Ipsilateral corticomotor responses are confined to the homologous muscle following cross-education of muscular strength

Cross-education of strength occurs when strength-training one limb increases the strength of the untrained limb and is restricted to the untrained homologous muscle. Cortical circuits located ipsilateral to the trained limb might be involved. We used transcranial magnetic stimulation (TMS) to determine the corticomotor responses from the untrained homologous (biceps brachii) and non-homologous (flexor carpi radialis) muscle following strength-training of the right elbow flexors. Motor evoked potentials were recorded from the untrained left biceps brachii and flexor carpi radialis during a submaximal contraction from 20 individuals (10 women, 10 men, aged 18-35 years; training group; n = 10 and control group; n = 10) before and after 3-weeks of strength-training the right biceps brachii at 80% of 1-repetition maximum (1-RM). Recruitment-curves for corticomotor excitability and inhibition of the untrained homologous and non-homologous muscle were constructed and assessed by examining the area under the recruitment curve (AURC). Strength-training increased strength of the trained elbow flexors (29%), resulting in a 18% increase in contralateral strength of the untrained elbow flexors (P 0.05). These findings show that the cross-education of muscular strength is spatially distributed; however, the neural adaptations are confined to the motor pathway ipsilateral to the untrained homologous agonist.