Which muscle fiber type is fast-twitch glycolytic and associated with high power?

• A. Type IIb/x
• B. Type I
• C. Type IIa
• D. Type IIb

Answer: (A)

The key idea is that fast-twitch glycolytic fibers are built for explosive power. These fibers contract very quickly because they have high myosin ATPase activity, and they rely mainly on anaerobic glycolysis for energy. That combination lets them generate a lot of force in a short time, which is why they’re tied to high-power actions like sprinting or jumping. However, because they don’t have many mitochondria or much oxidative capacity, they fatigue quickly.

Among the fiber types, the fast-twitch glycolytic type—often labeled Type IIb or Type IIx in humans and other animals—is the one that fits this description best. It’s the fastest and most glycolytic, delivering the highest peak power but with rapid onset of fatigue. Type IIa fibers are fast-twitch but more oxidative and fatigue-resistant, providing a blend of speed and endurance, while Type I fibers are slow-twitch and endurance-oriented. So the fast-twitch glycolytic option aligns with high power output in short bursts.