Contractile properties of rat soleus motor units following 14 days of hindlimb unloading

The purpose of this study was to compare the isometric contractile properties of rat soleus motor units after 14 days of hindlimb unloading (HU) to those under control conditions. The motor units (MU) were classified using two mechanical criteria: the presence or not of a sag during unfused tetani and the value of the twitch time-to-peak (TTP). Under control conditions, the soleus muscle was composed of 85% of slow-type (sag -, TTP > 20 ms) and 15% of fast-type (sag +, TTP < 20 ms) units. Following HU, these two populations were still present and results showed: (1) large decreases in their maximal tetanic tensions (of -67% and -60% for slow- and fast-type, respectively), and (2) changes in their relative proportions, i.e. a decrease in the percentage of slow-type units and a twofold increase in the percentage of fast-type units were observed. These latter changes might be the consequence of a complete transformation of slow-towards fast-type units. A third population appeared in the HU solei, 26% of the samples, combining the presence of a sag and speed-related properties between those of slow- and fast-type units. These slow-intermediate units might come from slow units partially transformed into a faster type during HU. Thus the present study showed that unloading conditions induced a reorganisation of the soleus motor unit profile. The complete or partial transformation of the motor units could be related to the changes in the electromyographical activity of the unloaded soleus.