Oxygen Utilization During The Contraction-Relaxation Cycle Of Intermittent Forearm Exercise

Abstract

Previous work from our laboratory showed that microvascular RBC volume was preserved and extraction of O2 continued to occur during the contraction phase of intermittent leg extension exercise. Limitations of these measures in the quadriceps include adipose tissue thickness and an inability to observe more than the most superficial portions of the muscle. The smaller muscle mass of the forearm is advantageous in that it possesses a smaller adipose pad and allows for a more comprehensive interrogation of the entire muscle. PURPOSE: The purpose of the current study was to characterize muscle oxygenation during the contraction/relaxation phases of intermittent forearm exercise. We hypothesized that O2 extraction by the muscle would not be limited to the relaxation phase of exercise. METHODS: Six healthy men (24 ± 2 yrs) completed two constant-power handgrip exercise tests at moderate (M: 40% peak power) and severe (S: 85% peak power) intensities for 10 minutes or until exhaustion. Using near-infrared spectroscopy (NIRS), measurements of total hemoglobin and myoglobin (total [Hb+Mb]), deoxygenated hemoglobin and myoglobin (deoxy [Hb+Mb]) and tissue O2 saturation (StO2) of the flexor digitorum superficialis were taken to determine changes in microvascular hematocrit, estimated microvascular O2 extraction and tissue oxygenation, respectively. Mean values for the relaxation phase and peak values during the contraction phase were calculated at both intensities. RESULTS: Total [Hb+Mb] (M: 103 ± 18.2 μM; S: 119 ± 26.9 μM) and deoxy [Hb+Mb] (M: 44.4 ± 17.9 μM; S: 53.7 ± 13.5 μM) were higher (P < 0.05) during severe intensity exercise compared to moderate intensity exercise. Regardless of intensity, the contraction phase elicited an increase (P < 0.05) in total [Hb+Mb] (M: 6.33 ± 3.88 μM; S: 7.59 ± 6.64 μM) as well as in StO2 (M: 3.69 ± 1.49 %; S: 5.31 ± 4.52 %;P = 0.01). In addition, deoxy [Hb+Mb] (M: 5.79 ± 8.75 μM; S: 7.99 ± 7.81 μM) tended to be greater (P = 0.08) during the contraction phase for both intensities. CONCLUSION: The increases in total [Hb+Mb] and StO2 during contraction suggest that arterial oxygenated blood may be shifted from larger NIRS-invisible vessels to smaller NIRS-visible ones by the contraction process, which may mask O2 extraction during contraction.