Task- and intensity-dependent modulation of arm-trunk neural interactions in the corticospinal pathway in humans

Abstract

Most human movements require coordinated activation of multiple muscles. Although many studies reported associations between arm, leg, and trunk muscles during functional tasks, their neural interaction mechanisms still remain unclear. Therefore, the aim of our study was to investigate arm-trunk or arm-leg neural interactions in the corticospinal tract during different arm muscle contractions. Specifically, we examined corticospinal excitability of the erector spinae (ES; trunk extensor), rectus abdominis (RA; trunk flexor), and tibialis anterior (TA; leg) muscles while participants exerted: (1) wrist flexion; and (2) wrist extension isometric contraction at various contraction intensity levels ranging from rest to 50% of maximal voluntary contraction (MVC) effort. Corticospinal excitability was assessed using motor evoked potentials (MEPs) elicited through motor cortex transcranial magnetic stimulation. Results showed that ES MEPs were facilitated even at low contractions (>5% MVC) during wrist flexion and extension, while stronger contractions (>25% MVC) were required to facilitate RA MEPs. The extent of facilitation of ES MEPs depended on contraction intensity of wrist extension, but not flexion. Moreover, TA MEPs were facilitated at low contractions (>5% MVC) during wrist flexion and extension, but contraction intensity dependence was only shown during stronger wrist extension contractions (>25% MVC). In conclusion, trunk extensor corticospinal excitability seems to depend on the task and the intensity of arm contraction, while this is not true for trunk flexor and leg muscles. Our study therefore demonstrated task- and intensity-dependent neural interactions of arm-trunk connections, which may underlie anatomical and/or functional substrates of these muscle pairs.

Significance Statement

Although it is known that most human movements require coordinated activation of multiple muscles, understanding of how they are controlled in the central nervous system still lacks. Our study investigated the characteristics of neural interactions of arm-trunk and arm-leg muscles in the corticospinal tract of human participants using motor evoked potentials elicited by transcranial magnetic stimulation. We showed that arm muscle contractions can facilitate corticospinal excitability of the trunk and leg muscles. Specifically, arm-trunk neural interactions depended on the task and intensity of arm movements. Our findings therefore suggest that corticospinal neurons have complex output patterns to distinct muscles in different body segments, which may depend on the anatomical and/or functional relationship of these muscle pairs.