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; 2200 m/~ 90%) and severe (SH; 4200 m/~ 79%) hypoxia in a cross-over design. Neuromuscular tests, including 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; η = 0.72). Both cerebral (P < 0.001; η = 0.86) and muscle (P < 0.01; η = 0.54) oxygenation decreased throughout the exercise, independent of condition (P ≥ 0.45; η ≥ 0.14). Compared to SL, muscle oxygenation was globally lower in MH and SH (P = 0.011; η = 0.36). Cardiovascular solicitation neared maximal values at exhaustion in all conditions. Peak twitch amplitude with single and paired electrical stimuli (P < 0.001; η ≥ 0.87), maximal torque (P < 0.001; η ≥ 0.48) and voluntary activation measured using transcranial magnetic stimulation (P ≤ 0.034; η ≥ 0.31) during brief and sustained MVCs were all reduced at exhaustion, independent of condition (P ≥ 0.196; η ≥ 0.15).

CONCLUSION

Despite reduced exercise capacity with increasing severity of hypoxia during exhaustive intermittent cycling, neuromuscular fatigue characteristics were not different at task failure and cardiovascular solicitation neared maximum values.