U" shape, or brackets "" around the shoulder girdle - wrapping from the lateral 1/3 of the clavicle to lateral 2/3 of the spine of the scapula. The medial border of the anterior deltoid is clearly demarcated by the deltopectoral groove and the cephalic vein which runs through it. The lateral border of the anterior deltoid ends at the distal end of the clavicle, where the middle deltoid begins on the acromion - in essence, the origin of these two heads are divided by the acromioclavicular joint. The origin of the middle and posterior deltoid are often separated by an indentation between the two, at the posterior angle of the acromion. The medial border of the posterior deltoid is met by the insertion of the lower trapezius on the inferior rim of the spine of the scapula.

"Slide12dj" by Anatomist90 - Own work. Licensed under CC BY-SA 3.0 via Commons - https://commons.wikimedia.org/wiki/File:Slide12dj.JPG#/media/File:Slide12dj.JPG

All 3 divisions of the deltoid muscle insert into the deltoid tuberosity on the lateral humerus. The broad origin and pointed insertion, give the deltoid a delta shape (from a lateral view). As the deltoid muscle "caps" the shoulder, it lies superficial to a complex network of insertions. The pectoralis major, latissimus dorsi, teres major and long head of the biceps brachii approach the anterior deltoid and pass underneath it on their way to the bicipital groove, where they are joined by the insertions of the subscapularis on the lesser tubercle and infraspinatus and teres minor on the greater tubercle. The middle deltoid covers the subacromial space, the insertion of the supraspinatus, parts of the subacromial bursae and much of the axillary nerves investing in the deltoid itself. The posterior deltoid covers the lateral portion of the infraspinatus, the origin of the teres major and teres minor, the long head of the triceps and the axillary nerve as it passes through the quadrangular space.

The origin of the brachialis abuts the insertion of the deltoids on the inferior and anterior sides of the deltoid tuberosity. The posterior portion of the deltoid tuberosity is abutted by the origin lateral head of the triceps brachii.

"Gray207" by Henry Vandyke Carter - Henry Gray (1918) Anatomy of the Human Body (See "Book" section below)Bartleby.com: Gray's Anatomy, Plate 207. Licensed under Public Domain via Commons - https://commons.wikimedia.org/wiki/File:Gray207.png#/media/File:Gray207.png

Palpation

Have your partner sit or stand with their arm(s) bare. Stand to the side of your partner with a lateral view of their arm. The deltoid is a superficial muscle. In most individuals you will be able to see the deltoid at the top of the humerus, without any further cuing or palpation. In very lean individuals, a little abduction will make the pointed insertion at the deltoid tuberosity visible.

Place your hands on the top of the clavicle and acromion. Using your thumb and forefinger, try to outline the "U" shaped origin of the deltoid from clavicle to the spine of scapula. Follow the deltoid to its most posterior fibers on the middle 1/3rd of the spine of the scapula and note how they meet the fibers of the trapezius muscles. Cuing your partner to abduct their shoulder will cause most of the deltoid to contract and become tense under your fingers - alternating between abduction and relaxation may aid your palpation as you follow the various fibers of the deltoid from origin to insertion. Further, using flexion to isolate the anterior deltoid, and extension to isolate the posterior deltoid may aid in your palpation.

Notice the well developed deltoids, including the indentation between the posterior and middle deltoids. https://s-media-cache-ak0.pinimg.com/736x/a4/bd/ea/a4bdea7bddd71c9856cd4591c0460936.jpg

Nerve:

The deltoids are innervated by the axillary nerve, via the posterior cord of the brachial plexus, originating from nerve roots C5 and C6 (8, 11). The anterior branch of the axillary nerve innervates the anterior and middle heads of the deltoid, and the posterior branch or the axillary nerve innervates the posterior deltoid, as well as the teres minor and long head of the triceps brachii (8, 22, 23).

"Gray810" by Henry Vandyke Carter - Henry Gray (1918) Anatomy of the Human Body (See "Book" section below)Bartleby.com: Gray's Anatomy, Plate 810. Licensed under Public Domain via Commons - https://commons.wikimedia.org/wiki/File:Gray810.png#/media/File:Gray810.png

Action:

  • Anterior: Shoulder flexion, horizontal adduction, internal rotation and will contribute to abduction when firing in synergy with the middle and posterior deltoid (8,11). The anterior deltoid may be the primary abductor of the shoulder when the humerus is externally rotated, for example, during an overhead press.
  • Middle: Abduction (8, 11)
  • Posterior: Shoulder horizontal abduction, extension, external rotation and will contribute to abduction when firing in synergy with the middle and anterior deltoid (of the 3 parts of the deltoid, the posterior deltoid has the smallest moment arm for abduction) (8 ,11). Some texts note a contribution to adduction when the humerus is fully externally rotated and abducted - for example, during a pull-up (13).

