Which fiber type is slow-twitch and fatigue-resistant?

• A. Type IIa
• B. Type IIx
• C. Type I muscles
• D. Type IIb

Answer: (C)

Slow-twitch fibers, Type I, are built for endurance. They have many mitochondria, high capillary density, and plenty of myoglobin, which support aerobic metabolism. This makes them capable of sustaining contractions for long periods with minimal fatigue. They contract more slowly and generate less force, but they’re the go-to fibers for activities like distance running or cycling, where steady effort over time is key. Fast-twitch types (Type IIa and IIx) provide more force but fatigue more quickly, with Type IIa being intermediate and Type IIx more explosive. Type IIb, when present in humans, is the fastest and most fatigable. So the slow-twitch and fatigue-resistant fiber type is Type I.