Human Skeletal Muscle Fiber Type Classifications

Abstract

Human skeletal muscle is composed of a heterogenous collection of muscle fiber types.1–3 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.2 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.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 understand the muscle fiber classification techniques. Muscle fibers are composed of functional units called sarcomeres.3 Within each sarcomere are the myofibrillar proteins myosin (the thick filament) and actin (the thin filament). The interaction of these 2 myofibrillar proteins allows muscles to contract (Fig. 1).4 Several classification techniques differentiate fibers based on different myosin structures (isoforms) or physiologic capabilities.1,2,5 The myosin molecule is composed of 6 polypeptides: 2 heavy chains and 4 light chains (2 regulatory and 2 alkali). A regulatory and an alkali light chain are associated with each of the heavy chains. The heavy chains contain the myosin heads that interact with actin and allow muscle to contract (Fig. 1).4 The myosin heavy chain in the head region also contains an adenosine triphosphate (ATP) binding site and serves as the enzyme (adenosinetriphosphatase [ATPase]) for hydrolyzing ATP into adenosine diphosphate (ADP) and inorganic phosphate (PI), which provides the energy necessary for muscle contraction. The thin filament is made of actin and 2 regulatory proteins, troponin and tropomyosin.3 When the muscle fiber receives a stimulus in the form of an action potential, Ca2+ is released from the sarcoplasmic reticulum. The calcium then binds to troponin and, through tropomyosin, exposes a myosin binding site on the actin molecule (Fig. 1).4 In the presence of ATP, the myosin head binds to actin and pulls the thin filament along the thick filament, allowing the sarcomere to shorten. As long as Ca2+ and ATP are present, the myosin heads will attach to the actin molecules, pull the actin, release, and reattach. This process is known as cross-bridge cycling. The speed at which cross-bridge cycling can occur is limited predominantly by the rate that the ATPase of the myosin head can hydrolyze ATP. Regulatory function of troponin and tropomyosin. Troponin is a small globular protein with 3 subunits (TnT, TnI, TnC). (A) Resting condition: Tropomyosin under resting conditions blocks the active sites of actin, preventing actin and myosin from binding. (B) Contraction: When troponin binds with Ca2+, it undergoes a conformational change and pulls tropomyosin from the blocking position on the actin filament, allowing myosin heads to form cross-bridges with actin. From Plowman SA, Smith DL. Exercise Physiology for Health, Fitness, and Performance. Boston, Mass: Allyn & Bacon; 1997:433. Copyright 1997 by Allyn & Bacon. Reprinted/adapted by permission. Regulatory function of troponin and tropomyosin. Troponin is a small globular protein with 3 subunits (TnT, TnI, TnC). (A) Resting condition: Tropomyosin under resting conditions blocks the active sites of actin, preventing actin and myosin from binding. (B) Contraction: When troponin binds with Ca2+, it undergoes a conformational change and pulls tropomyosin from the blocking position on the actin filament, allowing myosin heads to form cross-bridges with actin. From Plowman SA, Smith DL. Exercise Physiology for Health, Fitness, and Performance. Boston, Mass: Allyn & Bacon; 1997:433. Copyright 1997 by Allyn & Bacon. Reprinted/adapted by permission. Initially, whole muscles were classified as being fast or slow based on speeds of shortening.3 This division also corresponded to a morphological difference, with the fast muscles appearing white in some species, notably birds, and the slow muscles appearing red. The redness is the result of high amounts of myoglobin and a high capillary content.3 The greater myoglobin and capillary content in red