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 after a bout of fatiguing exercise and during the recovery period. 2. Fatigue was induced in eight healthy males by a maximal effort lasting 25 s performed on an isokinetic cycle ergometer at a constant pedal frequency of 60 revolutions per minute. The cranks of the ergometer were driven by an electric motor. Before and after the sprint, subjects allowed their legs to be passively taken round by the motor. During the passive movement the knee extensors were stimulated (4 pulses; 100 Hz). Peak voluntary force (PVF) during the sprint and peak stimulated forces (PSF) before and in recovery were recorded via strain gauges in the pedals. Recovery of voluntary force was assessed in a series of separate experiments in which subjects performed a second maximal effort after 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 maximum rate of force development (MRFD) was also reduced following fatigue to 68.8 +/- 11.0% (P < 0.05) of control and was fully recovered after 2 min. PVF was reduced to 72.0 +/- 9.4% (P < 0.05) of the control value following the maximal effort. After 3 min voluntary force had fully recovered. 4. The effect of changing the duration of the fatiguing exercise (10, 25 and 45 s maximal effort) resulted in an increased degree of voluntary force loss as the duration of the maximal effort increased. 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 fatigue in this type of dynamic exercise may be due to changes in the muscle itself and not to failure of central drive.