Study of motor cortex excitability in the load holding task

Abstract

Changes in the amplitude of hand muscle responses to a series of ten stimuli applied to the motor cortex has been studied in subjects holding a small load for 3 min. The amplitude of muscle responses and the background activity decreased with time as compared to the initial level. Regression analysis showed that the muscle response amplitude decreased with the number of stimuli to a greater extent than the background activity. Comparison of the parameters of hand muscle activity during load holding in the stable and unstable equilibrium positions showed that the decrease in the muscle response to motor cortex stimulation during load holding in a state of unstable equilibrium is less pronounced than during load holding in a state of stable equilibrium. For the forearm muscles, the muscle response amplitude and background activity decreased less with the number of stimuli, and this decrease did not depend on the stability of the load position. It may be supposed that the evoked responses decreased more rapidly than the background activity because the motor cortex is involved in the adjustment of the level of muscle activity at the stage of the development of the program for the performance of motor tasks and then transfers the control to subcortical structures.