SUPRAMAXIMAL ACTIVATION INCREASES MOTOR UNIT VELOCITY OF UNLOADED SHORTENING

It has been shown that the rate of tension generation (dP/dt) continue s to increase with increasing stimulation rates, even after maximal te tanic tension has been achieved. Since dP/dt is directly proportional to unloaded shortening velocity, it was questioned whether supramaxima l stimulation rates would increase shortening velocity. To test the re lationship of velocity and stimulation rate, slack tests were performe d on motor units isolated in the rat soleus muscles. For each motor un it 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 significa ntly 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 t o grade motor unit power production by grading force production and/or shortening velocity.