Co-contraction of Agonist and Antagonist Muscles Increased Corticospinal Excitability and Muscular Activity in Arm Muscles

Abstract

Background: This research aimed to explore how the co-contraction (MCo) of agonist and antagonist arm muscles influences the excitability of the corticospinal pathway. Methods: A randomized, controlled, within-subject crossover design was employed. A total of seventeen healthy male volunteers, aged 20 - 27 years, were enrolled in the study following a comprehensive eligibility screening process. Each participant completed four experimental sessions under controlled conditions. The protocol of this study included measuring the motor-evoked potential (MEP)'s area under the recruitment curve (AURC) and the pre-stimulus electromyography (EMG) in four states (rest, agonist activity, antagonist activity, and MCo of agonist and antagonist muscles) and at 100%, 120%, 140%, 160%, and 180% of the resting motor threshold (RMT), with 10 pulses per intensity (50 pulses total). Sessions were randomized and counterbalanced with ≥ 24-hour intervals to minimize carryover effects. Results: The study revealed that muscle activation states, particularly agonist activation and MCo, significantly increased pre-stimulus root mean square (RMS) amplitudes (P < 0.001). The MCo caused a significant increase in the total AURC compared to rest states [d = 4.15, P < 0.001 for biceps brachii (BB) and d = 2.47, P < 0.001 for triceps brachii (TB)]. Furthermore, antagonist muscle activity also increased the AURC of MEP relative to the resting state in the agonist muscle (d = 0.50, P < 0.001 for BB and d = 0.28, P < 0.001 for TB). Conclusions: The present study's findings indicate that the simultaneous contraction of two muscles can lead to greater increases in MEP's and the pre-stimulus EMG in the arm muscles. Furthermore, this research offers valuable methodological insights (standardized protocol development, enhanced experimental design, and upper-limb validation) for future transcranial magnetic stimulation (TMS) studies and lays the groundwork for investigating MCo-induced modulations in the corticospinal pathway.