| Despite a number of studies on healthy humans investigating the role of unloading the respiratory muscles on exercise endurance with heliox (He-O2), there is no agreement as to what extent exercise endurance is affected. Furthermore, the role of He-O2 on the components of fatigue (central vs peripheral) has not been investigated. This thesis was aimed at investigating the role of He-O2 on exercise endurance and neuromuscular function during exercise in healthy humans. Nine younger males (30--40 years old) and ten older males (55--70 years old) participated in the study. After screening tests (pulmonary function test and incremental exercise test), each subject performed two randomized constant power output (75% of peak power output) cycling tasks to exhaustion, breathing either medical air (Air) or He-O2 and, finally, the same test while breathing He-O2 for the same duration as the exercise during Air breathing. In younger subjects, exercise duration was not increased with He-O 2 relative to Air and the amount of peripheral quadriceps fatigue was similar for all conditions. In older subjects, in contrast, exercise duration was significantly increased (14%) during He-O2 relative to Air and peripheral quadriceps fatigue was similar for He-O2 and Air in spite of different exercise durations. However, the amount of peripheral fatigue was less for ISO (P<0.05). These findings suggest that in older subjects, He-O2 probably unloaded the respiratory muscles so they were able to exercise longer at the required intensity. In contrast, we think that the effect of heliox was absent at the required intensity in younger subjects because it either did not unload the respiratory muscles, or the cardiac output capability was not taxed to the same extent. In both groups, EMG activity did not reveal impairment of muscle activation showing that the central nervous system was still capable of activating the muscles. However, the muscle response to this activation was less than it had been prior to the exercise, confirming the presence of peripheral fatigue. We interpreted these findings to mean that peripheral muscle fatigue plays an important role in regulating exercise endurance in these individuals performing cycling exercise at 75% of maximal power output. |
