The study aimed at demonstration of sexual differences in morphology and innervation of the medial gastrocnemius muscle (MG) in male and female Wistar rats. The experimental project involved three stages: (1) the functional isolation and analysis of contractile properties of as many motor units (MUs) as possible, (2) recording the contraction force of the whole muscle evoked by stimulation of the sciatic nerve and, (3) the morphological examinations of MG. The experiments revealed that MG of males contained 46% of fast fatigable (FF) MUs, 40% of fast resistant (FR) MUs and 14% of slow (S) MUs, whereas in females 40% of FF, 37% of FR and 23% of S type MUs were found. The force of MUs in male muscle was higher, and the twitch time parameters were longer in males. Moreover, the muscle mass and force in males was signifi cantly higher. The relation of the muscle force to the mean value of MUs force revealed that male MG contained approximately 10% more MUs than in females. The male muscles which had approximately 1.5 times bigger mass than in females were composed of over 11 800 muscle fi bers, whereas in females 8 000 fi bers. These results strongly suggested that the average number of muscle fi bers in individual MUs of males was higher about 47% comparing to females. In addition, the mean diameter of muscle fi bers was 29% bigger in males. Finally, the cross-section area of muscle fi bers was also 15% larger in males.
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