Effect of Hypoxia on the Kinetics of Peripheral Muscle Oxygenation during Prolonged Exercise

Abstract

Near-infrared spectroscopy (NIRS) is a noninvasive method of measuring muscle oxygenation. We have examined relationships between muscle oxygenation and blood lactate concentration, substrate utilization, femoral artery blood flow, and systemic oxygen intake during prolonged exercise. However, we have not examined muscle oxygenation trends during prolonged exercise at different oxygen levels. PURPOSE: To clarify the effect of hypoxia (FiO2=0.16) on the local muscle oxygenation trends during prolonged cycle exercise at ventilatory threshold (VT) level. METHODS: Seven healthy males completed an orientation trial and a maximal exercise test, followed by two 30-min submaximal exercise bouts. During each exercise test, expiratory gas analysis was performed using an expiratory gas analyzer, and the muscle oxygenation trends of vastus lateralis muscle were determined by NIRS. After exercises, calibration was performed by cuff occlusion at 260 mmHg for 10 min. Cardiorespiratory parameters, forearm venous blood lactate concentration and epinephrine levels were measured at rest and during exercise. RESULTS: Arterial O2 saturation levels at rest and during exercise in the hypoxic condition were significantly lower than those in normoxia condition (p<0.05). VO2, HR and epinephrine levels during exercise in the hypoxic condition were significantly higher than those in normoxia condition (p<0.05). Muscle oxygenation decreased immediately after the onset of exercise and gradually increased, but it did not reach the resting level. This pattern of muscle oxygenation was similar for two gas conditions. However, the rate of decrease in muscle oxygenation was enhanced by reduced arterial O2 saturation. On the other hand, after the initial drop, the rate of increase in muscle oxygenation during exercise did not change for two gas conditions. These results indicate that the rate of decrease in muscle oxygenation is sensitive to oxygen delivery. CONCLUSIONS: The significant decrease in muscle oxygenation induced by hypoxia during prolonged exercise at VT level was due to a greater release of oxygen by oxyhemoglobin at the capillaries via Bohr effect.