Corticospinal influences on the distal muscles of the hand in conditions of inertial loading.

Electromyographic activity and synchronous discharges in the muscles of the wrist induced by transcranial magnetic stimulation of the motor cortex as the thumb and index finger were used to hold a handle bearing a weight were studied during performance of a number of motor tasks. When the subject increased grip force, for example, in response to increases in the weight of the attached load or by voluntarily squeezing the handle, the evoked response increased proportionally to muscle activity. If the subject moved the hand holding the handle up and down with an amplitude of 10 cm and a frequency of 0.5-1 Hz, grip force changed in accordance with the predicted inertial loading. The muscle response in the adductor pollicis muscle increased to a greater extent than the activity in the muscle. The response to sudden inertial loading consisted of a reflex increase in grip force, the muscle response increasing to a lesser extent than activity in the muscle. This suggests that larger increases in evoked muscle responses on up and down movement of the hand with a load are associated with anticipatory changes in grip force. These results are assessed from the point of view of the involvement of the motor cortex in generating anticipatory changes in muscle activity in the distal muscles.