Oxygen availability affects exercise capacity, but not neuromuscular fatigue characteristics of knee extensors, during exhaustive intermittent cycling

Purpose: To compare the effects of different hypoxia severities on exercise capacity, cardio-respiratory, tissue oxygenation and neuromuscular fatigue characteristics in response to exhaustive intermittent cycling. Methods: Eleven well-trained cyclists, repeated supra-maximal cycling efforts of 15 s (30 of anaerobic power reserve, 609±23 W), interspersed with 45 s of passive rest until task failure. The exercise was performed on separate days in normoxia (SL; simulated altitude/end-exercise arterial oxygen saturation = 0 m/~96), moderate (MH; 2,200 m/~90) and severe (SH; 4,200 m/~79) hypoxia in a cross-over design. Neuromuscular tests, during brief (5 s) and sustained (30 s) maximal isometric voluntary contractions of the knee extensors, were performed at baseline and exhaustion. Results: Exercise capacity decreased with hypoxia severity (23±9, 16±6 and 9±3 cycle efforts in SL, MH and SH, respectively P<0.001; η2=0.72). Both cerebral (P<0.001; η2=0.86) and muscle (P<0.01; η2=0.54) oxygenation decreased throughout the exercise, independent of condition (P≥0.45; η2≥0.14). Compared to SL, muscle oxygenation was globally lower in MH and SH (P=0.011; η2=0.36). Cardiovascular solicitation neared maximal values at exhaustion in all conditions. Peak twitch amplitude with single and paired electrical stimuli (P<0.001; η2≥0.87), maximal torque (P