    • Note that the anterior and posterior deltoid are antagonists to one another.
    • The anterior and posterior divisions have a fusiform fiber arrangement suggesting these muscles are more apt to contribute to speed than strength, while the middle deltoid is comprised of multipennate bands of muscle suggesting adaptation to produce more force at the expense of speed (8). Perhaps, the variations in fiber arrangement is based on the relative contribution to force for a given joint action - the middle deltoid receives help from far few and smaller muscles during abduction of the shoulder, than the anterior and posterior deltoid receives during flexion or extension.
    • Although the anterior and posterior deltoid have a fiber arrangement and moment arm that alludes to the ability to perform internal and external rotation, EMG studies conflict regarding their recruitment during activity including these actions (19).

Deltoid - Cadaver Disection - https://c2.staticflickr.com/4/3286/3018650468_1566997531_b.jpg

Integrated Function:

Stabilization: Shoulder and acromioclavicular joint

Eccentrically Decelerates:

  • Anterior: Shoulder extension, horizontal abduction, external rotation and will decelerate adduction when firing in synergy with the middle and posterior deltoid.
  • Middle: Adduction
  • Posterior: Shoulder horizontal adduction, extension, internal rotation and may aid in decelerating adduction when firing in synergy with the middle and anterior deltoid.

Synergists:

  • Abduction: During shoulder abduction the middle deltoid works synergistically with the anterior and posterior deltoid and the supraspinatus. This action often occurs in conjunction with scapular upward rotation and activation of the serratus anterior, upper trapezius and lower trapezius.

    • It was believed that the supraspinatus initiated abduction and the deltoids contributed the necessary force needed to overcome resistance; however, EMG studies show the middle deltoid and supraspinatus are recruited together during abduction with a similar increase in activity to peak activation between 90 - 180º of abduction (15).

  • Flexion and Horizontal Adduction: The anterior deltoid, coracobrachialis, pectoralis major and biceps brachii commonly work as a functional unit (8).
  • Extension: The prime more of extension is the latissimus dorsi, with aid from synergists: posterior deltoid, teres major and long head of the triceps brachii.
  • Internal Rotation: The anterior deltoid may contribute to internal rotation, although this is not its primary role. The subscapularis and pectoralis major are more active and produce more force during internal rotation.
  • External Rotation: The posterior deltoid may contribute to external rotation, although this is not its primary role. The posterior deltoid should stay relatively inactive during external rotation with the teres minor and infraspinatus providing the majority of the force necessary to overcome resistance.

Synergistic Dominance:

  • The posterior deltoid may become synergistically dominant for the infraspinatus and teres minor in those who exhibit Upper Body Dysfunction (UBD). It is thought that the lengthened position of the infraspinatus and teres minor reduces force production, the over-activity of the subscapularis alters reciprocal inhibition leading to relative inhibition, resulting in a relative increase in firing rate of the posterior deltoid and increase in force production during external rotation and shoulder stability during arm elevation. This results in altered glenohumeral arthrokinematics which may be a contributing factor to various shoulder pathologies and UBD.

http://cdn.muscleforlife.com/wp-content/themes/mfl/tim/timthumb.php?src=http://cdn.muscleforlife.com/wp-content/uploads/2015/10/rotator-cuff-exercises.jpg&w=640&q=85

Subsystems

    • The anterior deltoid is not traditionally viewed as a muscle that plays a role in an integrated core subsystem; however, the continuation of the abdominal fascia into the pectoral fascia (see "Fascial Integration" below) and then the deltoid fascia (especially the fascia over-lying the anterior deltoid) alludes to a continuation of the Anterior Oblique Subsystem (AOS) that envelopes and recruits from the entire anterior kinetic chain. We can see a synergistic relationships between the anterior deltoid, pectoralis major and the AOS during all throwing motions and pushing motions. Even the kyphotic and rounded position of an individual with significant Upper Body Dysfunction (UBD) implies that these muscles may be linked with a propensity toward adaptive shortening and over-activity.
    • Although it is common for the AOS, pectoralis major and the anterior deltoid to become adaptively shortened, some individuals will adopt an anterior pelvic tilt, shoulder instability, and/or prime mover inhibition (20), which may imply that there is a need for integration and conditioning of the AOS. When creating a program to correct one of these dysfunctions, integrated exercise designed to increase AOS recruitment (legs with push) should be part of a rehabilitation and/or a resistance training program.

