Muscle fatigue induced by stimulation with and without doublets.

Muscles are usually stimulated by shocks delivered at some constant rate. However, human thenar motor units generate optimum force per pulse when excited by impulse trains that begin with one or two short interpulse intervals ("doublets"), followed by longer intervals. Our aim was to determine whether the rate of force and force-time integral reduction during fatigue of thenar muscles is influenced by an initial doublet, and/or the number of pulses per train. We first matched thenar force-time integral using two different pulse patterns, one of which began with a doublet. Fatigue induced by trains that contained a doublet resulted in slower rates of force and force-time integral reduction and smaller increases in half-relaxation time than that evoked by bursts of 40-HZ stimulation. When the force was measured in each protocol after equal numbers of pulses had been delivered, the force loss was still significantly less for pulse trains containing a doublet. These results have useful implications when designing stimulation to strengthen weak muscles or to drive paralyzed muscles.