Human skeletal muscle fiber type classifications

Human skeletal Muscle is composed of a heterogenous collection of muscle fi ber types.(1-3) This range of muscle fiber types allows for the wide variet y of capabilities that human muscles display. In addition, muscle fibers ca n adapt to changing demands by changing size or fiber type composition. Thi s plasticity serves as the physiologic basis for numerous physical therapy interventions designed to increase a patient's force development or enduran ce. Changes in fiber type composition also may be partially responsible for some of the impairments and disabilities seen in patients who are decondit ioned because of prolonged inactivity, limb immobilization, or muscle dener vation.(2) Over the past several decades, the number of techniques availabl e for classifying muscle fibers has increased, resulting in several classif ication systems. The objective of this update is to provide the basic knowl edge necessary, to read and interpret research on human skeletal muscle. Muscle fiber types can be described using histochemical, biochemical, morph ological, or physiologic characteristics; however, classifications of muscl e fibers by different techniques do not always agree.(1) 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 un derstand the muscle fiber classification techniques.