http://victoriawellness.com/wp-content/uploads/versus_anterior_sling.jpg

Arthrokinematics:

  • Shoulder: All three head of the deltoids may contribute to superior glide of the humeral head on the glenoid fossa (3, 10). The considerable force vector for superior translation created by the deltoid, adds greater significance to the inferior glide and role of the infraspinatus, teres minor, subscapularis and potentially supraspinatus in stabilizing the glenohumeral joint during elevation of the arm. Although the anterior and posterior deltoid have a fiber arrangement and moment arm that alludes to the ability to perform anterior and posterior roll, EMG studies conflict regarding their recruitment during internal and external rotation (19). Based on fiber arrangement, function and behavior during postural dysfunction it seems likely that the anterior deltoid contributes to posterior glide of the humerus by tipping the humeral head back during flexion and the posterior deltoid contributes to anterior glide by tipping the humeral head forward during extension.
  • In a study by Lawrence et al., shoulder pain patients exhibited arthrokinematic dysfunction that included excessive inferior and anterior glide (17). This may be the result of prime mover inhibition of the anterior and middle deltoid (16) resulting in a decrease in a superior translatory force, with a relative increase in activity and the development of trigger points in the posterior deltoid (16, 18) resulting in an increase in anterior glide.

http://drkamaldeep.files.wordpress.com/2010/12/clip_image002_thumb2.jpg?w=591&h=498

Fascial Integration:

My Fascial Hypothesis: Large fascial sheaths not only play a role in the transmission of mechanical force, but may also play a role in dictating the function of muscular synergies. This is likely caused by reducing or increasing tone of invested musculature via reflex arcs formed between mechanoreceptors embedded in the connective tissue and the attached musculature. In this way my view of fascia differs slightly from noted expert on the subject Tom Myers. I think of these large fascial sheaths (specifically the thoracolumbar fascia, iliotibial band, and abdominal fascial sheath) as natures “mother board.” A place for mechanical information to be communicated to the nervous system for more efficient recruitment of the muscular system. Despite having a slightly different philosophy it does not change the fact that fascia plays an important communicative role in the human body and we have Tom Myers to thank for his work.

  • Deltoid and Pectoral Fascia: The deltoid fascia is fairly superficial, running nearly continuous with a thin lamina covering the surface of the pectoralis major, known as the pectoral fascia (20). The division between the pectoral fascia and deltoid fascia is clearly demarcated by the deltopectoral groove and the cephalic vein. Further, the deltoid fascia wraps underneath the anterior deltoid and continues into the complex network of axillary fascia and the superficial fascia covering the biceps brachii (20). The insertion of the deltoid was found to be fused with the lateral intermuscular septum, and a fusion was observed between anterior band of deltoid muscle and the insertion of pectoralis major (23).  The synergy of muscles implicated by this continuity of superficial fascia is a functional unit recruited during flexion and horizontal adduction - the anterior deltoid, coracobrachialis, pectoralis major and biceps brachii.

https://s3.amazonaws.com/classconnection/895/flashcards/2612895/png/brachial_fascia1310955901310-14FB35612807664468D.png

  • As mentioned above under the heading "Subsystems", the deltoid fascia may be a continuation of an anterior axial fascia that extends from the Anterior Oblique Subsystem (AOS). The deltoid fascia, running continuous with the pectoral fascia, continues to the well developed pectoral fascia that is continuous with the fibrous sheaths of the rectus abdominis and external obliques and eventually the adductors via the pubis and adductor tendons. Further examination of both superficical and deep layers of the anterior axial fascia alludes to a synergy of muscles involved in all pushing, throwing, and spiral motions.

