Current intensity-dependent effects of acute and repeated transcranial direct current stimulation on knee muscular endurance

Objective This study aimed to investigate the acute and prolonged effects of anodal tDCS at different intensities applied over the primary motor cortex (M1) on knee muscular endurance. Methods Seventy healthy adults were randomized into four groups (1 mA, 2 mA, 3 mA, and sham) and received 10 sessions of 20 min tDCS over 4 weeks. The endurance index (EI), calculated as the ratio of work output during the last five to the first five of 15 maximal isokinetic contractions, was assessed at baseline (T0), immediately after the first session (T1), 24 h after the 10th session (T2), and at 1-week follow-up (T3). Results Among the 70 healthy participants, a non-linear dose-response pattern was observed. Only 2 mA tDCS produced significant enhancements in endurance, specifically for left knee flexion, outperforming sham stimulation at T1 and both sham and 3 mA stimulation at T2. Notably, 1 mA and 3 mA showed negligible or attenuated efficacy. Conclusions These findings underscore that tDCS-induced endurance gains are contingent upon optimized intensity and hemispheric targeting, rather than simply applying higher currents. The observed non-linear behavioral response pattern warrants further investigation with neurophysiological measures to elucidate the underlying cortical excitability dynamics.