Effects of respiratory muscle work on exercise performance

The normal respiratory muscle effort at maximal exercise requires a signifi cant fraction of cardiac output and causes leg blood flow to fall. We quest ioned whether the high levels of respiratory muscle work experienced in hea vy exercise would affect performance. Seven male cyclists [maximal O-2 cons umption ((V) overdot O-2) 63 +/- 5 ml . kg(-1) min(-1)] each completed 11 r andomized trials on a cycle ergometer at a workload requiring 90% maximal ( V) over dot O-2. Respiratory muscle work was either decreased (unloading), increased (loading), or unchanged (control). Time to exhaustion was increas ed 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 (V) over dor O-2 caused hyperventi lation, and reduced the rate of change in perceptions of respiratory and li mb discomfort throughout the duration of exercise. These findings demonstra te that the work of breathing normally incurred during sustained, heavy-int ensity I exercise (90% (V) over dot O-2) has a significant influence on exe rcise performance. We speculate that this effect of the normal respiratory muscle load on performance in trained male cyclists is due to the associate d 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 p erceived.