Enhancing corticospinal facilitation from sEMG-triggered paired intermittent theta burst stimulation and neuromuscular electrical stimulation in young and older adults

Background Pairing intermittent theta burst stimulation (iTBS) with neuromuscular electrical stimulation (NMES) can increase corticospinal excitability (CSE) of a relaxed muscle. Whether initiating stimulation with surface electromyographic activity (sEMG-triggered) during an intentional holding task enhances this effect, and whether such facilitation is preserved with ageing, remains unknown. Objective To compare the effects of different paired and unpaired stimulation protocols on corticospinal facilitation in young and older adults, and to examine acute effects on hand function in older adults. Methods Young and older adults received iTBS alone, sEMG-triggered iTBS + NMES, and sEMG-triggered sham iTBS + NMES targeting the abductor pollicis brevis (APB) of the dominant hand on separate occasions. A subset of young adults also received non-triggered iTBS + NMES. Stimulation was delivered while participants held a small cylinder between the thumb and index finger. Motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation were recorded at baseline and up to 30 min post-stimulation. In older adults, hand function was assessed using maximum voluntary contraction (MVC), Nine Hole Peg Test (NHPT), and Box and Block Test (BBT). Results sEMG-triggered iTBS + NMES produced stronger MEP facilitation than iTBS alone or sham iTBS + NMES in both age groups. sEMG-triggered and non-triggered iTBS + NMES produced comparable facilitation in young adults. No group-level improvements were observed in MVC, NHPT, or BBT. Conclusions sEMG-triggered iTBS + NMES reliably enhanced short-term corticospinal facilitation in young and older adults. The findings suggest that intentional motor involvement during stimulation, rather than brief muscle activation per se, contributes to the observed facilitation.