Musculo-skeletal constraints on corticospinal input to upper limb motoneurones during coordinated movements

Five participants generated pronation and supination movements of the left forearm, at a frequency of 2.5 Hz, in time with an auditory metronome. On alternating trials, the participants were required to coordinate either maximum pronation or maximum supination with each beat of the metronome. We also manipulated the mechanical context in which the movements were performed. In each block of trials, the external axis of rotation was located either adjacent to the radius or adjacent to the ulna. 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. During these movements, motor potentials were evoked in the quiescent muscles of the right limb by transcranial magnetic stimulation of the left motor cortex. Our results indicated that the amplitude of the EMG activity recorded from the muscles of the left limb was strongly contingent upon the mechanical context in which the movements were performed. In addition, for three of the four muscles examined (FCR, PT and BE). the degree of association between the amplitude of the motor potential evoked in the right limb, and the level of EMG recorded contiguously from the homologous muscle of the left limb, was influenced profoundly by the location of the external axis of rotation. These findings are discussed in light of the mechanisms by which the excitability of the descending pathways from the higher motor centres to the spinal motoneurones may be modulated by changes in peripheral afferent input, arising from alterations in the configuration of the skeletal musculature. (C) 2000 Elsevier Science B.V. All rights reserved.