Supramaximal activation increases motor unit velocity of unloaded shortening

It has been shown that the rate of tension generation (dP/dt) continues to increase with increasing stimulation rates, even after maximal tetanic tension has been achieved. Since dP/dt is directly proportional to unloaded shortening velocity, it was questioned whether supramaximal stimulation rates would increase shortening velocity. To test the relationship of velocity and stimulation rate, slack tests were performed on motor units isolated in the rat soleus muscles. For each motor unit tested, two slack tests were performed at two different stimulation rates: one rate yielded a maximal tetanic tension with a ''slow'' dP/dt (PO) and the other rate yielded a maximal tetanic tension with a ''fast'' dP/dt (RG). The two stimulation rates (PO and RG) had significantly different effects (p<.05) on motor unit shortening velocity, with the RG rate yielding a shortening velocity greater than that of PO by an average of 13+/-6%. This suggests that rate coding could be used to grade motor unit power production by grading force production and/or shortening velocity.