Effect of knee flexion on muscle oxygen saturation in adults during a 40-s isometric squat

Monitoring internal load is essential to understanding fatigue and training adaptation in athletes. This study aimed to evaluate the effect of different knee flexion angles during a 40-s isometric squat on muscle oxygen saturation (SmO₂) responses. Seventeen physically active participants (9 males and 8 females: 28 ± 9 years old, 174 ± 12 cm, and 70.1 ± 16.6 kg) performed three randomized squats at 80°, 90°, and 100° knee angles. Muscle excitation (i.e., RMS) was measured using surface electromyography (EMG), and SmO₂ was recorded with near-infrared spectroscopy technology. Results indicated that SmO₂ differed only between the most extreme angles (80° vs. 100°), with significantly greater deoxygenation at 80°, emerging in the final 20% of the squat (p < 0.01 and 95% CI of the differences [6, 21%]). In contrast, the RMS of the squat at 80° was higher than at 90° (p = 0.03 and 95% CI [0.4, 8.5%]) and at 100° (p < 0.001 and 95% CI [5.8, 14.2%]). The findings suggest that 90° knee flexion elicits an intermediate metabolic and muscular excitation response. A 40-s isometric squat is sufficient to reveal condition-specific differences in SmO₂, though differences only emerge toward the end of the task. Thus, using a goniometer for posture control may be sufficient in field settings, offering a practical approach for load monitoring and fatigue assessment during isometric strength testing.