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

## TL;DR

This study shows how different knee angles during a squat affect muscle oxygen levels and muscle activity in physically active adults.

## Contribution

The study reveals that extreme knee angles during isometric squats cause significant differences in muscle oxygen saturation and excitation.

## Key findings

- Muscle oxygen saturation differences emerged only between 80° and 100° knee angles during the squat.
- Higher muscle excitation was observed at 80° knee flexion compared to 90° and 100°.
- A 40-s isometric squat can reveal condition-specific differences in muscle oxygen saturation toward the end of the task.

## Abstract

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.