Inspiratory muscles do not limit maximal incremental exercise performance in healthy subjects

We investigated whether the inspiratory muscles affect maximal incremental exercise performance using a placebo-controlled, crossover design. Six cyclists each performed six incremental exercise tests. For three trials, subjects exercised with proportional assist ventilation (PAV). For the remaining three trials, subjects underwent sham respiratory muscle unloading (placebo). Inspiratory muscle pressure (Pmus) was reduced with PAV (−35.9 ± 2.3% versus placebo; P < 0.05). Furthermore, V˙O2 and perceptions of dyspnea and limb discomfort at submaximal exercise intensities were significantly reduced with PAV. Peak power output, however, was not different between placebo and PAV (324 ± 4 W versus 326 ± 4 W; P > 0.05). Diaphragm fatigue (bilateral phrenic nerve stimulation) did not occur in placebo. In conclusion, substantially unloading the inspiratory muscles did not affect maximal incremental exercise performance. Therefore, our data do not support a role for either inspiratory muscle work or fatigue per se in the limitation of maximal incremental exercise.