Relationshp between oxygenation and myoelectric activity at vastus lateralis and lateral gastrocnemius muscles

In order to find the cause behind the difference in the pattern of oxygenation between thigh and calf muscles during an incremental cycling exercise, we investigated the relationship between oxygenation and surface myoelectric activity. Five healthy male subjects performed the five 6-min cycling exercises of 50, 100, 150, 200 and 250 watts. We measured oxygenated hemoglobin/myoglobin (oxy-Hb/Mb) on continuous wave near infrared spectroscopy (NIR) and myoelectric activity with surface electrodes (EMG). The NIR probe and electrodes were positioned on the vastus lateralis muscle (VL) and lateral gastrocnemius muscle (LG) of the right leg. The relative change in oxy-Hb/Mb level was estimated by regarding the oxy-Hb/Mb in the resting condition as 100% and that obtained during thigh occlusion as 0%. The mean values of oxy-Hb/Mb and integrated EMG (iEMG) determined ranged from 5'30 " to 6'00 " at each work rate. In the VL, the oxy-Hb/Mb level decreased slightly at 50 and 150 watts (99.2 +/- 6.8% and 84.6 +/- 12.9%), followed by a more prominent decrease at 200 and 250 watts (74.9 +/- 11.9 and 65.2 +/- 9.6%). The iEMG increased slowly at 50 and 150 watts (24.9 +/- 8.8 and 38.0 +/- 8.8 mu V), and a rapid increment occurred at 200 and 250 watts (49.6 +/- 13.0 and 57.3 +/- 19.6 mu V). In the LG, however, the oxy-Hb/Mb level was decreased slightly at 50 and 200 watts (97.3 +/- 11.1 and 87.4 +/- 6.4%) and a rapid decrement occurred at 250 watts (74.1 +/- 6.0%). The iEMG increased slowly at 50 and 200 watts (18.3 +/- 9.4 and 30.5 +/- 8.8 mu V) , and a rapid increment occurred at 250 watts (38.5 +/- 8.9 mu V). There was a significant negative correlation between the oxy-Hb/Mb level and iEMG in the VL (r = -0.587, p < 0.001) and in the LG (r = -0.599, p < 0.001). From these results, we speculated that the difference in the relative change of oxy-Hb/Mb between the vastus lateralis and the lateral gastrocnemius muscle was due to the difference of increase pattern of muscle activity accompanied by an increase in exercise intensity in these two muscles.