RESPONSE OF ARM FLEXOR MUSCLES TO MAGNETIC AND ELECTRICAL BRAIN-STIMULATION DURING SHORTENING AND LENGTHENING TASKS IN MAN

1. The responses of the brachioradialis and biceps brachii muscles to non invasive magnetic and electrical stimulation of the human motor cortex have been investigated. during performance of different tasks. 2. Both muscles were simultaneously active during elbow flexor isometric torque or forearm flexion lifting a weight (shortening contraction), or extension breaking the fall of the weight (lengthening contraction). The forearm extensor triceps brachii muscle was not engaged in any task. By using different weights, comparable levels of EMG activity were obtained in the same muscle across tasks. 3. Both magnetic (7 subjects) and electrical (3 subjects) brain stimulation (at about 1.5 times the motor threshold) produced larger responses during shortening, and smaller responses during lengthening, in the brachioradialis muscle with respect to isometric contractions, in spite of equal background EMG levels. Responses evoked in the biceps brachii by either stimulation mode were smaller during lengthening but not significantly enhanced during shortening. No consistent differences in the task-related modulation of the responses were present between electrical or magnetic stimulations. No significant changes in the evoked responses occurred during passive elbow flexion or extension. 4. In three subjects, the H reflex was evoked in the brachioradialis by stimulation of the radial nerve during performance of the same tasks. The pattern of task-related modulation of the reflex amplitude paralleled that obtained for brain stimulation. 5. The opposite modulation induced by the shortening and lengthening tasks both in magnetically and electrically evoked motor responses, and in the H reflex, suggests that task-related changes in excitability of the cortical neurones play a minor role. The cortical control of the two different tasks might be exerted by an appropriate 'setting' action on the excitability of the spinal circuit, or by a change in the excitability profile of the motoneuronal pool.