Increased leg muscle fatigability during 2 mA and 4 mA transcranial direct current stimulation over the left motor cortex

Transcranial direct current stimulation (tDCS) using intensities <= 2 mA on physical and cognitive outcomes has been extensively investigated. Studies comparing the effects of different intensities of tDCS have yielded mixed results and little is known about how higher intensities (> 2 mA) affect outcomes. This study examined the effects of tDCS at 2 mA and 4 mA on leg muscle fatigability. This was a double-blind, randomized, sham-controlled study. Sixteen healthy young adults underwent tDCS at three randomly ordered intensities (sham, 2 mA, 4 mA). Leg muscle fatigability of both legs was assessed via isokinetic fatigue testing (40 maximal reps, 120 degrees/s). Torque- and work-derived fatigue indices (FI-T and FI-W, respectively), as well as total work performed (TW), were calculated. FI-T of the right knee extensors indicated increased fatigability in 2 mA and 4 mA compared with sham (p = 0.01, d = 0.73 and p < 0.001, d = 1.61, respectively). FI-W of the right knee extensors also indicated increased fatigability in 2 mA and 4 mA compared to sham (p = 0.01, d = 0.57 and p < 0.001, d = 1.12, respectively) and 4 mA compared with 2 mA (p = 0.034, d = 0.37). tDCS intensity did not affect TW performed. The 2 mA and 4 mA tDCS intensities increased the fatigability of the right knee extensors in young, healthy participants, potentially from altered motor unit recruitment/discharge rate or cortical hyperexcitability. Despite this increase in fatigability, the TW performed in both these conditions was not different from sham.