Effects of Single-Session High-Definition Transcranial Alternating Current Stimulation (HD-tACS) Combined with Robot-Assisted Hand Exercises on Corticospinal Excitability in Chronic Stroke.
Meng H, Houston M, et al. • American journal of physical medicine & rehabilitation • 2026
HD-tACS combined with robot-assisted hand exercises may enhance corticospinal tract excitability compared with robot-assisted hand exercises alone after a single session in both healthy and stroke subjects.
Key Findings
Results
HD-tACS significantly increased FDS MEP amplitudes in healthy adults by 15.3%.
Twelve healthy adults participated in the study.
Stimulation was delivered over the primary motor cortex (M1) FDS hotspot at 2mA and 21 Hz for 30 minutes.
The increase in FDS MEP amplitudes was statistically significant in the healthy group.
All subjects performed robot-assisted grasp-and-open hand exercises during both active and sham stimulation.
A randomized crossover design was used, with each participant receiving both active and sham conditions.
Results
In chronic stroke subjects, FDS MEP amplitudes increased by approximately 22.3% after HD-tACS, but this change did not reach statistical significance.
Seven individuals with chronic stroke participated in the study.
Stimulation was applied ipsilesionally over the M1 FDS hotspot.
The ~22.3% increase was numerically larger than the 15.3% seen in healthy adults but was not statistically significant, likely due to the small sample size.
The preliminary nature of the study and small stroke sample size are acknowledged as limiting factors.
Results
When MEP amplitudes were normalized as percentage change, HD-tACS significantly increased FDS MEP amplitudes in both healthy and stroke groups compared with sham stimulation.
This finding held across both the healthy group (n=12) and the stroke group (n=7).
Normalization as percentage change revealed a significant effect that was not observed in the raw amplitude analysis for the stroke group.
This suggests that relative changes in corticospinal excitability were meaningfully different between active and sham conditions in both populations.
Results
No significant modulation of extensor digitorum communis (EDC) MEP amplitudes was observed in either healthy or stroke groups.
EDC MEP amplitudes were measured alongside FDS MEP amplitudes in both groups.
Neither the healthy group nor the stroke group showed statistically significant changes in EDC MEP amplitudes following HD-tACS.
This indicates that the excitability changes were muscle-specific, affecting the flexor but not the extensor muscle of the hand.
Methods
The study used a randomized crossover design with HD-tACS delivered at 2mA and 21 Hz over the M1 FDS hotspot for 30 minutes during concurrent robot-assisted hand exercises.
HD-tACS was applied using high-definition electrodes targeting the primary motor cortex FDS hotspot.
Stimulation parameters were 2mA intensity and 21 Hz frequency.
Duration of each stimulation session was 30 minutes.
Robot-assisted grasp-and-open hand exercises were performed concurrently during both active and sham stimulation sessions.
The crossover design ensured each participant served as their own control.
What This Means
This research suggests that combining a type of non-invasive brain stimulation called high-definition transcranial alternating current stimulation (HD-tACS) with robot-assisted hand exercises can boost the excitability of the neural pathways connecting the brain to hand muscles. In the study, healthy adults showed a statistically significant 15% increase in the brain's ability to activate a key hand-gripping muscle (flexor digitorum superficialis) after a single 30-minute session of active stimulation compared to a sham (fake) stimulation session. People with chronic stroke showed an even larger increase of about 22%, though this did not reach statistical significance, likely because of the very small number of stroke participants.
When the researchers accounted for individual differences by looking at relative (percentage) changes rather than raw measurements, both healthy people and stroke survivors showed significantly greater increases in brain-to-muscle signaling with active HD-tACS compared to sham. Importantly, the stimulation appeared to selectively affect the gripping muscle rather than the hand-opening muscle, suggesting the effect may be specific to the targeted brain area. No safety concerns were reported.
This research matters because restoring hand function is one of the most important goals in stroke rehabilitation, and current therapies often have limited effectiveness. This preliminary study suggests that pairing HD-tACS with robotic hand therapy could be a promising combination to amplify the brain's responsiveness during rehabilitation. However, the authors stress that these findings are preliminary due to the very small sample sizes, and larger studies are needed before any firm conclusions can be drawn about the usefulness of this approach for stroke recovery.
Meng H, Houston M, Rao A, Rho J, Francisco G, Li S. (2026). Effects of Single-Session High-Definition Transcranial Alternating Current Stimulation (HD-tACS) Combined with Robot-Assisted Hand Exercises on Corticospinal Excitability in Chronic Stroke.. American journal of physical medicine & rehabilitation. https://doi.org/10.1097/PHM.0000000000003108