Oxygen supply-consumption balance in the thigh muscles during exhausting knee-extension exercise

The purpose of this study was to investigate the difference in muscle oxygenation between the individual muscles involved in an exhaustive knee-extension exercise. Eight active women performed exercise by extending the knee joint from 90 degrees to 30 degrees (60 extensions min-1) at 20%, 30%, and 40% maximum voluntary contraction (MVC). Changes in oxy-(delta HbO2), deoxy-(delta Hb), and total (delta HbT) hemoglobin concentrations, and oxygen saturation (delta SO2NIRS = HbO2/HbT) in the vastus lateralis (VL) and rectus femoris (RF) muscles were measured with a spatially resolved near-infrared spectrometer (NIRS). The delta SO2NIRS in the VL and RF decreased rapidly from the pre-exercise control value (VL: 75.6 +/- 0.9%; RF: 81.6 +/- 1.6%) at the onset of exercise at three different intensities, although no significant difference in delta SO2NIRS was found between the two muscles at this time. However, the delta SO2NIRS decreased more rapidly thereafter and reached a lower value at exhaustion in the VL than in the RF. The difference in delta SO2NIRS between the VL (-10.3 +/- 1.7%) and RF (-4.0 +/- 1.0%) was significant (p < 0.05) when exercise intensity was 30% MVC. When the decreases in delta HbO2 and delta HbT (p < 0.05) were compared at different exercise intensities, the values at 30% and 40% MVC were significantly lower (delta HbO2: p < 0.01; delta HbT: p < 0.05) than those at 20% MVC in the VL, but there was no significant difference in any of the parameters in the RF, or in delta Hb in the VL. These results suggest that the muscle oxidative response to exhaustive knee-extension exercise differed between the VL and RF muscles. At exhaustion, oxygen saturation decreased to a lower level in the VL than in the RF, and an intensity-dependent difference in muscle oxygenation parameters was observed at 30% MVC in the VL but not in the RF muscles.