Effect of transcranial direct current stimulation over the primary motor cortex on short-term balance acquisition in healthy individuals.

Background: The primary motor cortex (M1) is central to motor learning processes, and an increasing number of studies have suggested its role in balance control. However, the specific role of M1 in balance control remains unclear, and a causal contribution to improvements in balance ability after balance training has not yet been proven. Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique that modifies brain activity and enables to probe the involvement of M1 in balance learning. The current study aims to explore the role of M1 in the acquisition of balance skills by applying tDCS during short-term perturbation-based balance training.

Methods: Thirty-four participants were randomly assigned to one of three groups receiving balance training combined with tDCS: anodal tDCS, sham tDCS, and a control group without stimulation. All participants were involved in a structured, three-session perturbation-based balance training program completed within one week. During these sessions, the assigned tDCS protocol was applied over the M1 leg area concurrently with the training sessions. We analyzed electroencephalography (EEG) and balance ability during balance perturbations and changes in cortico-spinal excitability at rest. Balance perturbations were applied by translating the standing surface forward and backward. An acoustic signal was given two seconds before perturbation in an additional condition to reveal the effect of perturbation anticipation on reactive cortical responses.

Results: The results indicate that balance ability, measured by center of mass (COM) displacement and joint excursions, was improved in forward perturbation across all groups, with the anodal stimulation group showing the largest improvement relative to baseline performance following training. Moreover, the anodal stimulation group showed a significant decrease in alpha band power following forward perturbations compared to baseline values after training. N1 latency was reduced across all participants in both perturbation directions after training. However, only the anodal stimulation group showed a significant reduction in backward perturbations compared to baseline values. While training did not induce any significant change in short-interval intracortical inhibition (SICI) measured by Transcranial Magnetic Stimulation (TMS), it increased intracortical facilitation (ICF) in the right tibialis anterior (TA) muscle across all groups, independent of the stimulation condition.

Conclusions: This study provides evidence that tDCS over the M1 area facilitates balance skill acquisition, possibly by facilitating motor preparation and execution and improving the efficiency of sensorimotor integration processes, as shown by decreased alpha power and N1 latency. These findings may have implications for the potential use of tDCS in improving balance control.