Transcranial current direct stimulation reduces cold pain perception but not acute muscle pain

Stimulation of muscle pain receptors by release of algesic substances during high intensity exercise is the cause of acute muscle pain. Peripheral signals are processed in the brain and then perceived as pain sensation. Some authors have proposed that an athletes' ability to tolerate exercise-induced muscle pain could represent an important factor in long lasting, high intensity exercise (5, 6). Non-invasive techniques such as the transcranial direct current stimulation (tDCS) have been previously shown to relieve pain perception (1, 4), and so we investigated whether tDCS administration would lead to an improvement in exercise performance. Pain response was monitored during exercise (PAIN-EXE) and a cold pressor test (CPT), (PAIN-CPT) in two separate studies (A and B respectively). In study A, following full ethical approval, 9 participants performed a cycling time to exhaustion (TTE) at a 70% of their peak power output while in study B, 7 subjects underwent a CPT with an 8 min cut-off time. Both studies involved a control (CON), placebo (SHAM) and experimental (tDCS) session in a single-blind, randomised, counter-balanced design. tDCS stimulation for 10 min at 2.0 mA was delivered by placing anodal electrode above the left motor cortex (M1) with the cathodal electrode placed above dorsolateral right prefrontal cortex (1). Ratings of perceived exertion (RPE) were monitored during the TTE using Borg 6-20 scale. PAIN-EXE and PAIN-CPT were assessed using the 10 points numerical Cook scale (3). An isotime of 6 min, plus the final min, for both the TTE and CPT were used in order to include all participants in the subsequent analyses. A one-way ANOVA with repeated measures was used to assess TTE duration. Two-way ANOVA with repeated measures was used to analyse RPE, PAIN-EXE and PAIN-CPT data. All data are presented as means ± SD in Fig. 1.No significant differences (p>0.05) in exercise duration, RPE and PAIN-EXE were found in the TTE. However, PAIN-CPT in the tDCS session was significantly lower (p<0.05) compared with the other conditions (5.6±2.8 CON, 6.0±3.0 SHAM, 5.5±2.7 tDCS).These findings demonstrate that tDCS is capable of inducing an analgesic effect in response to cold pain stimuli but not for exercise-induced muscle pain.