Which statement correctly describes slow-twitch and fast-twitch fibres?

• A. Type I fibres are fast, fatigue quickly; Type II fibres are slow, fatigue-resistant.
• B. Type I fibres are slow, fatigue-resistant and rely on oxidative metabolism; Type II fibres are fast, high force but fatigue quickly.
• C. Type I fibres are anaerobic; Type II fibres are aerobic.
• D. Type I fibres are not present in humans; Type II control all movements.

Answer: (B)

This question tests understanding of how slow-twitch and fast-twitch fibres differ in speed, endurance, and energy use. Slow-twitch fibres (Type I) contract slowly, are highly fatigue-resistant, and rely on oxidative (aerobic) metabolism. They have lots of mitochondria, capillaries, and myoglobin, which supports long, endurance activities. Fast-twitch fibres (Type II) contract quickly and generate high force, but they fatigue more rapidly because they depend more on glycolytic energy systems for quick bursts. The statement that Type I fibres are slow, fatigue-resistant and rely on oxidative metabolism, while Type II fibres are fast and capable of high force but fatigue quickly, matches these characteristics. Other options mix up these traits (for example, labeling Type I as fast or anaerobic, or saying Type I fibres don’t exist in humans), which does not fit how these fibres actually behave.