Summary • During muscular contraction, the smallest unit that can be activated is the motor unit, and the muscle fibers within that unit have identical characteristics. • Human skeletal muscle possesses three distinct fiber types: type I, `slow-twitch` oxidative fibers are slow in force generation and have an oxidative profile (rich in oxidative enzyme expression, mitochondria and capillary supply); type Ila, `fast-twitch` oxidative fibers are fast in force generation but have similar oxidative profiles to type I fibers; and IId/x, `fast-twitch` with a glycolytic metabolic profile (i.e., rich in glycolytic enzyme expression and poor in mitochondria and capillary supply). • Athletes from endurance sports tend to have a high proportion of type I fibers in the trained musculature, although fiber type alone does not predict endurance performance capability. • There is currently no direct experimental evidence for `fiber-type transformation` in humans.
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