oa The effect of graded hypoxia on the development of neuromuscular fatigue during maximal intermittent dynamic leg extensions

Hypoxia has been shown to exacerbate the performance loss during Maximal Intensity Intermittent Exercise (MIIE). However, the effect of hypoxia on the development of central and peripheral mechanisms of fatigue during MIIE is unknown. The aim of the study is to explore the development of both central and peripheral fatigue during multiple bouts of MIIE under varying severity levels of hypoxia. On separate days, 14 healthy men performed four bouts of 6 x 5 maximal intensity, isokinetic leg extensions at 300°/s (15 s passive rest separating reps, 100 s separating sets) under normoxia (simulated altitude/fraction of inspired 02: 0m/0.21%), moderate (3000m/14.4%), and severe hypoxia (5400m/10.1%). Neuromuscular assessments which included electromyography and measurements of voluntary and evoked contractions of the knee extensors were conducted immediately pre and post each bout of exercise. Performance loss was dependent on the severity of hypoxia with a main effect of condition on the mean peak power during each bout of exercise (p<.005) and the percent decrement in mean peak power across each bout (normoxia; 2.9 ± 1.3 %, moderate hypoxia; 5.2 ± 1.5 %, severe hypoxia; 10.3 ± 2.0 %, p=<.01). Despite a main effect of time, the reduction in RMS activity was not different between conditions (-10.4% all conditions compounded). Maximal voluntary contraction force, % voluntary activation and potential twitch force (Q,tw,pot) all decreased across time (p=<.05), with the end exercise percent reduction in Q,tw,pot being greater in severe hypoxia compared to normoxia (41.3 ± 3.0 % v 28.0 ± 3.2 % p=<.05). In conclusion, performance decrements during maximal intermittent dynamic leg extensions are exacerbated by hypoxia and result from both central and peripheral adjustments. However, central adjustments do not appear to limit excessive development of peripheral fatigue below a critical threshold.