Acute knee extensor torque and muscle oxygenation responses to isometric testing in exercise and non-exercised legs...

Portable near-infrared spectroscopy (NIRS) provides a noninvasive measure of local muscle metabolic demand through muscle oxygen saturation (SmO₂). This study aimed to evaluate acute SmO₂ responses in exercising and non-exercising muscles during an isometric test conducted before and after exercise. Twelve physically active men (25 ± 6 years; 1.75 ± 0.07 m) performed three maximal voluntary contractions (MVC) and a 45-s isometric contraction at 70% MVC (IC70%) before and after completing five sets of 30 concentric knee extensions at 180°·s^-1, with 60 s rest between sets, on an isokinetic dynamometer. SmO₂ was recorded in the vastus lateralis (exercised and non-exercised leg) and triceps brachii. Significant differences in SmO₂ were observed between exercised and non-exercised legs both pre- (p = 0.048) and post-exercise (p < 0.001), whereas no significant changes were found in the triceps brachii or non-exercising vastus lateralis. Acute responses to isometric exercise differed between SmO₂ and torque measures. While the torque of the IC70% did not change following the exercise protocol, SmO₂ showed a higher deoxygenation response during exercise. The isometric test did not indicate any redistribution of muscle oxygenation from non-exercising to exercising muscles. These results provide novel insights into the dissociation between mechanical and microvascular responses during isometric exercise and support the potential of this test as a practical tool for monitoring training loads.