Twitch force and action potentials of single motor units in medial gastrocnemius muscle of the rat.

On the basis of a sag test in unfused tetanus we classified fast and slow motor units (MUs) in medial gastrocnemius muscle of the rat. The following parameters of motor units were investigated: single twitch force, duration of the action potentials, maximum-minimum amplitude time (M-MAT), action potentials amplitude and their latencies after stimulus (to the onset of potentials) and the diameter of MU territory in muscle medio-lateral direction. Differences were found between fast and slow MUs in all of the properties. A correlation was found between MU twitch forces and the diameters of territories and amplitudes of action potentials in the whole investigated MU population, as well as between the duration and the M-MAT of the action potentials.