Inducing ipsilateral motor‐evoked potentials in the biceps brachii muscle in healthy humans

To assess reticulospinal tract excitability, high‐intensity transcranial magnetic stimulation (TMS) has been used to elicit ipsilateral motor‐evoked potentials (iMEPs). However, there is no consensus on robust and valid methods for use in human studies. The present study proposes a standardized method for eliciting and analysing iMEPs in the biceps brachii. Twenty‐four healthy young adults participated in this study. Electromyography (EMG) electrodes recorded contralateral MEPs (cMEPs) from the right and iMEPs from the left biceps brachii. A dynamic preacher curl task was used with ~15% of the subject's one‐repetition maximum load. The protocol included maximal compound action potential (M‐max) determination of the right biceps brachii muscle, TMS hotspot determination, and four sets of five repetitions where 100% stimulator output was delivered at an elbow angle of 110° of flexion. We normalized cMEP amplitude by M‐max (% M‐max) and iMEP by cMEP amplitude ratio (ICAR). Clear iMEPs above background EMG were observed in 21 subjects (88%, ICAR = .31 ± .19). Good‐to‐excellent agreement (intraclass correlation coefficient [ICC] = .795–1.000) and low bias (.01–.08 mV and .60–1.11 ms) were demonstrated when comparing two different analysis methods (i.e. fixed time‐window vs. manual onset detection) to determine the cMEP and iMEP amplitude and latency, respectively. Most subjects demonstrated clear iMEPs above background EMG triggered at a pre‐determined joint angle during a light‐load dynamic preacher curl exercise. Similar results were obtained when comparing a single‐trial manual identification of iMEP and a semi‐automated time‐window data analysis approach.