Human skeletal muscle is composed of a heterogenous collection of muscle fiber types. This range of muscle fiber types allows for the wide variety of capabilities that human muscles display. In addition, muscle fibers can adapt to changing demands by changing size or fiber type composition. This plasticity serves as the physiologic basis for numerous physical therapy interventions designed to increase a patient's force development or endurance. Changes in fiber type composition also may be partially responsible for some of the impairments and disabilities seen in patients who are deconditioned because of prolonged inactivity, limb immobilization, or muscle denervation. Over the past several decades, the number of techniques available for classifying muscle fibers has increased, resulting in several classification systems. The objective of this update is to provide the basic knowledge necessary to read and interpret research on human skeletal muscle.

Muscle fiber types can be described using histochemical, biochemical, morphological, or physiologic characteristics; however, classifications of muscle fibers by different techniques do not always agree. Therefore, muscle fibers that may be grouped together by one classification technique may be placed in different categories using a different classification technique. A basic understanding of muscle structure and physiology is necessary to understand the muscle fiber classification techniques.