Acute Fatigue Impairs Heart Rate Variability and Resting Muscle Oxygen Consumption Kinetics

Featured Application: Importantly, measurements obtained through VOT-NIRS can vary significantly post-exercise, which should be considered by researchers before application. This study evaluated the influence of acute fatigue on heart rate variability (HRV) and muscle oxygen saturation (SmO₂) at rest, as well as the reliability of SmO₂ data measured using near-infrared spectroscopy (NIRS) during a vascular occlusion test (VOT). Twelve physically active subjects participated. Measurements included perceived muscle soreness using the visual analog scale (VAS pain), HRV parameters, variables of resting SmO₂ (desaturation and resaturation), and reoxygenation kinetics (mean response time, MRT) through a VOT-NIRS located in the vastus lateralis (VL). Measurements were taken at three points: 24 h before, before exhaustive exercise, and 30 min after exhaustive exercise. The results indicated that acute fatigue increased resting muscle oxygen consumption in desaturation (+22 SmO₂) and resaturation (+18 SmO₂), improved MRT (−15 s), and elevated sympathetic nervous system (SNS) activity, as observed in the R-R interval (−262 ms) and SNS index (+0.5). HRV significantly influenced desaturation (r² = 0.69), resaturation (r² = 0.60), and MRT (r² = 0.54). Reliability was established with an ICC of 0.49 and 0.63 for desaturation and resaturation, respectively. Real changes in desaturation and resaturation should be considered ≥ 7% SmO₂ at rest and ≥ 11% SmO₂ to avoid daily fatigue interference. In conclusion, acute fatigue increases resting SmO₂ consumption and is associated with higher SNS activity and increased VAS pain.