Abstract: BACKGROUND: Transcranial direct current stimulation can enhance human motor performance by modulating cortical excitability, but its impact on the performance of unstable resistance exercise remains unclear. 
OBJECTIVE: To investigate the effects of transcranial direct current stimulation on unstable barbell bench press performance and electromyographic activity. 
METHODS: A randomized, self-controlled crossover design was employed. Twenty-two male college students were randomly assigned to receive either active anodal transcranial direct current stimulation or sham stimulation. After transcranial direct current stimulation interventions, subjects performed a fatigue test involving the unstable barbell bench press. Performance during the unstable barbell bench press task was assessed by recording both the number of completed repetitions and barbell acceleration, serving as indicators of load capacity and movement stability control. In addition, surface electromyographic signals were collected from the right biceps brachii, triceps brachii, anterior deltoid, posterior deltoid, and pectoralis major during the exercise. A paired t-test was used to examine differences in repetition counts between true and sham stimulation conditions. Repeated measures analysis of variance was applied to analyze differences in triaxial acceleration, agonist muscle activation levels, and antagonist co-activation levels. 
RESULTS AND CONCLUSION: (1) No significant difference in the number of barbell bench press repetitions was observed between the true and sham stimulation conditions. However, the mean amplitude of Y-axis acceleration was substantially lower in the true stimulation group than the sham stimulation group. (2) True transcranial direct current stimulation markedly increased the activation level of the anterior deltoid and the co-activation level of the posterior deltoid. Except for the posterior deltoid, the activation and co-activation levels of the tested muscles were markedly higher during the latter half of the exercise compared with the initial half. (3) There were no significant interaction effects between transcranial direct current stimulation intervention and exercise phase on the measured parameters. (4) These findings indicate that transcranial direct current stimulation has no significant impact on muscle endurance performance during unstable barbell bench press exercises, but can enhance movement stability. This improvement may be attributed to the significant increase in the activation levels of the anterior deltoid and the co-activation of the posterior deltoid, leading to greater shoulder joint stiffness and stability. In competitive sports and clinical rehabilitation training, transcranial direct current stimulation can be employed to modulate performance during unstable resistance exercise according to the training goals. 

Key words: transcranial direct current stimulation, true stimulation, sham stimulation, unstable resistance exercise, movement stability, surface electromyography