FATIGUE AND RECOVERY OF VOLUNTARY AND ELECTRICALLY ELICITED DYNAMIC FORCE IN HUMANS

1. Percutaneous electrical stimulation of the human quadriceps muscle has been used to assess the loss of central activation immediately aft er a bout of fatiguing exercise and during the recovery period. 2. Fat igue was induced in eight healthy males by a maximal effort lasting 25 s performed on an isokinetic cycle ergometer at a constant pedal freq uency of 60 revolutions per minute. The cranks of the ergometer were d riven by an electric motor. Before and after the sprint, subjects allo wed their legs to be passively taken round by the motor. During the pa ssive movement the knee extensors were stimulated (4 pulses; 100 Hz). Peak voluntary force (PVF) during the sprint and peak stimulated force s (PSF) before and in recovery were recorded via strain gauges in the pedals. Recovery of voluntary force was assessed in a series of separa te experiments in which subjects performed a second maximal effort aft er recovery periods of different durations. 3. Peak stimulated forces were reduced to 69.8 +/- 9.3% immediately after the maximal effort, (P < 0.05), but had returned to pre-exercise values after 3 min. The max imum rate of force development (MRFD) was also reduced following fatig ue to 68.8 +/- 11.0% (P < 0.05) of control and was fully recovered aft er 2 min. PVF was reduced to 72.0 +/- 9.4% (P < 0.05) of the control v alue following the maximal effort. After 3 min voluntary force had ful ly recovered. 4. The effect of changing the duration of the fatiguing exercise (10, 25 and 45 s maximal effort) resulted in an increased deg ree of voluntary force loss as the duration of the maximal effort incr eased. This was associated with an increased reduction in PSF measured immediately after the exercise. 5. The close association between the changes in stimulated force and voluntary force suggests that the fati gue in this type of dynamic exercise may be due to changes in the musc le itself and not to failure of central drive.