Effects of prolonged hypobaric hypoxia on human skeletal muscle function and electromyographic events

This study tested the hypothesis that a prolonged decrease in arterial oxyg en pressure in resting Or contracting skeletal muscles alters their ability to develop force through an impairment of energy-dependent metabolic proce sses and also through an alteration of electrophysiological events. The exp eriment was conducted during a 32-day simulated ascent of Mt. Everest (8848 m altitude) (Everest III Comex '97), which also allowed testing of the eff ects of re-oxygenation on muscle function. Maximal voluntary contractions ( MVCs) of the flexor digitorum, and static handgrips sustained at 60% of MVC , were performed by eight subjects before the ascent (control), then during the stays at simulated altitudes of 5000 m, 6000 m and 7000 m, and finally day after the return to 0 m. The evoked muscle compound action potential ( M-wave) was recorded at rest and during the manoeuvres at 60% of MVC. The c hanges in median frequency of electromyographic (EMG) power spectra were al so studied during the contraction at 60% of MVC. In four individuals, trans ient re-oxygenation during the ascent allowed us to test the reversibility of hypoxia-induced MVC and M-wave changes. At rest, a significant decrease in M-wave amplitude was noted at 5000 m. This effect was associated with a prolonged M-wave conduction time at 6000 m and an increased M-wave duration at 7000 m, and persisted after the return to Om. Re-oxygenation did not mo dify the changes in M-wave characteristics. A significant decrease in MVC w as measured only during the ascent(- 10 to -24%) in the nondominant forearm of subjects who underwent re-oxygenation; this intervention slightly impro ved muscle strength at 6000 m and 7000 m. During the ascent and after the r eturn to 0 m, there was a significant reduction of the median frequency dec rease throughout contraction at 60% of MVC compared with the EMG changes me asured before the ascent. It is concluded that prolonged exposure to hypoxi a slows the propagation of myopotentials and alters sensorimotor control du ring sustained effort. Re-oxygenation did not affect the hypoxia-induced EM G changes and had a modest influence on muscle strength.