How many categories of muscle fiber are identified?

• A. Three
• B. Two
• C. Four
• D. Five

Answer: (A)

Muscle fibers are categorized by how fast they contract and how they generate energy. In humans, the standard classification identifies three distinct types: slow-twitch oxidative fibers (Type I), fast-twitch oxidative glycolytic fibers (Type IIa), and fast-twitch glycolytic fibers (Type IIx). Type I fibers are fatigue resistant and rely on aerobic metabolism, Type IIa blend power with endurance, and Type IIx generate high power but fatigue quickly due to anaerobic metabolism. Some species have a Type IIb category, but in humans that isn’t used as a separate group in routine classifications. So the commonly identified number of categories is three.