RESPIRATORY MUSCLE UNLOADING AND ENDURANCE EXERCISE PERFORMANCE

Abstract

Ventilatory demand increases throughout endurance exercise. It was hypothesised that if the normal loads on the respiratory muscles contribute to the limitation of endurance exercise, then a reduction of these loads (resistive unloading) should improve exercise performance. Inspiratory (I) and expiratory (E) unloading is possible at rest and moderate exercise levels, with the recent development of a feedback controlled loading/unloading device [Younes. et al: JAP 62:2491,1987]. This thesis is concerned with the specific modifications that were necessary in this device, to apply respiratory muscle unloading (FWUL) throughout the high ventilatory levels of prolonged exercise. The device was also used in a study in which seven healthy males performed constant load heavy exercise on two days, one with and one without (control) respiratory muscle unloading. With FWUL, mouth pressure was made positive with inspiration and negative with expiration proportional to flow, to provide a mean unloading of 1.7 cmH20· 1-1• s, effectively eliminating 73% of the total airflow resistance. The order of the two tests was randomised and they were separated by 4 days. The subjects exercised at high exercise levels (- 90% maximum oxygen consumption) and flow proportional unloading was applied throughout these exercise levels. There was no significant difference (paired t test) in exercise duration between the control (mean ± SE, 11.4 ± 1.2 min) and FWUL (12.6 ± 2.1 min) tests. Repeated measures ANOVA revealed no difference in minute ventilation, tidal volume, respiratory frequency, heart rate and oxygen uptake, between the two tests. To confirm respiratory muscle unloading, intraoesophageal pressure (Ppl) was measured throughout exercise in 5 subjects. Respiratory muscle output (Pmus) was calculated by using Ppl and the measured values for chest wall elastic recoil, and normal values for chest wall resistive pressures. Peak and mean Pmus were significantly less with unloading (A(I) - 18%, 16%; A(E) - 36%, 37%, respectively), throughout exercise. The unloading device with its modifications was thus successfully used to unload the respiratory muscles at the high ventilatory levels of endurance exercise. The lack of improvement in exercise performance with unloading, in spite of a fall in respiratory muscle output, indicates that the normal resistive loads do not significantly contribute to endurance exercise limitation.