Oxygen Utilization During The Contraction-relaxation Of Isometric Knee Extension Exercise

Abstract

PURPOSE: Sufficiently high intramuscular pressures during contraction can lead to increased resistance to perfusive oxygen delivery and therefore increased fractional oxygen extraction in order to maintain muscle VO2. Near-infrared spectroscopy (NIRS) has been used to estimate fractional oxygen extraction via deoxygenated heme concentrations (deoxy-[heme]) and changes in microvascular hematocrit via changes in total heme concentrations (total-[heme]). We tested the hypotheses that during severe and extreme intensities 1) deoxy-[heme] would decrease during contraction compared to relaxation and 2) total-[heme] would be unchanged during the contraction-relaxation cycle. METHODS: Four subjects (2 men, 24.0 ± 3.7 yrs, 81.9 ± 27.4 kg, 168.9 ± 10.2 cm) completed two isometric knee extension tests to failure at 40% (severe) and 70% (extreme) MVC. NIRS was placed on the right vastus lateralis to continuously measure deoxy-[heme] and total-[heme] during relaxation and contraction. Deoxy-[heme] and total-[heme] were averaged during the final 5 contraction-relaxation cycles and compared using paired t-tests. RESULTS: Average deoxy-[heme] during contraction and relaxation at 40% was 44.8 ± 31.8 υM and 44.4 ± 31.6 υM, respectively. Average deoxy-[heme] during contraction and relaxation at 70% was 41.9 ± 24.5 υM and 40.1 ± 23.6 υM, respectively. Average total-[heme] during contraction and relaxation at 40% was 127.0 ± 106.4 υM and 124.2 ± 105.2 υM, respectively. Average total-[heme] during contraction and relaxation at 70% was 116.0 ± 89.4 υM and 115.0 ± 90.5 υM, respectively. Although statistical significance was not detected (p=0.06), all four subjects demonstrated an increase in deoxy-[heme] at 70% MVC, but not at 40%. CONCLUSIONS: These data suggest that the contraction-relaxation cycle may result in fluctuations in fractional oxygen extraction during extreme but not severe isometric exercise. Furthermore, the contraction-relaxation cycle does not appear to alter microvascular hematocrit.