Training adaptations differ in fast-twitch (Type II) versus slow-twitch (Type I) muscle fibers. Which statement is most accurate?

Training adaptations differ in fast-twitch (Type II) versus slow-twitch (Type I) muscle fibers. Which statement is most accurate?

• A. Type I fibers become more oxidative with endurance training.
• B. Type IIa fibers decrease oxidative capacity with training.
• C. Type IIx fibers convert to Type IIb with training.
• D. Type IIb fibers are common in human training adaptation.

Answer: (A)

Endurance training improves the oxidative capacity of slow-twitch fibers because these fibers are specialized for sustained aerobic work. With regular aerobic activity, Type I fibers increase mitochondrial density and oxidative enzyme activity, gain more capillaries, and enhance fat oxidation, all of which boost their ability to sustain exercise for longer and with less fatigue. The notion that Type IIa fibers lose oxidative capacity with training isn’t supported, since these fast-twitch fibers can become more oxidative as a result of endurance or mixed training. In humans, a distinct Type IIb fiber isn’t a common adaptation; instead, there can be a shift from Type IIx toward Type IIa with endurance training, not a conversion to IIb. So, Type I fibers becoming more oxidative with endurance training is the most accurate description.