Effects of tDCS combined with unstable resistance training on knee muscle strength and neuromuscular control in community-dwelling older adults: a randomized controlled trial

Background Age-related declines in knee muscle strength are not fully explained by muscle loss and may also involve alterations in neuromuscular control. Unstable resistance training (URT) increases stabilization and sensorimotor demands during exercise. Transcranial direct current stimulation (tDCS) may modulate corticospinal excitability during training. This study investigated whether tDCS combined with URT enhances knee strength and electromyography (EMG)-derived neuromuscular control in older adults. Methods Ninety community-dwelling older adults were randomized to URT with a-tDCS, URT with sham tDCS, URT, stable resistance training (SRT), or non-exercise control (CON). The four training groups completed three sessions per week for 8 weeks. Knee extension and flexion relative peak torque were assessed using isokinetic dynamometry at 60°/s and 120°/s. Surface EMG quantified normalized root mean square (RMS) amplitudes and thigh co-activation, and residualized change-score analyses examined associations between co-activation and knee strength changes. Results At 120°/s, significant group × time interactions were observed for knee extension ( F (4, 69) = 34.496, p < 0.001, η p ² = 0.667) and flexion relative peak torque ( F (4, 69) = 36.474, p < 0.001, η p ² = 0.679). Knee extension and flexion torque increased in all training groups and were higher in the URT + a-tDCS group than in other groups after the intervention (all p < 0.05). At 60°/s, knee extension and flexion torque increased in all training groups and were higher than in the CON group (all p < 0.05). Thigh co-activation decreased in the URT + a-tDCS group at 60°/s. In the URT group, co-activation changes were negatively correlated with changes in extension ( r = − 0.647, p = 0.009) and flexion torque ( r = − 0.576, p = 0.025) at 120°/s. Conclusion Eight weeks of URT combined with a-tDCS produced greater gains in knee strength at 120°/s than the other interventions in community-dwelling older adults. Changes were observed in quadriceps RMS amplitude and thigh co-activation. The exploratory association between reduced thigh co-activation and greater strength gains was identified only in the URT group, suggesting that the contribution of co-activation to strength adaptation may differ across interventions. Trial registration Chinese Clinical Trial Registry, ChiCTR2500112261, registered on November 12, 2025 ( http://www.chictr.org.cn/ ).