slow-twitch (ST) fiber, a muscle fiber that develops less tension more slowly than a fast-twitch fiber. The ST fiber is usually fatigue resistant and has adequate oxygen and enzyme activity. Studies indicate that world-class endurance runners apparently have high percentages of ST fibers. It is called red muscle because of the abundance of capillaries serving the fiber muscle. The muscle also contains high amounts of the protein myoglobin that functions to store oxygen inside the muscle cell. See also fast-twitch (FT) fiber.
