Type II Muscle Fibers

Type II muscle fibers are skeletal muscle fibers characterized by relatively fast contractile properties. They are commonly called fast-twitch fibers and are identified primarily by the fast myosin heavy chain isoforms they express. In adult human limb and trunk muscles, the principal Type II subtypes are Type IIA and Type IIX fibers .^1,2

Compared with Type I fibers, Type II fibers generally have faster myosin ATPase activity and greater maximum shortening velocity, supporting rapid force development and power production. Their metabolic properties vary by subtype. Type IIA fibers combine substantial oxidative and glycolytic capacities, whereas Type IIX fibers generally have lower oxidative capacity and higher glycolytic capacity. Type IIA fibers are therefore typically more fatigue resistant than Type IIX fibers .^1,2

Type II fibers should not be treated as a single uniform category. Human skeletal muscles contain mixtures of Type I, Type IIA, and Type IIX fibers, as well as hybrid fibers that express more than one myosin heavy chain isoform. Fiber proportions also vary between muscles and individuals and can change with factors such as muscle activity and training .^1,2

An important terminology distinction concerns Type IIB fibers. True Type IIB fibers occur in several other mammals, particularly small mammals, but the fast fibers historically called Type IIB in human limb muscle are now recognized as Type IIX. Classification by myosin isoform also should not be assumed to predict every metabolic or mechanical property of an individual fiber .^1,2