Does transcranial direct current stimulation enhance the hypoalgesic effect of exercise?

Exercise produces a decrease in pain sensitivity via an effect called exercise-induced hypoalgesia (EIH). Transcranial direct current stimulation (tDCS), acting on similar analgesic mechanisms as EIH, represents a potential complementary intervention that may amplify the effects of exercise on pain. This study aimed to explore if anodal tDCS could enhance the effect of exercise on pain compared to exercise alone. A total of 35 healthy participants aged 19-37 years completed a familiarisation session followed by two separate sessions where active and sham tDCS was applied in a randomised cross-over design. The familiarisation session involved familiarisation to the pain assessment and exercise tasks, while the subsequent tDCS sessions involved pain sensitivity assessment, exercise and either anodal tDCS or sham tDCS. tDCS doses were applied at 2 ​mA over the primary motor cortex for 10 ​min, with the reference electrode placed over the contralateral supraorbital area. The exercise task involved a sustained isometric grip strength contraction at 35% of maximal voluntary contraction (MVC) until volitional exhaustion. Pain sensitivity was evaluated as pressure pain threshold before tDCS, after tDCS, and after exercise. Across both tDCS conditions, pain threshold was higher after exercise when compared to pre- and post-tDCS measurement. This increase in pain threshold did not differ between active and sham tDCS conditions. Our findings suggest that the hypoalgesic effects of active anodal tDCS over the motor cortex prior to exercise are no greater than the effects of sham tDCS prior to exercise.