Development and reversal of fatigue in human tibialis anterior.

The lack of easily measurable, objective physiologic indices of muscle fatigue in humans has hampered the evaluation of interventions that putatively inhibit muscle fatigue. In 6 healthy subjects, isometric fatigue of right and left tibialis anterior (TA) was produced using 180 s of intermittent electrical stimulation to the motor point (40 Hz, 650-ms or 250-ms trains, 1 train/s). The time course, magnitude, and reproducibility of fatigue and recovery from fatigue was established, and the effect of duty cycle measured. During fatigue, force fell rapidly in all subjects over the first 70 s, then fell more slowly. Recovery was also biphasic--force increased rapidly over the initial 30 s then slowly--and was incomplete at 300 s. Comparing duty cycles of 0.65 versus 0.25, the longer duty cycle caused a greater fall in force (-65% versus -29%; P < 0.001) and lower recovery (P < 0.02). In all subjects, left leg muscles fatigued more than right. We conclude that this noninvasive fatigue and recovery protocol is highly reproducible and that it may prove useful for evaluating the effects of pharmacologic or restorative interventions on human muscle fatigue.