Summation of tension in motor units of the soleus muscle of the cat.

In the cat soleus muscle which is exclusively composed of slow motor units the discrepancy between the sum of individual tensions and the tension on combined stimulation of several motor units was found to be much less than previously reported for slow motor units of peroneus longus. In peroneus the tension on combined stimulation was systematically larger than the value predicted from the sum of individual tensions. For both muscles it was possible to reduce the difference between observed and expected values by comparing the tension on combined stimulation with the sum of tensions, not of single motor units, but of groups of units. It is concluded that whenever tension is measured for single motor units, especially slow units in mixed muscles, the values obtained may be modified by frictional forces. The size of the effect appears to vary from one preparation to the next.