During the first weeks of daily endurance exercise, which is likely to happen?

• A. A decrease in capillary density
• B. A progressive increase in specific muscle proteins that improve muscle function
• C. A reduction in mitochondrial enzymes
• D. An increase in muscle fiber stiffness

Answer: (B)

Early endurance training makes the muscle more efficient at producing energy through aerobic pathways. A central early change is an upregulation of proteins involved in oxidative metabolism—mitochondrial enzymes and other components of the energy-producing machinery—so the muscle can generate ATP more effectively during sustained activity. This progressive increase in specific muscle proteins that support function best explains the improvement in endurance with daily training. While capillary growth supports oxygen delivery and can rise with continued training, the dominant quick adaptation is the rise in mitochondrial enzymes and related oxidative proteins. An increase in muscle fiber stiffness isn’t a typical early adaptation of endurance work, so it doesn’t fit as well.