Heterogeneous oxygenation in nonexercising triceps surae muscle during contralateral isometric exercise

To test whether changes in oxygenation of a resting skeletal muscle, evoked by a static contraction in a contralateral muscle, is uniform within a given skeletal muscle, we used near-infrared spectroscopy (NIRS). Seven subjects performed 2 min static knee extension exercise at 30% of maximal voluntary contraction. Changes in oxygenated hemoglobin (HbO₂) were monitored using multiple-channel NIRS (40 channels, 13 sources and 12 detectors) attached on the contralateral nonexercising triceps surae muscle. Changes in HbO₂ were expressed as a percentage of total labile signals. To characterize the distribution of changes in HbO₂, channels were compared between their positions on the triceps surae muscle, and represented as 'proximal versus distal' and 'lateral versus medial' portions. During static muscle contraction, the averaged changes in HbO₂ of all channels were correlated with those in calf blood flow (plethysmography; R²=0.188, P<0.05) and with calf vascular conductance (R²=0.146, P< 0.05). HbO₂ did not differ significantly between the lateral and medial portions of the triceps surae muscle. In contrast, the decrease of HbO₂ in the proximal portion of the muscle was greater than that of the distal portion (P< 0.05). These results indicate that the changes in oxygenation of a resting muscle, evoked by static contraction of the contralateral muscle, are heterogeneous.