Jason I. Pagan1, Kylie K. Harmon1, Ryan M. Girts1, Gabriela Rodriguez1, Rob J. MacLennan2, Jesus Hernandez Sarabia2, Nicholas Coker3, Joshua C. Carr4, Xin Ye5, Jason M. DeFreitas2, Matt S. Stock1. 1University of Central Florida, Orlando, FL. 2Oklahoma State University, Stillwater, OK. 3Springfield College, Springfield, MA. 4Texas Christian University, Fort Worth, TX. 5University of Hartford, West Hartford, CT.

BACKGROUND: Transcranial magnetic stimulation (TMS) is a non-invasive technique that is commonly used to study corticospinal plasticity. However, TMS measurements vary within and between individuals, and there are a variety of unique methodological challenges associated with studying the lower limbs. Perhaps the most prominent of these challenges is that, unlike the upper limbs, the motor representation of the lower limbs is deep and oriented near the midline, rendering it difficult to isolate the behavior of specific muscles. The purpose of this study was to determine the test-retest reliability of corticospinal excitability (CE) of three distinct muscles. METHODS:Twenty-nine participants (mean ± SD age= 23 ± 4 years; 15 males and 14 females) reported to the laboratory for 2 visits that were separated by ≥ 48 hours but ≤ 1 week. Isometric strength testing of the right knee extensors was performed with an isokinetic dynamometer (knee joint angle = 110°) while bipolar surface electromyographic (EMG) signals from the rectus femoris, vastus lateralis, and tibialis anterior were recorded. A double-cone coil was used to deliver single TMS pulses over the portion of the motor cortex which demonstrated the greatest rectus femoris EMG peak-to-peak amplitude (i.e., hotspot) while participants performed an isometric knee extension corresponding to 10% of their maximal voluntary contraction torque. Twenty pulses were delivered at a stimulator output corresponding to 120% of active motor threshold (AMT). Motor evoked potential peak-to-peak amplitude was used to quantify CE. Statistical analyses included paired samples t-tests, Cohen’s d effect sizes, intraclass correlation coefficients (SEM), and the standard error of measurement (SEM). RESULTS: Excellent test-retest reliability was observed for AMT (p = .699, d = .075, ICC = .919, SEM = 5.2%) and vastus lateralis CE (p = .365, d = .185, ICC = .973, SEM = 16.2%). In contrast, CE reliability was lower for the rectus femoris (p = .093, d = .336, ICC = .841, SEM = 41.6%) and tibialis anterior (p = .059, d = .381, ICC = .896, SEM = 48.0%). The high ICCs may be attributed to substantial variance between participants. CONCLUSION: Reliability of lower limb TMS measures may be muscle specific. Rectus femoris AMT and vastus lateralis CE may be useful for assessing neuromuscular adaptations to exercise and rehabilitation interventions.

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