Which statement best describes the adaptation associated with high-intensity interval training?

• A. Hypertrophy of fast muscle fibers
• B. Increase in capillary density
• C. Mitochondrial biogenesis
• D. Decrease in oxidative enzymes

Answer: (C)

High-intensity interval training drives adaptations that boost the muscle’s aerobic energy production by increasing mitochondrial content. The repeated, near-maximal efforts create metabolic stress that activates signaling pathways, especially PGC-1α, which promotes mitochondrial biogenesis. More mitochondria and higher oxidative enzyme levels expand the muscle’s capacity to generate ATP through oxidative metabolism, improving endurance and VO2max. While some capillary remodeling can occur and fast-twitch fibers are recruited during intense bouts, the defining adaptation is the growth of mitochondria and the associated oxidative capacity. Decreases in oxidative enzymes would run opposite to the observed response, and while capillary density can rise, the most characteristic HIIT change is mitochondrial biogenesis.