Decomposition of motor unit tetanic contractions of rat soleus muscle: Differences between males and females.

Mathematical decomposition of tetanic contractions of slow motor units (MUs) of the rat heterogeneous medial gastrocnemius muscle revealed immense variability of twitch-shape responses to successive pulses, contrary to results obtained for fast MUs. The aim of this study in rat soleus muscle, almost exclusively composed of slow MUs, was to reveal whether such variability of twitch-shape decomposed components was a common property of slow MUs in the two studied muscles, and whether ranges of the force amplitude or time parameters of these decomposed twitches showed sex differences. Unfused tetanic contractions evoked by stimulation at variable interpulse intervals were analyzed for 10 MUs of males and 10 MUs of females. Significantly higher variability between parameters of the decomposed responses was observed for male soleus MUs, as the mean ratio of forces of the strongest decomposed twitch and the first (the weakest) decomposed twitch amounted to 3.8 for males and 2.8 for females. The ratios of the contraction times of the longest decomposed to the first twitch were much more similar between male and female MUs, 2.6 and 2.9, respectively. Consequently, the mean ratio of the force-time area for the strongest decomposed to the first twitch was much bigger in male than female MUs (7.35 vs. 5.07, respectively). Our observations indicate that high variability of responses to successive stimuli is a general property of slow MUs in different rat muscles, but the mechanisms of summation of individual twitches into tetanic contractions of MUs are not identical for male and female rats.