A 17 year old high school cross-country runner has been training aerobically for six months in preparation for the upcoming season. Which of the following adaptations will occur in the muscles during that time?

A 17 year old high school cross-country runner has been training aerobically for six months in preparation for the upcoming season. Which of the following adaptations will occur in the muscles during that time?

• A. Increased concentration of glycolytic enzymes
• B. Hyperplasia of Type II fibers
• C. Transformation from Type I to Type II fibers
• D. Hypertrophy of Type I fibers

Answer: (D)

Aerobic endurance training mainly strengthens the slow-twitch (Type I) fibers because they are used most during prolonged aerobic activity. After six months of consistent endurance work, these fibers increase their capacity to generate energy through oxidative processes. That means more mitochondria, greater capillary density supplying oxygen, and higher activity of oxidative enzymes. As the muscles adapt to sustain repeated, moderate-intensity contractions over long periods, the Type I fibers can also grow a bit larger due to increased contractile protein content, which is why they hypertrophy. The other possibilities don’t fit the typical aerobic adaptation pattern. Increasing glycolytic enzymes would reflect a shift toward faster, more glycolytic energy pathways used in high-intensity efforts. Increasing the number of fibers (hyperplasia) isn’t a common adaptation in humans. Transforming Type I to Type II would imply a shift toward a less endurance-oriented fiber type, which would not occur with sustained aerobic training.

Aerobic endurance training mainly strengthens the slow-twitch (Type I) fibers because they are used most during prolonged aerobic activity. After six months of consistent endurance work, these fibers increase their capacity to generate energy through oxidative processes. That means more mitochondria, greater capillary density supplying oxygen, and higher activity of oxidative enzymes. As the muscles adapt to sustain repeated, moderate-intensity contractions over long periods, the Type I fibers can also grow a bit larger due to increased contractile protein content, which is why they hypertrophy.

The other possibilities don’t fit the typical aerobic adaptation pattern. Increasing glycolytic enzymes would reflect a shift toward faster, more glycolytic energy pathways used in high-intensity efforts. Increasing the number of fibers (hyperplasia) isn’t a common adaptation in humans. Transforming Type I to Type II would imply a shift toward a less endurance-oriented fiber type, which would not occur with sustained aerobic training.