  • The function of this synergy can be observed during some of the most powerful activities in sport (e.g. throwing a baseball, swinging a bat, an axe chop, the “Heisman Maneuver”, linemen pushing, etc). The synergy above varies a great deal in function and behavior relative to postural dysfunction, when compared to the “Serape Effect” or “Spiral Lines” discussed by Tom Myers (6) and others. However, I have found consideration of the fascial network above to be practically significant. That is, the clavipectoral, anterior axillary fascia and deltopectoral fascia are often a source of restriction in those with Upper Body Dysfunction (UBD), and those same individuals often exhibit signs of AOS dominance or over-activity. It is easy to imagine a relationship between adaptive shortening and over-activity in any one of the muscles listed above, and the resulting restriction throughout this fascial network. Further research is needed to clarify the muscles involved in this synergy and what role the anterior axial fascia plays.
  • Deltoid and Trapezius Fascia: A fascial continuity exists between the trapezius fascia (an extension of the superficial layer of the deep cervical fascia) and the deltoid fascia - consider the link between the origin of the deltoid and insertion of the trapezius muscle on the spine of the scapula and acromion (20). The origin of the deltoid overlaps and mirrors the insertion of the trapezius muscle, from the medial surface of the posterior deltoid fascia and the infraspinous fascia to the anterior deltoid fascia's strong investment into the deep brachial fascia (23). This fascial synergy is noted by Tom Myers as part of the "Superficial Back Arm Line" (6). The deltoids and trapezius muscle work synergistically during elevation of the arm to ensure optimal scapulohumeral rhythm, glenohumeral congruence and clearance of the acromion shelf as the arm is elevated beyond 90º. The strongest fascial connection is likely between the upper trapezius, superior acromioclavicular (AC) ligament, and the anterior deltoid (10). This relationship is highlighted every time we reach for something – shoulder flexion via the anterior deltoid, stabilization of the AC joint via increased tension on the superior acromioclavicular ligament, and upward rotation of the scapula via the upper trapezius.

Cadaver Dissection of Superficial Back Arm Line (Trapezius, Deltoids, Extensor Mass - Anatomy Trains: 2nd Edition by Tom Myers Cadaver Dissection of Superficial Back Arm Line (Trapezius, Deltoids, Extensor Mass - Anatomy Trains: 2nd Edition by Tom Myers

Behavior in Postural Dysfunction:

Several clinicians and texts have alluded to the propensity of the deltoid to become dominant, short and over-active (7, 8); however, dysfunction likely results in prime mover inhibition of the anterior and middle deltoid (1, 8, 11, 12, 15, 16, 17), and a relative increase in posterior deltoid activity (16, 18). The internally rotated position adopted by those who exhibit UBD, likely has little if any affect on deltoid length and extensibility.

In Upper Body Dysfunction (UBD)

Although the anterior and middle deltoid are large prime movers of the upper body, their relative contribution to postural dysfunction is small. Relative to other muscles of the shoulder complex, few texts reference techniques specific to release, lengthening or activation of these muscles. The posterior deltoid does have a propensity toward trigger point development (8, 18) and is often targeted using various release and stretching techniques.

In a study on swimmers with shoulder pain, an over-all decrease in EMG activity was noted in the anterior and middle deltoid (16). Relative to the anterior and middle deltoid, posterior deltoid activity increased (remain unchanged, despite a decrease in performance) (16). Further, in a study on muscle recruitment and latent trigger points, the development of latent trigger points in the posterior deltoid lead to higher levels of EMG activity in the posterior deltoid during shoulder flexion (18).

The increase in posterior deltoid activity may be the result of synergistic dominance, due to infraspinatus and teres minor inhibition, in those who exhibit Upper Body Dysfunction (UBD). The lengthened position of the infraspinatus and teres minor reduces force production, the over-activity of the subscapularis in UBD alters reciprocal inhibition leading to relative inhibition, resulting in a relative increase in firing rate of the posterior deltoid and increase in recruitment, force production and activity during external rotation and during arm elevation to aid in stabilization.

Considering arthrokinematic dysfunction, a study by Lawrence et al. showed that shoulder pain patients exhibited excessive inferior and anterior glide of the humeral head relative to the glenoid fossa (17). This behavior could be at least partially explained by anterior and middle deltoid inhibition and an increase in activity of the posterior deltoid. The resultant change in force couple relationships would result in a decrease in superior translatory force, with the relative increase in posterior deltoid activity contributing to excessive anterior glide.

