Corticomotor excitability changes seen in the resting forearm during contralateral rhythmical movement and force manipulations: A TMS study

The aim of this study was to examine changes in corticomotor excitability to a resting wrist extensor muscle during contralateral rhythmical isotonic and static isometric wrist contractions (flexion/extension) at different loads and positions, using transcranial magnetic stimulation (TMS). TMS-induced motor-evoked potentials (MEPs) were recorded from the relaxed right extensor carpi radialis (ECR) and flexor carpi radialis (FCR) respectively, while the left arm underwent unimanual manipulations. Rhythmical isotonic (0.5 Hz) flexion and extension movements of the left wrist under 3 load conditions (no, low and high force) and a frequency matched passive movement condition were collected, along with isometric flexion/extension contractions in each position (low and high force). TMS was delivered at eight positions (4 in the flexion phase and 4 in the extension phase) during the continuous movement conditions and each of these positions was sampled with isometric contraction. The potentials evoked by TMS in right ECR were potentiated when the left ECR was engaged, independent of position within that phase of contraction or contraction type (isotonic and isometric). Motor cortical excitability of the resting right ECR increased as load demands increased to the left wrist. Passive rhythmical movement did not influence excitability to the resting ECR implying that voluntary motor drive is required. Our findings indicated that the increase in corticomotor drive during both rhythmic isotonic and static isometric contractions of the opposite limb is likely mediated by interhemispheric interactions between cortical motor areas. Improving our understanding of these cortical networks can be useful in future methods to enhance neuroplasticity through neurorehabilitation methods.