Muscle Deoxygenation And Electromyographic Activity During Isolated Muscle Exercise In Hypoxia

Abstract

It is well known that acute hypoxia significantly increased the rates of development of exercise-induced muscle fatigue. It is hypothesized that muscle deoxygenation is accelerated during exercise in hypoxia, and that this would be related to the rate of increase in electromyographic (EMG) activity as an index of motor unit recruitment. PURPOSE: The aim of the present study was to elucidate the changes in muscle deoxygenation and EMG activity during isolated muscle exercise in hypoxia and the relationship between the changes in deoxygenation and EMG activity. METHODS: Seven healthy male subjects performed intermittent, unilateral, isometric, submaximal quadriceps muscle contractions [60% maximal voluntary contraction (MVC), 5 sec of contraction/5 sec of rest] while breathing normoxic (FIO2=0.21) or hypoxia (FIO2=0.11-0.12) gas mixture. MVC was obtained before and at the end of each minute of submaximal exercise until exhaustion, defined as the inability to exert or maintain 60% of rested MVC. The change in muscle deoxygenation was measured by near-infrared spectroscopy. Surface EMG and muscle deoxygenation were simultaneously recorded from the vastus lateralis muscle. RESULTS: The deoxygenation index (deoxygenated Hb/Mb - oxygenated Hb/Mb) during exercise in hypoxia was larger (P<0.05) than that in normoxia. The rate of increase in iEMG during exercise in hypoxia was greater (P<0.05) than in normoxia. The rate of percent reduction of MVC in hypoxia was larger (P<0.05) than normoxia. To clarify the relationship between parameters during exercise in hypoxia and normoxia, the magnitude of the changes in deoxygenation index and iEMG at end of first three minutes during submaximal exercise were calculated individually as the difference between those obtained in hypoxia and normoxia (delta = hypoxia - normoxia). A significant positive correlation was observed between delta deoxygenation index and delta iEMG at the second (r=0.71, P<0.05) and third (r=0.75, P<0.05) minutes during exercise. CONCLUSIONS: These results suggest that the rates of rise of muscle activity and development of fatigue during submaximal isolated muscle exercise in acute hypoxia are linked to the magnitude of muscle deoxygenation.