Cortical and Subcortical Neural Interactions Between Trunk and Upper-limb Muscles in Humans

•Upper-limb muscle contractions facilitated corticospinal, but not subcortical neural circuits of the remote trunk muscles.•Trunk muscle contractions facilitated both corticospinal and subcortical neural circuits of the remote upper-limb muscles.•Corticospinal neurons may have complex outputs to multiple muscles which interact within cortical and spinal circuits. Activities of daily living require simultaneous and coordinated activation of trunk and upper-limb segments, which involves complex interlimb interaction within the central nervous system. Although many studies have reported associations between activity of trunk and limb muscles during functional tasks, evidence on cortical and subcortical contributions to trunk-limb neural interactions is still not fully clear. Therefore, the aim of this study was to examine interactions between trunk and upper-limb muscles in the: (i) corticospinal circuits by using motor evoked potential (MEP) elicited through transcranial magnetic stimulation; and (ii) subcortical circuits by using cervicomedullary motor evoked potential (CMEP) elicited through cervicomedullary junction magnetic stimulation. Responses were evoked in the erector spinae (trunk) and flexor carpi radialis (upper-limb) muscles in twelve able-bodied individuals: (1) while participants were relaxed; (2) during trunk muscle contractions while arms were at rest; and (3) during upper-limb muscle contractions while the trunk was at rest. Our results showed that trunk muscle CMEP responses were not affected by upper-limb muscle contractions, while MEP responses were modulated. This indicates that at least the subcortical circuits may not attribute to facilitation of the trunk muscles during upper-limb contractions. On the other hand, in the upper-limb muscles, both CMEP and MEP responses were modulated during trunk contractions. These results indicate that cortical and subcortical mechanisms attributed to facilitation of upper-limb muscles during trunk contractions. In conclusion, our study demonstrated evidence that trunk-limb neural interactions may be attributed to cortical and/or subcortical mechanisms depending on the contracted muscle.