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1 John Rankin Laboratory of Pulmonary Medicine, Department Preventive Medicine, University of Wisconsin, Madison, Wisconsin 53705; and 2 Department of Kinesiology, Kansas State University, Manhattan, Kansas 66506
The
normal respiratory muscle effort at maximal exercise requires a
significant fraction of cardiac output and causes leg blood flow to
fall. We questioned whether the high levels of respiratory muscle work
experienced in heavy exercise would affect performance. Seven male
cyclists [maximal O2 consumption
(
O2) 63 ± 5 ml · kg
1 · min1] each
completed 11 randomized trials on a cycle ergometer at a workload
requiring 90% maximal O2. Respiratory
muscle work was either decreased (unloading), increased (loading), or
unchanged (control). Time to exhaustion was increased with unloading in 76% of the trials by an average of 1.3 ± 0.4 min or 14 ± 5% and decreased with loading in 83% of the trials by an average of
1.0 ± 0.6 min or 15 ± 3% compared with control
(P < 0.05). Respiratory muscle unloading during
exercise reduced O2, caused
hyperventilation, and reduced the rate of change in perceptions of
respiratory and limb discomfort throughout the duration of exercise.
These findings demonstrate that the work of breathing normally incurred
during sustained, heavy-intensity exercise (90%
O2) has a significant influence on
exercise performance. We speculate that this effect of the normal
respiratory muscle load on performance in trained male cyclists is due
to the associated reduction in leg blood flow, which enhances both the
onset of leg fatigue and the intensity with which both leg and
respiratory muscle efforts are perceived.
blood flow distribution; dyspnea; work of breathing
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