Effects of high-altitude exposure on supraspinal fatigue and corticospinal excitability and inhibition.

Abstract

While acute hypoxic exposure enhances exercise-induced central fatigue and can alter corticospinal excitability and inhibition, the effect of prolonged hypoxic exposure on these parameters remains to be clarified. We hypothesized that 5days of altitude exposure would (i) normalize exercise-induced supraspinal fatigue during isolated muscle exercise to sea level (SL) values and (ii) increase corticospinal excitability and inhibition. Eleven male subjects performed intermittent isometric elbow flexions at 50% of maximal voluntary contraction to task failure at SL and after 1 (D1) and 5 (D5)days at 4350m. Transcranial magnetic stimulation and peripheral electrical stimulation were used to assess supraspinal and peripheral fatigues. Pre-frontal cortex and biceps brachii oxygenation was monitored by near-infrared spectroscopy. Exercise duration was not statistically different between SL (1095±562s), D1 (1132±516s), and D5 (1440±689s). No significant differences were found between the three experimental conditions in maximal voluntary activation declines at task failure (SL -16.8±9.5%; D1 -25.5±11.2%; D5 -21.8±7.0%; p>0.05). Exercise-induced peripheral fatigue was larger at D5 versus SL (100Hz doublet at task failure: -58.8±16.6 versus -41.8±20.1%; p<0.05). Corticospinal excitability at 50% maximal voluntary contraction was lower at D5 versus SL (brachioradialis p<0.05, biceps brachii p=0.055). Cortical silent periods were shorter at SL versus D1 and D5 (p<0.05). The present results show similar patterns of supraspinal fatigue development during isometric elbow flexions at SL and after 1 and 5days at high altitude, despite larger amount of peripheral fatigue at D5, lowered corticospinal excitability and enhanced corticospinal inhibition at altitude.