Safety and effects of a therapeutic 15 Hz rTMS protocol administered at different suprathreshold intensities in able-bodied individuals

Background and objectives: High-frequency repetitive transcranial magnetic stimulation (HF-rTMS) remains a promising strategy for neurorehabilitation. The stimulation intensity (SI) influences the after-effects observed. Here, we examined if single sessions of a HF-rTMS protocol, administered at different suprathreshold SI, can be safely administered to able-bodied (AB) individuals. Methods: Six right-handed men were included in this pilot study. HF-rTMS was delivered over the left M1, in 10 trains of 75 biphasic stimuli at 15 Hz, at 105 to 120% of the individual resting motor threshold (RMT). Participants and EMG were monitored to control for signs of spread of excitation and brief EMG burst (BEB) post-stimulation. TMS side-effects questionnaires and the numeric rating scale (NRS) were administered during each session. Additionally, we assessed CSE and motor performance changes with measures of resting (rMEP) and active (aMEP) motor evoked potential and grip strength and the Box and Block test (BBT) scores, respectively. Results: Overall, the sessions were tolerated and feasible without any pain development. EMG analysis during HF-rTMS revealed increased BEB frequency with SI. Statistical models revealed an increase of CSE at rest (rMEP) but not during active muscle contraction (aMEP). No linear relationship was observed between HF-rTMS intensity and rMEP increase. No significant changes were highlighted for motor performance measures. Conclusion: Although feasible and tolerable by the AB tested, the results demonstrate that when administered at suprathreshold SI, 15 Hz-rTMS reveals signs of persistent excitation, suggesting that safety precautions and close monitoring of participants should be performed when testing such stimulation protocols.