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

High-frequency repetitive transcranial magnetic stimulation (HF-rTMS) remains a promising strategy for neurorehabilitation. The stimulation intensity (SI) influences the aftereffects observed. Here, we examined whether single sessions of a 15 Hz rTMS protocol, administered at suprathreshold SI, can be safely administered to able-bodied (AB) individuals. Six right-handed men were included in this pilot study. HF-rTMS was delivered over the right M1, in 10 trains of 75 biphasic stimuli at 15 Hz, at 105-120% of the individual resting motor threshold (RMT). To assess safety, electromyography (EMG) was monitored to control for signs of spread of excitation and brief EMG burst (BEB) after stimulation. Additionally, TMS side effects questionnaires and the numeric rating scale (NRS) were administered during each session. We assessed corticospinal excitability (CSE) and motor performance changes with measures of resting (rMEP) and active (aMEP) motor evoked potential and grip strength and box and blocks test (BBT) scores, respectively. Overall, the sessions were tolerated and feasible without any pain development. However, EMG analysis during 15 Hz rTMS administration 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 15 Hz rTMS SI and rMEP increase. No significant changes were highlighted for motor performance measures. Although feasible and tolerable by the AB individuals tested, the results demonstrate that when administered at suprathreshold intensities (≥ 105% RMT) the 15 Hz rTMS protocol reveals signs of persistent excitation, suggesting that safety precautions and close monitoring of participants should be performed when testing such combinations of high-intensity and high-frequency stimulation protocols. The results also give insight into the nonlinear existent relationship between the SI and HF-rTMS effects on CSE.NEW & NOTEWORTHY The results of this pilot study show the effects of a therapeutically promising 15 Hz repetitive transcranial magnetic stimulation (rTMS) protocol, administered at different suprathreshold intensities in able-bodied individuals. Although tolerable and feasible with a neuromodulatory potential, 15 Hz rTMS might result in persistent excitability that needs to be closely monitored if administered at suprathreshold stimulation intensity. These results reaffirm the importance of feasibility studies, especially in translational animal-to-human research.