The internally rotated position of the humerus in those who exhibit UBD may have an affect on length, but given the questionable contribution to internal and external rotation (6) I doubt that many individuals will exhibit adaptive shortening of the anterior deltoid, or require lengthening and stretching techniques for the posterior deltoid. Even if adaptive shortening has occurred, it is likely that stretches used for the pectoralis major and latissimus dorsi will be sufficient to increase or maintain length of the anterior and posterior deltoid.

In Lumbo Pelvic Hip Complex Dysfunction (LPHCD) and Sacroiliac Joint Dysfunction (SIJD) the deltoids do no play a role.

In Lower Leg Dysfunction (LLD) the deltoids do not play a role.

In Short, the anterior deltoid and middle deltoid have a propensity toward prime mover inhibition and under-activity, and the posterior deltoid has a propensity to become over-active. This implies that release techniques for the posterior deltoid should be part of a human movement professionals repertoire. Based on their negligible contribution to internal and external rotation of the humerus, and the fact that these muscles are lengthened in stretches for other commonly short muscles of the upper body, it is doubtful that specific lengthening techniques for the deltoid are needed. Prime mover inhibition is likely best addressed in a resistance training program using large multi-joint movement patterns. Clinically, I have found that exercises traditionally used for the chest and back are sufficient for maintaining and increasing shoulder strength. The regular addition of exercise specific to the deltoids seems to predispose individuals to pain, inflammation and chronic shoulder pathologies (such as impingement), especially in overhead athletes. Further, mobilization of the deltoid, pectoral and trapezius fascia (for example, using Instrument Assisted Soft Tissue Mobilization (IASTM) may be beneficial for restoring mobility and optimal function. Further, the use of mobilization techniques for the glenohumeral, acromioclavicular and sternoclavicular joints may reduce arthrokinematic dyskinesis and aid in restoring optimal function of the deltoids. Manual mobilizations should be part of the repertoire of any licensed professional (PT, ATC, DC, DO) treating pathologies related to the shoulder.

Clinical Implications:

  • Shoulder Pain

    • Impingement syndrome
    • Anterior Instability
    • Bursitis

  • Sternoclavicular Joint Pain
  • Acromioclavicular Joint Pain
  • Trigger Points
  • Upper Body Dysfunction leading to Scapular or Thoracic Spine Pain

Signs of Altered Length/Tension and Tone:

Trigger Point Palpation of the Deltoid:

  • Palpation of active trigger points in the deltoid does not generally refer pain distally, but spreads locally in the region of the affected muscle. "The deltoid muscle may develop satellite trigger points from key points in other muscles, such as the infraspinatus and supraspinatus (8)."

Deltoid Trigger Points

Specific Techniques for Strengthening the Deltoids

Note: The deltoids play a large role in exercises intended for chest and back (push and pull) strength. Specific exercises for the deltoids may not be necessary

Specific Techniques for Posterior Deltoid Extensibility:

Posterior Shoulder Self-Administered Release:

Posterior Shoulder Stretch (Sleeper Stretch):

Modified Sleeper Stretch:

Bibliography:

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  15. Inman VT, Saunders JB, Abbot JC, Observations on the function of the shoulder jugular). (Etymology Online)">joint. J Bone jugular). (Etymology Online)">Joint Surg 26:1 – 30, 1944
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  17. Lawrence, R.L., Braman, J.P., Staker, J.L., Laprade, R.F., Ludewig, P.M. (2014) Comparison of 3-dimensional shoulder complex kinematics in individuals with and without shoulder pain, Part 2: Glenohumeral jugular). (Etymology Online)">joint. Journal of Orthopaedic & Sports Physical Therapy 44(9). 646-B3
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  20. Robert Schleip, Thomas W. Findley, Leon Chaitow, Peter A. Huijing. Fascia: The Tensional Network of the Human Body © 2012 Elsevier Ltd.
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  22. Gurushantappa PK, Kuppasad S. (2015). Anatomy of the axillary nerve and its clinical importance: a cadaveric study. Journal of Clinical and Diagnostic Research. 9(3): AC13-AC17
  23. Wysiadecki G, Polguj M, Zytowski A, et al. (2014). Morphology and proposed model of innervation of the human deltoid muscle: a pilot study.  Folia Morphol. 73(2): 216-223.

© 2015 Brent Brookbush

Questions, comments, and criticisms are welcome and encouraged.