Effects of transcranial alternating current stimulation on common neural drive to motoneurons during gait in healthy young adults

Abstract

Background: Non-invasive brain stimulation, such as transcranial direct current stimulation, has been of interest as a therapeutic tool to modulate cortical excitability. However, there is little evidence that such stimulation during gait can modulate the corticospinal control of human walking. Since previous reports revealed the rhythmic activation of human motor cortex during gait, it is expected that oscillatory brain stimulation, such as transcranial alternating current stimulation (tACS), canmodulate the cortical control of muscle activities during gait. Recently, coherence analysis of paired surface electromyography (EMG) has been used to quantitatively evaluate the common neural drive from the motor cortex to spinal motoneurons during gait, which can be applied to investigate the corticospinal control of walking. Purpose: This study aimed to investigate the effects of tACS during gait on corticospinal excitability and EMG/EMG coherence in healthy young adults. Methods: Ten healthy young adults participated in this study. All participants received tACS over the leg area of the left primary motor cortex (M1) during treadmill walking for 10minutes. They also performed treadmill walking for 5minutes before and after tACS to measure surface EMG. TACS with a current intensity of 2mA (ranging from −1 to 1mA) was applied for 10min at the frequency that corresponds to the gait cycle during treadmill walking before tACS (0.87± 0.60Hz). To evaluate corticospinal excitability, transcranialmagnetic stimulationwas applied over the leg area of the left M1 before and after tACS. The averages of 10 peak-to-peak amplitudes of motor evoked potentials (MEPs) of fixed intensity (SI 1mV)weremeasured from the right tibialis anterior (TA) and lateral gastrocnemius (LG) muscles. To evaluate the common neural drive to muscles, EMG/EMG coherence was calculated from the following muscle pairs: the proximal and distal ends of the TA muscle (TA-TA) and the LG and medialis gastrocnemius muscles (LG-MG) on the right leg. The areas of coherence curve were calculated in the beta (13–30Hz) and gamma (30–60Hz) frequency bands before and after tACS. MEP amplitudes from the TA and LG muscles before and after tACS were compared. The relationships between MEP amplitude changes and changes in the area of beta and gamma-band EMG/EMG coherence of each muscle were investigated. Results:MEP amplitudes in the TA muscle were significantly increased after tACS, although therewas no significant change in MEP amplitudes in the LG muscle. There was a significant positive correlation between change in MEP amplitude of the TA muscle and change in area of beta-band TA-TA coherence. Conclusion(s): TACS increased corticospinal excitability in the TA muscle. Furthermore, tACS also induced gaitspecific plasticity that modulated the neural drive from M1 to TA motoneurons during gait, corresponding to changes in corticospinal excitability in the TA muscle. Implications: TACS can modulate the common neural drive from the motor cortex to spinal motoneurons during gait and may be an effective tool for improving gait function in patients with central nervous system disorders.