Intra- and inter-session reliability of ipsilateral motor pathway excitability measurements in male and female adults

Examining ipsilateral motor pathways, purportedly reflecting cortico-reticulospinal excitability, has received growing attention in rehabilitation and exercise science. However, the reliability of measures used to assess its excitability, such as ipsilateral motor-evoked potential (iMEP), remains largely unverified. Fifty-three participants (26 males, 27 females) completed two laboratory visits one week apart. Dominant arm contralateral MEP (cMEP) and non-dominant arm iMEP, along with maximal compound muscle action potential (M-max), from both m.biceps brachii were recorded. After determination of preacher curl one-repetition maximum (1-RM), participants performed four sets of five preacher curl repetitions at 30% of 1-RM, with 30 s rest between sets. Transcranial magnetic stimulation was delivered at 100% of maximum stimulator output during the concentric phase of the contraction (110° of flexion). Intraclass correlation coefficients (ICCs) for all variables indicated good-to-excellent intra-session (ICC ≥ 0.88) and inter-session reliability (ICC ≥ 0.80). Notably, the ICCs for iMEP and cMEP amplitudes demonstrated excellent reliability for both intra-session (ICC ≥ 0.98) and inter-session (ICC ≥ 0.91) measurements. iMEP presence was reliable (ICC = 0.87) and had similar variability as iMEP amplitude. However, ICC for iMEP/cMEP ratio (ICAR) was 0.83 and there was substantial inter-session variability (coefficient of variation = 56.8%; relative standard error of measurement = 31.7%). Sex did not influence the reliability of the measures. iMEP and cMEP measured during preacher curl exhibited excellent intra-session and good-to-excellent inter-session reliability in both sexes. Thus, this test is a useful tool to evaluate cortico-reticulospinal and corticospinal excitability in humans, although caution is advised if using ICAR.