Task-specificity Of Corticospinal Excitability: The Influence Of Contractile Properties

Abstract

Transcranial magnetic stimulation (TMS) is increasingly applied to investigate neurophysiological function during exercise interventions. Although often assessed in testing modalities that are different than the intervention, neurophysiological function may be task-specific. PURPOSE: To compare neurophysiological function between an isometric squat (SQT) and knee extension (KE). METHODS: Twenty-two young adults (2 women, 20 right-footed, age: 25±5yrs, BMI: 25.9±3.1, VO2: 46.2±8.8 ml·kg-1·min-1) performed isometric SQT (N=7) or KE (N=15), with hip-, knee- and ankle-joints fixed at 90° as part of a larger study, exposing participants to operational stress for a 5-day period. Lower extremity strength and muscle activity (RMS) were recorded during maximum voluntary contractions (MVCs), using a linear force transducer and electromyography (EMG) sensors placed over the vastus lateralis, respectively. Motor-evoked-potential (MEP)-based stimulus response curves (SRC) were derived using TMS and a double cone coil placed over the dominant motor cortex leg hotspot during intermittent isometric contractions at 15% MVC. Forty stimuli were applied for two rounds of SRC, with stimulator output (SO) ranging from 5-100% in 5% increments and random order. Since neurophysiological function did not differ across days, grand averaged responses were compared using multivariate ANOVAs or Mann-Whitney U. RESULTS: Greater force and muscle activity were evident for KE compared to SQT (Force: 1303.9±407.0 vs. 812.8±189.5N, p=0.01; EMGRMS: 0.04±0.003 vs. 0.06±0.01, p=0.03). KE corticospinal excitability was twice as high compared to SQT (1.4±0.7 vs. 0.7±0.4mV, p=0.04), but similar when normalized to muscle activity (KE: 25.3±13.9 vs. SQT: 17.4±10.7 mV·EMGRMS-1, p=0.21). No difference was evident in SRCV50 and SRCSLOPE (54±9 vs. 62±14%, p=0.12 and 5.0±1.9 vs. 4.2±1.1, p=0.35). CONCLUSION: Contractile and corticospinal excitability appear to be task-specific, discouraging the assessment of neurophysiological function in modalities that are different from the intervention. Contractile function further seems to influence corticospinal excitability, which may reflect the underlying differences in neuromechanics between the two movements. Supported by the Department of Defense W81XWH-16-PHTBIRP-CR3A.