Motor unit organization of human medial gastrocnemius.

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Abstract
The properties of 57 motor units in human medial gastrocnemius were studied using controlled intramuscular microstimulation, glycogen depletion and muscle biopsy. Motor units could be divided into 3 classes based on their mechanical properties. Type S units were slow, small and fatigue-resistant. Type FR units were fast, intermediate in size and fatigue-resistant. Type FF units were fast, large and fatigable. Glycogen depletion of a number of type S and FF units revealed them to be composed of type 1 and type 2b muscle fibers, respectively. During slowly increasing voluntary contractions where units are recruited in order of size, type 1 and 2a muscle fibers would probably be employed at low force levels, followed by type 2b muscle fibers in stronger contractions.