Tissue oxygenation by near-infrared spectroscopy and muscle blood flow during isometric contractions of the forearm

The relationship between tissue oxygenation measured by near-infrared spectroscopy (NIRS) and forearm muscle blood flow (FBF) measured by Doppler ultrasound was tested during isometric contractions at 10 and 30% maximal voluntary contraction (MVC) under conditions of normoxia and hypoxia (14% inspired O-2). Six subjects maintained contractions at 10% MVC for 5 min and at 30% for 2 min in both gas conditions. FBF was elevated during exercise at 10% MVC in hypoxia compared to normoxia, but there was no further increase in flow at 30% MVC. Median power frequency calculations from electromyographic recordings suggested progressive development of fatigue throughout both 10 and 30% MVC contractions. NIRS indicated no change in muscle oxygenation at 10% MVC, but deep venous blood O-2 saturation was reduced in normoxia and more so in hypoxia. At 30% MVC, both NIRS and venous O-2 saturation were reduced, with no effect of hypoxia on the NIRS signal. While NIRS might provide an indication of muscle oxygenation during isometric exercise, the conflicting findings for NIRS and direct Venous blood sampling at 10 vs. 30% MVC suggest caution in the application of this noninvasive technique.