Anodal transcranial direct current stimulation alters elbow flexor muscle recruitment strategies

Background Transcranial direct current stimulation (tDCS) is known to reliably alter motor cortical excitability in a polarity dependent fashion such that anodal stimulation increases cortical excitability and cathodal stimulation inhibits cortical excitability. However, the effect of tDCS on agonist and antagonist volitional muscle activation is currently not known. Objective This study investigated the effect of motor cortical anodal tDCS on EMG/force relationships of biceps brachii (agonist) and triceps brachii (antagonist) using surface electromyography (EMG). Methods Eighteen neurologically intact adults (9 tDCS and 9 controls) participated in this study. EMG/force relationships were established by having subjects perform submaximal isometric contractions at several force levels (12.5%, 25%, 37.5%, and 50% of maximum). Results Results showed that anodal tDCS significantly affected the EMG/force relationship of the biceps brachii muscle. Specifically, anodal tDCS increased the magnitude of biceps brachii activation at 37.5% and 50% of maximum. Anodal tDCS also resulted in an increase in the peak force and EMG values during maximal contractions as compared to the control condition. EMG analyses of other elbow muscles indicated that the increase in biceps brachii activation after anodal tDCS was not related to alterations in synergistic or antagonistic muscle activity. Conclusions Our results indicate that anodal tDCS significantly affects the voluntary EMG/force relationship of the agonist muscles without altering the coactivation of the antagonistic muscles. The most likely explanation for the observed greater EMG per unit force after anodal tDCS appears to be related to alterations in motor unit recruitment strategies.