Final answer:
A high school cross country runner training aerobically for 6 months will experience hypertrophy of Type I fibers, which are adapted for endurance through increased aerobic metabolism and mitochondrial content.
Step-by-step explanation:
The 17-year-old high school cross country runner training aerobically for 6 months will likely experience hypertrophy of Type I fibers. Type I fibers are also known as slow oxidative (SO) fibers, and they use aerobic metabolism to produce ATP, which allows for prolonged periods of exercise such as long-distance running. Through endurance training, the athlete's muscles will adapt by increasing the number of mitochondria, myoglobin content, and capillary networks within Type I muscle fibers. This contrasts with exercises relying on anaerobic metabolism, which would increase the concentration of glycolytic enzymes and potentially hypertrophy of Type II fibers, predominantly used in high-intensity, short-duration events such as sprinting.
During the 6 months of aerobic training, the high school cross country runner will experience several adaptations in their muscles. One of these adaptations is an increase in the concentration of glycolytic enzymes, which are used in anaerobic metabolism. Additionally, there may be hypertrophy of Type I (slow-twitch) fibers, which use aerobic metabolism to produce low power contractions over long periods. However, there is unlikely to be hyperplasia of Type II (fast-twitch) fibers or transformation from Type I to Type II fibers.