Recovery from fatigue of human diaphragm and limb muscles

This study was designed to compare the recovery from fatigue of human inspiratory and limb muscles using repeated maximal static contractions. Series of 18 maximal contractions of 10 sec duration were performed with a duty cycle of 50% for maximal inspiratory efforts (against a shutter at FRC), and with duty cycles of 5%, 10%, 20% and 50% for the elbow flexors in repeated studies on 6 subjects. The peak inspiratory pressure at the end of the series declined to 86.7% +/- 5.3% (mean +/- S.D.) of its initial value: maximal force of the elbow flexors declined to 83.5% +/- 7.0% (5% duty cycle), 80.0% +/- 5.5% (10% duty cycle), 70.0% +/- 9.3% (20% duty cycle), and 66.4% +/- 8.0% (50% duty cycle). Thus, the elbow flexors required approximately a 10-fold reduction in duty cycle to maintain over a series of contractions a force generating capacity comparable to that of the diaphragm. A small degree of 'central' fatigue developed progressively during all series of contractions but did not correlate with duty cycle. Fatigue-induced changes in twitch contraction properties varied with changes in duty cycle. Our major conclusions are that the human diaphragm has a marked capacity to recover from fatigue and that this may have been underestimated in previous studies from this and other laboratories.