Skeletal muscle contains two primary types of fibers: fast twitch (FT) and slow twitch (ST). On average, ST fibers comprise 45-55% of the muscle fibers in arm and leg muscles of children and adults. However, elite athletes display patterns of muscle type distribution that are characteristic of the energy demands of their sport. Link to Exercise Pysiology-muscle fiber types for more information.
|Fast twitch fibers have a high capacity for anaerobic metabolism, a low oxidative capacity, and are quick to fatigue. FT fibers are activated during short term, high intensity physical activity characterized by muscle contractions performed at high frequencies (one contraction per second or faster). The stop-and-go, change-of-pace movements common in team sports, as well as sprinting and other high frequency, forceful muscle actions depend on the action of FT fibers. Thus, FT muscle fibers provide the high rate of ATP production required for the fast, powerful movements displayed by this ice hockey player. Due to heavy reliance on anaerobic metabolism for ATP generation in FT fibers, muscle glycogen is depleted more rapidly in FT than ST fibers. Additionally, rates of muscle glycogen resynthesis in FT fibers after repeated bouts of high intensity exercise are significantly higher than glycogen resynthesis in ST fibers after prolonged exercise.|
|Slow twitch fibers are well supplied with capillaries and rely primarily on oxidative metabolism. ST fibers resist fatigue and are recruited during prolonged, low to moderate intensity activity. Athletes, who have high aerobic and endurance capacities, such as this cross country skier, may have percentages of ST fibers in leg gastrocnemius muscles as high as 90-95%.|