Transcranial direct current stimulation of the SMA enhances intra-limb anticipatory postural adjustments

Anticipatory postural adjustments (APAs) represent a fundamental component of the voluntary movement. APAs circuitry is not fully understood, but several studies suggest that the supplementary motor area (SMA) may be deeply involved. To confirm its engagement in APA generation, and verify the possibility to interfere with it, we applied trascranial direct current stimulation (tDCs) over the SMA, looking for effects on the postural control of the upper-limb during a brisk index-finger flexion. Indeed, such a movement is preceded by inhibitory APAs carved in Biceps and Anterior Deltoid EMG, and by an excitatory APA on Triceps (Caronni & Cavallari, Exp Brain Res 2009). Subjects (n=12) performed 3 sessions of 30 index-finger flexions, while sitting on a chair with the right upper-arm along the body, elbow at 90° and hand prone. Before each session, subjects underwent tDCs (1mA, 20min) with CATHODAL, ANODAL or SHAM configuration. During ANODAL stimulation the average APAs in Biceps and Triceps were greater than during SHAM stimulation (+26.5% and +66%, respectively) and during CATHODAL stimulation (+20.5% and +63.4%). ANOVA p<0.02 Tukey p<0.05, for both muscles. Instead, the APA recorded on Anterior Deltoid was unchanged in all sessions. These results confirm the SMA involvement in APA generation and suggest that tDCs, which has been shown to be a valuable tool for treating different neurological conditions, may be as well employed to modulate APAs during postural rehabilitation.