Transcranial direct current stimulation improves isometric time to exhaustion performance of lower limbs.

Supraspinal fatigue is defined as the inability of the motor cortex (M1) to produce an adequate neural drive to excite and drive motoneurons adequately, and could contribute to the decrease in force production capacity (2). Recently, research studies have applied the use of transcranial direct current stimulation (tDCS) to manipulate corticospinal excitability in order to improve endurance performance (1). These interventions can be inhibitory (cathodal) or excitatory (anodal). Since there is no consensus on the standard placement of electrodes for improving endurance performance, we therefore tested the effect of two electrodes configurations. Nine subjects underwent a control (CON), placebo (SHAM) and two different tDCS configurations sessions in a double blind, randomised and counterbalanced design. In one tDCS session, the anodal electrode was placed over the left M1 and the cathodal on contralateral forehead (HEAD) while for the other montage, the anodal electrode was placed over the left M1 and cathodal electrode above the contralateral shoulder (SHOULDER). tDCS was delivered for 10 min at 2.0 mA, after which participants performed an isometric time to exhaustion (TTE) of the right knee extensors at 20% of the maximal voluntary contraction (MVC). Peripheral and central parameters were examined respectively by femoral nerve stimulation and M1 excitability via TMS at baseline, after tDCS application and immediately after TTE. Heart rate (HR), ratings of perceived exertion (RPE), and leg muscle PAIN were monitored during the TTE. A one-way ANOVA with repeated measures was used to assess TTE duration, while two-way ANOVA with repeated measures was used to analyse central and peripheral parameters, HR, PAIN, and RPE. None of the central and peripheral parameters showed any difference between conditions after tDCS stimulation (p>0.05). MVC significantly decreased after TTE (p<0.05) due to presence of central and peripheral fatigue, whilst motor evoked potential area (MEP) and cortical silent period increased after TTE (p<0.05) independently of the experimental condition. TTE was longer in the SHOULDER condition (p<0.05) although HR and PAIN did not present any difference between conditions (p>0.05). However, RPE was significantly lower in the SHOULDER condition (p<0.05). This is the first study showing an improvement of isometric TTE performance of the lower limbs after tDCS stimulation and further demonstrates that anodal tDCS over M1 improves isometric endurance performance of the knee extensors. Our findings suggest that SHOULDER montage is more effective than HEAD montage to improve endurance performance.