The Effects of Transcranial Direct Current Stimulation During Extended Reality Exercises for Cortical, Neuromuscular, and Clinical Recovery of Stroke Survivors

<b>Background:</b> Rehabilitation methods that include anodal transcranial direct current stimulation (<i>a</i>tDCS) and extended reality (XR) exercises have been used to enhance neural networks and improve functional performance in stroke patients, but the neuromuscular and neurophysiological mechanisms underlying these improvements are not fully understood. The purpose of this study was to examine the effects of <i>a</i>tDCS during XR rehabilitation exercises on cortical, neuromuscular, and clinical outcomes of stroke survivors. <b>Methods:</b> Nineteen chronic stroke survivors were placed into either a transcranial direct current stimulation (tDCS) or a <i>Sham</i> group, without significant (<i>p</i> > 0.73) differences in the baseline levels of disability between groups. The tDCS group received active <i>a</i>tDCS and the <i>Sham</i> group received sham <i>a</i>tDCS applied on the ipsilesional primary motor cortex (M1) while performing a 10-session XR rehabilitation program. Surface electromyography (EMG) activity was recorded from deltoid and rectus femoris of the paretic limb without and with the application of active/sham <i>a</i>tDCS on the M1. Shoulder abduction and hip flexion active maximum joint range of motion (ROM_max), electroencephalography (EEG)-derived brain symmetry index (BSI) and functional/clinical tests were assessed before and after the rehabilitation program. <b>Results:</b> EMG activity was ~ 31% greater during hip flexion of the paretic limb with the application of active <i>a</i>tDCS than without <i>a</i>tDCS (<i>p</i>=0.04). Paretic hip flexion ROM_max increased by ~ 26%, BSI decreased by ~ 72% (indicating greater brain symmetry) and timed up and go (TUG) functional test improved by ~ 11% from before to after the rehabilitation program for the tDCS group only (<i>p</i> < 0.05). No other significant differences (<i>p</i> > 0.05) were observed. <b>Conclusion:</b> It seems that the application of active <i>a</i>tDCS targeted the ipsilesional M1 representation of the quadriceps, which potentiated muscle activation in the paretic rectus femoris during XR exercises and resulted in greater motor recovery in hip flexion movements. The EEG-derived BSI results also indicate that <i>a</i>tDCS was effective in reorganizing the ipsilesional hemisphere brain activity after stroke.