Ipsilateral contraction increases map area and decreases motor threshold for contralateral hand muscle.

Ipsilateral motor pathways projecting from primary motor cortex (M1) to the distal upper limb exist in humans, and activation of these ipsilateral pathways also facilitate corticomotor excitability of the contralateral homologous muscle. Handedness and biological sex potentially influence the motor cortical representations, however their influence on ipsilateral representations remain largely unexplored. Here, we investigated the effects of ipsilateral contraction on hand motor representation size and excitability, and examined whether these effects were dependent on handedness and/or biological sex. In 80 individuals (40 right-handed, 40-left handed, equal males and females), we performed transcranial magnetic stimulation (TMS) to obtain motor threshold and motor map area of the contralateral first dorsal interosseus (FDI) muscle of each hemisphere, at (1) rest and during (2) ipsilateral FDI muscle contraction at approximately 10% of maximum voluntary contraction. Results showed that ipsilateral contraction increased motor map area and decreased motor threshold within M1. These changes were independent of handedness and biological sex, and similar between hemispheres. Our findings demonstrate that an ipsilateral hand contraction increases the size and excitability of motor representations within ipsilateral M1, and these increases are irrespective of one's handedness and biological sex.