What evidence indicates recruitment of type 2a muscle fibers?

• A. Evidence of lactate in blood and muscles
• B. Increased capillary density
• C. Decrease in mitochondrial content
• D. Elevated resting heart rate

Answer: (A)

High‑intensity work that recruits fast‑twitch fibers drives glycolysis, and Type IIa fibers rely on this glycolytic pathway when the demand outpaces aerobic capacity. The byproduct of that glycolytic activity is lactate, which you can measure as lactate in blood and within muscles. So observing lactate accumulation signals that fast‑twitch fibers are actively contributing to force production.

In contrast, increased capillary density is a long-term adaptation from endurance training and reflects improved oxidative support, not a direct, immediate sign of which fibers are working. A decrease in mitochondrial content would reduce oxidative capacity, not indicate recruitment of these fibers. An elevated resting heart rate points to overall cardiovascular status or conditioning, not specifically which muscle fibers are being recruited at a given moment.

So lactate presence is the clearest indicator that Type IIa fibers are being recruited during high‑intensity activity.