What is the primary adaptation observed with sprint training in untrained individuals?

• A. Hypertrophy of fast muscle fibers
• B. Hypertrophy of slow muscle fibers
• C. Increase in mitochondrial biogenesis
• D. Decrease in glycolytic enzymes

Answer: (A)

High-intensity sprint work primarily loads fast-twitch motor units, so in someone who hasn’t trained before the most noticeable change is the growth in the size of fast-twitch (type II) muscle fibers. This hypertrophy reflects increased protein synthesis in those fibers in response to the repeated, maximal or near-maximal power efforts, which boosts their cross-sectional area and force production. Mitochondrial biogenesis is more characteristic of endurance, steady-state aerobic training that pushes the oxidative capacity of muscle, not the rapid, glycolytic sprint stimulus. A decrease in glycolytic enzymes isn’t expected; in fact, those enzymes often rise to support quick energy production during sprints. Hypertrophy of slow fibers isn’t the primary adaptation because slow-twitch fibers aren’t the main drivers during maximal sprinting.