[Corticospinal influence on the hand distal muscles under weight load]

Baseline and synchronous evoked EMG activity were studied in wrist muscles of human subjects under conditions of transcranial magnetic stimulation of the motor cortex during the performance of different motor tasks. During recording, a subject had to hold a loaded handle with the thumb and index finger. When a subject increased the grip force (intentionally or under conditions of increasing handle load), the synchronous EMG response increased proportionally to the muscle activity. If the subject moved the hand with the handle up and down with the amplitude of 10 cm and frequency of 0.5-1.0 Hz, the grip force changed in accordance with the expected inertia load. The EMG response in the abductor pollicis brevis increased to a larger degree than the baseline activity. In case of a grip force increase in response to a sudden unexpected rise of the inertia load, the evoked EMG response increased to a lesser degree than the muscle baseline activity. The results suggest that the increase in the evoked response during voluntary periodical movement of the handle is related to the anticipatory grip force adjustment. The involvement of the motor cortex in the anticipatory activity of distal muscles is discussed.