Division of motor units in medial gastrocnemius muscle of the rat in the light of variability of their principal properties.

Ninety-seven motor units of medial gastrocnemius muscle of the rat were examined. Among the examined motor units, fast (F) and slow (S) units were distinguished on the base of the presence or absence of sag reflex in unfused tetanus, induced by stimulation at 40 Hz. The fatigue test, in turn, permitted to distinguish fast fatigable (FF) and fast resistant (FR) units. Fatigue index was calculated using two techniques, the results of which differed significantly from each other. The following basic features of motor units were examined: contraction time, half-relaxation time and twitch force. A more detailed analysis of all characteristics of the units disclosed that their classification into fast and slow units on the base of the sag test alone was unjustified. A more appropriate classification resulted when multiple properties of the units were considered. The distinguished types of the units differed significantly from each other in the examined properties. On the other hand, a significant individual variability in the properties was also shown to exist within each type of the units. Various characteristics of the motor units showed correlation with each other in the whole population of motor units and/or their individual types.