Muscle Fiber Types

Skeletal muscle fiber cells are highly adaptable to the stress that exercise imposes; improving their capacity to perform a similar activity in subsequent bouts. These cells have specific characteristics and unique adaptative abilities that allow them to improve their capacity to generate force in varied conditions. Understanding how to target these unique adaptive abilities, by altering training variables, may aid in implementing a more effective stimulus for better outcomes in fitness, performance, and physical rehabilitation settings.

This course reviews muscle fiber types including fiber type classifications and sub-classifications; as well as, pure and hybrid fibers (a.k.a. transitional fibers.). Further, this course details single muscle fiber twitch speeds and characteristics, fiber type proportion changes, fiber type adaptations to training, fiber types and athletic performance, and fiber type changes correlated with the aging process and exercise for older adults.

One of the most popular segments of this course is our table listing fiber type proportions by muscle group (snippet below). For example, did you know that the soleus is primarily type I muscle fiber, the gluteus maximus has a nearly even proportion of type I and type II fibers, and the sternocleidomastoid is primarily type II muscle fibers?

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, etc.

Additional Courses:

• Muscle Cell Structure and Function
• Muscle Fiber Dysfunction and Trigger Points

Caption: Staining of a muscle to highlight muscle fiber types

Snippets from the Course:

Pure Fiber Types

• Type I a.k.a. red fibers, slow/oxidative (SO) fibers, slow twitch fibers, myosin heavy chain (MHC) I fibers
• Type IIA a.k.a. pink fibers, fast/oxidative (FOG fibers), "intermediate" fibers, MHC IIA fibers
• Type IIB/X a.k.a. white fibers, fast/glycolytic (FG fibers), fast twitch fibers, or MHC IIx fibers or MHC IIb fibers (Note: Type IIb fibers is actually a misnomer, as this myosin chain configuration does not exist in humans).

Possible MHC Isoform Combinations (a.k.a. hybrid Fibers or transitional fibers):

• Type I and IIA
• Type IIA and IIB/X
• Type I, IIA, IIB/X

Fiber Type Proportions by Muscle (Segment of Table):