Oxygenation and neuromuscular activation of the quadriceps femoris including the vastus intermedius during a fatiguing contraction

Introduction: The purpose of this study was to assess oxygenation and neuromuscular activation of the quadriceps femoris muscle group, including the vastus intermedius (VI), during a fatiguing contraction. Methods: Eleven healthy men performed an isometric knee extension at 50% of the maximum voluntary contraction until failure. Near-infrared spectroscopy was used to measure tissue oxygen saturation (StO(2)), and surface electromyography was used to quantify the median frequency (MF) of the four individual quadriceps femoris muscles. Results: A significant decrease in StO(2) began at 25% of time to exhaustion in all four muscles, and StO(2) of the VI was significantly higher than StO(2) of the vastus medialis (VM) from 25% of time to exhaustion until 95% of time to exhaustion. The MF of the VI did not significantly decrease during the fatiguing task. We found a significant correlation between MF and StO(2) in the VI (r = 0.745, P<0.01) and a significant correlation between StO(2) and time to exhaustion in the VM and rectus femoris (RF) (VM, r = 0.684, P<0.05; RF, r = 0.635, P<0.05). Conclusion: These results suggest that muscle O-2 saturation in the VI is associated with neuromuscular fatigue during a fatiguing isometric contraction, and the level of oxygenation in the VM and RF is a key to sustaining a fatiguing contraction.