Effects of electrical stimulation and voluntary exercise on muscle oxygenation assessed by NIRS

In this study, we investigated the effect of low-frequency ES (electric stimulation) to muscle oxygenation level by NIRS (near infrared spectroscopy) from comparing with V-Ex (voluntary exercise). Ten subjects performed ES and V-Ex test in supine position on a bed with 90 degrees flexion of the right knee joint and fixation of the right ankle to the end of the bed with a strap. NIRS probe was placed on middle point of the vastus lateralis, and four electrodes were placed across the motor point of the rectus femoris and vastus lateralis. Stimulation voltage was started at 20 V (20 Hz, pulse duration: 200 micros, duty-cycle: 1s-1s), and then was increased at a rate of 3 V/30 s until maximal tolerance level. V-Ex (isometric knee extension) was performed with same posture as in ES, and exercise pattern was set at a 1-s contraction and 1-s relaxation cycle. Exercise intensity was started at 5% MVC (maximal voluntary contraction) and was increased at a rate of 5% MVC/30 sec until exhaustion. In ES and V-Ex, tissue oxygenation index was decreased with decrease in O2Hb (oxy-hemoglobin) and increase in HHb (deoxyhemoglobin), and muscle oxygenation levels at the end of test were very alike. Oxygen consumption, heart rate, systolic and diastolic blood pressure in ES and V-Ex increased significantly, however, the degree of change in ES was significantly lower than V-Ex. Blood lactate was significantly increased in both tests. Adrenaline and noradrenaline were significantly increased in V-Ex, even though they showed no change in ES. These results lead us to believe that ES is an effective technique activated muscle hypoxia and glycolytic pathway metabolism with low stress on respiratory, circulatory and sympathetic nervous systems.