Which fiber type is fatigue resistant and has intermediate contraction speed?

• A. Type I
• B. Type IIA
• C. Type IIB
• D. Type IIx

Answer: (B)

In skeletal muscle, contraction speed and fatigue resistance vary by fiber type. Type IIa fibers are fast-twitch but more oxidative than the purely glycolytic fast fibers, giving them a middle ground: they contract relatively quickly while resisting fatigue better than the fastest glycolytic fibers. They have higher mitochondrial content and capillary density, supporting sustained activity through both aerobic and anaerobic metabolism. Type I fibers are slow to contract and extremely fatigue-resistant but respond slowly, while the fastest fibers (Type IIx/IIb) contract rapidly but fatigue quickly due to their glycolytic metabolism. So the fiber type that is fatigue resistant and has intermediate contraction speed is Type IIa.