Motor unit control in human vastus lateralis muscle during fatigue

We investigated the motor unit firing behavior of the vastus lateralis muscle during a sequence of isometric constant-torque contractions maintained at 20% of the maximum voluntary contraction and repeated to exhaustion. Electromyographic signals were recorded via quadrifilar fine wire electrodes and decomposed into their constituent action potentials to obtain the motor unit firing times. In addition, we measured the whole-muscle mechanical properties during the fatigue protocol using electrical stimulation. In four out of five subjects, the firing rate of motor units first decreased within the first 10% - 20% of the endurance time and then increased. The firing rate increase was accompanied by an increase in the number of motor units recruited to reach and maintain the constant target torque. The elicited twitch and tetanic torque responses first increased and then decreased. In one subject, the initial decrease in firing rate was absent, but so was the increase in the torque response. Overall, the two processes followed the same time course and complemented each other. These data are consistent with an activation of the motor unit pool via a common drive that adjusts to compensate for the potentiation and diminution of the mechanical output of the muscle fibers.