Attenuation of Muscle Deoxygenation Precedes EMG Threshold in Normoxia and Hypoxia

Abstract

The aim of this study was to investigate the relationship between the attenuation point of muscle deoxygenation (APMD) and the EMG threshold (EMGT) during incremental cycling exercise under different fractions of inspired O2 (FIO2). Nine male subjects performed ramp cycling exercise tests (20 W·min(-1)) to exhaustion under normoxic and hypoxic conditions (FIO2 = 0.12). Pulmonary O2 uptake (VO2), muscle deoxygenation, and EMG activity in the vastus lateralis muscle were simultaneously measured during the tests, and both APMD and EMGT were calculated. Hypoxia significantly reduced peak VO2 (VO2peak). At the same absolute exercise intensity and at VO2peak, muscle deoxygenation, but not EMG activity, was significantly greater in hypoxia. VO2 at APMD was significantly decreased in hypoxia (P < 0.01). Similarly, VO2 at EMGT was significantly lower in hypoxia than in normoxia (P < 0.01). In addition, VO2 was lower at APMD than at EMGT under both conditions (P < 0.01). However, the relationships between APMD and EMGT were significant under both normoxic (r = 0.95, P < 0.01) and hypoxic (r = 0.89, P < 0.01) conditions. These results suggest that the attenuation of muscle deoxygenation near VO2peak is related to and precedes changes in neuromuscular activity under normoxic and hypoxic conditions.