Overlap in the cortical representation of hand and forearm muscles as assessed by navigated TMS

Abstract

The representation of upper limb muscles in the motor cortex is complex. It contains areas of excitability that may overlap between muscles. We expected the cortical representations of synergistic muscle pairs to overlap more than those of non-synergistic muscles. To detail this, we used navigated transcranial magnetic stimulation in eight hand and forearm muscles of twenty healthy participants. We transformed the cortical representations of muscles to a template MRI to allow for group analysis. We found that the amount of overlap in cortical representations differed significantly between within-hand and within-forearm muscle combinations. Most synergistic muscle pairs, both within the hand, within the forearm and between them, had a larger overlap than non-synergistic muscle pairs. Our study supports the largely overlapping nature of cortical representations of upper limb muscles. We can particularly underscore that the overlap is elevated in muscles that usually act in a synergistic manner.