Muscle fiber types

Muscle fiber types are categories of skeletal muscle fiber defined by contraction speed and how they supply energy.^{1Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 25.} Type I (slow-twitch) fibers use energy efficiently and rely heavily on aerobic metabolism, resisting fatigue but developing force slowly.^{1Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 25.} Type II (fast-twitch) fibers produce force quickly but tire fast; within this group, IIa fibers have more aerobic capacity and capillaries than IIx fibers and resist fatigue better.^{1Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 25.}

Most muscles contain a mix: the soleus averages more than 80% Type I fibers, while the triceps is about 60% Type II.^{2Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 14.} Type I content in the vastus lateralis ranges from 5% to 90% between people, and about 45% of fiber-type variation is thought to be genetic.^{3Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 177.}

In practice

• Type II fibers show about 50% more hypertrophy than Type I fibers with training, though responses vary widely between clients.^{2Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 14.}
• Training shifts fast-twitch fibers from IIx toward IIa; in one eight-week study, IIx fibers fell from about 18% to 7% of the total.^{4Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 111.} Detraining reverses that shift.^{4Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 111.}
• Fiber type can’t be judged by eye; measuring it requires a muscle biopsy.^{5Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 21.}

Where sources differ

Essentials of Strength Training and Conditioning considers a switch between Type I and Type II less probable and says the evidence is insufficient.^{4Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 111.} NSCA’s Essentials of Personal Training points to biopsy studies and a twin study as compelling evidence that long-term endurance training, and likely resistance training, can change the mix.^{6Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 91.}