Muscle Fiber Distribution and Trainability

In the average male and female, about 52% of the muscle fibers are Type I, with the fast-twitch fibers divided into 33% Type IIa and 13% Type IIb. There is great variation in fiber-type distribution across the population.

(Note: "Type IIb" is this module's source terminology — the current correct term for this fiber type in humans is Type IIx; see muscle-fiber-types for the full nomenclature update. The percentages and trainability findings below are unaffected by the label change.)

On the basis of studies comparing identical to fraternal twins, the distribution of fast and slow fibers seems to be genetically fixed. However, the capacity of a muscle fiber to produce ATP aerobically (its oxidative capacity) seems to be easily altered by endurance training. In some elite endurance athletes, Type IIb fibers cannot be found — they have been converted to the oxidative version, Type IIa. The increase in mitochondria and capillaries in endurance-trained muscle allows an individual to meet ATP demands aerobically, with less glycogen depletion and lactate formation.

• Average distribution: ~52% Type I, ~33% Type IIa, ~13% Type IIb (current term: Type IIx).
• Wide variation across individuals.
• Fast/slow fiber distribution is genetically fixed (twin studies).
• Oxidative capacity is readily altered by endurance training.
• Endurance training can convert Type IIb → Type IIa (more mitochondria/capillaries).
• Result: ATP met aerobically with less glycogen depletion and lactate formation.

• muscle-fiber-types — definitions of Type I/IIa/IIb, and the IIb → IIx nomenclature update.
• aerobic-system — the oxidative capacity that training boosts.
• motor-unit-recruitment — recruitment of the various fiber types.