Slow motor units in female rat soleus are slower and weaker than their male counterparts.

The aim of the study was to investigate sex-related differences in contractile properties, parameters of action potentials, and mechanisms of force regulation of motor units in the rat soleus muscle, which is a frequent experimental model in animal research. It was revealed that the mean mass of the muscle in males was bigger than in females, by approximately 80%. However, the relation of the muscle mass to the body mass was not significantly different. These results correspond to approximately twice as much tetanic force per motor unit in male rats, and higher maximal contractile output, reflected by the force-time area per stimulus pulse. On the other hand, no differences were observed with respect to twitch forces of motor units. Thus the twitch-to-tetanus ratio was significantly higher in females. Additionally, the contraction and the half-relaxation times were significantly longer in female motor units, which might be due to differences in muscle architecture. The force-frequency curve in males was shifted rightwards with respect to females, indicating that the same relative level of tetanic force could be achieved at considerably lower stimulation frequency in females. The analysis of motor unit action potentials revealed about four times higher amplitudes in male rats, whereas the time parameters of action potentials were similar. The motor units in male and female rat soleus are considerably different and these observations should be taken in the consideration in various experiments on the muscle.