Effects of Varying Levels of Normobaric Hypoxia on Muscle Oxygenation During Leg Exercise

Abstract

Previous studies have looked at muscle tissue oxygenation under a wide range of conditions including temperature extremes and in various disease states. PURPOSE: The purpose of this study was to examine the muscle tissue saturation response under varying normobaric hypoxic conditions during different intensities of aerobic exercise (walking and running). METHODS: Twelve healthy, fit participants (27.3 ± 7.1 yrs, 87.5 ± 10.2 kgs) were recruited and completed testing randomly in three different conditions simulating varying levels of hypoxia: 3000 m (MID), 5000 m (HIGH) and while breathing ambient air (LOW). A Higher Peak Mag10® hypoxic generator was used to produce normobaric hypoxia. During the HIGH, MID, and LOW conditions subjects walked for 5 min at 3 mph and then ran for 5 min at 6 mph on a motorized treadmill. Near-infrared spectroscopy (Portamon, Artinis Inc.) was used to measure oxygenated hemoglobin, deoxygenated hemoglobin, total hemoglobin, and tissue saturation index (TSI) throughout the exercise. The Portamon was placed over the vastus lateralis and secured. TSI is used as an index of muscle tissue oxygenation. In this study, the slope of the decline in TSI was determined and used to quantify the muscle oxygenation response. RESULTS: Overall, TSI decreased at a greater rate for running than walking in all three conditions. The decline in TSI was greatest (p<0.05) in the HIGH running condition (−0.023 ± .006) compared to the MID (−.018± .004) and LOW conditions (−.007± .002). There were no differences in TSI slope for the three walking conditions. CONCLUSIONS: These data suggest that muscle oxygenation declined at the fastest rate with running at the greatest level of hypoxia. This probably reflects a greater utilization of oxygen under severe hypoxic conditions in the muscle. Furthermore, running during all levels of hypoxia produced a greater rate of oxygen utilization than walking. Taken together, running at high altitude provides the greatest hypoxic stress to exercising leg muscle.