Examination and quantification of motor evoked potentials in the non-target resting leg

Transcranial magnetic stimulation (TMS) is a research tool that can noninvasively assess efficiency of the nervous system. However, TMS of the lower limb presents considerable challenges. Guidelines to streamline data collection procedures are warranted. As a secondary analysis of previously published data (PMID: 37267320), this study aimed to determine to what extent motor evoked potentials (MEPs) are evident from the resting ipsilateral leg when the vertex is stimulated. A secondary aim was to provide methodological insight into how to identify MEPs in the resting ipsilateral limb when the contralateral limb is active. Thirteen males and 14 females underwent TMS of the lower limb. A double cone coil was used to deliver 20 pulses to the vertex of the scalp at both 120% and 130% active motor threshold (AMT) during submaximal contractions of the dominant leg. MEP amplitude of the rectus femoris (RF), vastus lateralis (VL), biceps femoris (BF), tibialis anterior (TA), and medial gastrocnemius (MG) were examined in the resting ipsilateral leg. MEPs were evaluated as breaking a threshold of ≥ 50 µV, ≥ double baseline electromyography (EMG) amplitude, or ≥ triple baseline EMG amplitude. Our findings indicate that 1) MEPs are apparent in the muscles of the resting ipsilateral leg, detectable from 30 ± 38% (120% AMT; BF) to 96 ± 13% (130% AMT; TA) of the time; 2) 130% AMT results in a greater percentage of MEPs, and 3) MEP identification is most apparent at the threshold of ≥ double baseline EMG. Stimulation at the vertex allowed response detection in multiple lower limb muscles.