Four weeks of sprint training in previously untrained individuals results in which muscular adaptation?

• A. Hypertrophy of fast muscle fibers
• B. Hypertrophy of slow muscle fibers
• C. Increase in capillary density of slow fibers
• D. No change in fiber size

Answer: (A)

High-intensity sprint work recruits the fast-twitch muscle fibers, which are geared for rapid, powerful contractions. When previously untrained individuals start sprint training, these fast fibers are stimulated repeatedly at high effort, prompting adaptations that increase their size and strength. This hypertrophic response comes from increased contractile protein content and structural remodeling within the fast fibers, improving their capacity for force production and power.

Endurance-type changes, like hypertrophy of slow (slow-twitch) fibers or greater capillary density, are more typical of longer-duration, lower-intensity training that continuously engages oxidative metabolism; four weeks of sprinting doesn’t target those adaptations as its primary stimulus. A lack of any fiber size increase would also be unlikely in this context, since one of the earliest muscular adaptations to high-intensity training in previously untrained people is growth of the fast fibers.