In skeletal muscle, a single, multinucleated cell which appears banded or striated when viewed under a light microscope. A single muscle contains between 10 000 and 450 000 fibres. Each muscle fibre is less than 0.1 mm in diameter, but most extend the length of the muscle. This means that a fibre in the thigh may be more than 35 cm long. Several systems have been used to classify muscle fibre type, but most recognize three main types: two types of fast-twitch fibres (FT) and one type of slow-twitch fibre (ST). FT fibres can reach peak tension in about 50 ms and are associated with speed and power activities; ST fibres take approximately 110 ms to reach peak tension, and are associated with endurance activities. Elite marathon runners have more than 90% ST fibres in their gastrocnemius muscle. Elite sprinters, however, have only about 25% ST fibres in this muscle. Muscle fibre composition appears to be determined early in life, but as we age our muscles tend to lose FT fibres which increases the percentage of ST fibres. Extreme and prolonged training may enable one fibre type to take on characteristics of the opposite type or it may even convert one fibre to the other type.
Muscle fibre
