Intensity-dependent transcranial direct current stimulation effects on lower limb strength: optimizing acute and prolonged gains in explosive force and maximal strength.

Regarding transcranial direct current stimulation (tDCS), higher current intensities are not always associated with larger strength enhancement. This study systematically examined acute and chronic effects of tDCS intensity between 1 mA and 3 mA over the primary motor cortex (M1) on lower limb explosive and maximal strength. Seventy participants were randomly allocated to four groups receiving 10 sessions of different tDCS intensities (1 mA, 2 mA, 3 mA, or sham) to the M1 for 20 min, with a 48 h interval between sessions, for four weeks. Lower limb muscle strength was assessed at baseline (T0), immediately after the first session (T1), 24 h after the tenth sessions (T2), and at the 1 week follow-up (T3), using the countermovement jump (CMJ) for CMJ height, and an isokinetic muscle strength test for peak torque (PT) of the bilateral knee and ankle. There was a significant improvement in explosive and maximal strength after the multiple sessions of 2 mA and 3 mA tDCS, maintained for at least 1 week. But the single session of tDCS at intensities between 1 mA and 3 mA over the M1 did not sufficient to result in acute effects on lower limb explosive and maximum knee PT.