Which muscle fiber type is slow-twitch and associated with long-distance endurance such as a 10k run?

• A. Type 2b
• B. Type 1
• C. Type 2a
• D. Type 2x

Answer: (B)

The key idea is that long-distance endurance relies on slow-twitch, or Type I, muscle fibers. These fibers are highly oxidative, with many mitochondria, high capillary density, and abundant myoglobin, which lets them efficiently use oxygen to produce energy over extended periods. They contract more slowly and are resistant to fatigue, making them ideal for a 10k-style effort that requires sustained, moderate-intensity work.

In contrast, fast-twitch fibers are geared toward quick, powerful actions but fatigue quickly. Type IIa fibers are fast oxidative-glycolytic—somewhat endurance-capable but not as fatigue-resistant as Type I. Type IIx (often called IIb) fibers are fast glycolytic, delivering high force for short bursts and tiring rapidly, which doesn’t suit steady long-distance work.