Muscle activation patterns are determined, in part, by the mechanical context in which movements are executed. In bilateral movements of the forearm (pronation-supination), the position of the axis of rotation of the lower arm has a profound influence upon the stability of interlimb coordination. These findings suggest that the nature of the coupling between the limbs is strongly determined by musculo-skeletal constraints. In the present study we sought to investigate one facet of this coupling: the modulation of corticospinal input to upper limb motoneurones, that results from the voluntary movement of the contralateral limb. Five participants performed rhythmic pronation-supination movements of the left forearm in time with an auditory metronome. In separate blocks of trials, the external axis of rotation was located either above or below the long axis of the forearm. Surface electromyographic (emg) recordings were obtained from the flexor carpi radialis (FCR), extensor carpi radialis (ECR), pronator teres (PT), and biceps brachii (BB) muscles of both arms. Motor potentials were evoked in the (quiescent) muscles of the right arm by transcranial magnetic stimulation of the left motor cortex. We calculated the degree of association (correlation coefficients) between the r.m.s. emg amplitudes recorded from the muscles of the moving limb, and the amplitudes of the motor evoked potentials obtained from homologous muscles. It was evident that the excitability of the contralateral motor pathways was influenced by the mechanical context in which the movements were performed. When the axis of rotation was above the limb, correlation coefficients for FCR (F(1, 4) = 12.47, p<0.05, f = 0.79) and PT (F(1, 4) = 7.76, p<0.05, f = 0.46) were lower than when the axis of rotation was below the limb. In contrast, the correlation coefficients for BB were higher when the axis of rotation was above the limb, than when the axis was below the limb (F(1, 4) = 6.41, p=0.06, f = 0.47). The correlation coefficients obtained for ECR were not influenced by the mechanical context (F(1, 4) < 1, p>0.20, f = 0.1).
Supported by the Australian Research Council.