Vereschaka I. Activation of the shoulder belt and shoulder muscles of the humans in the two-joint isometric force

Thesis is devoted to study central motor commands (CMCs) arriving to the shoulder belt and shoulder muscles during creation of isometric contractions by hand in conditions of the fixed arm position in horizontal plane. Amplitudes of rectified and averaged EMGs recorded from the muscles were used to estimate intensities of the CMCs. It has been shown that preferred activity of the agonists muscles for a given force direction was accompanied by various extent of co-activation of the antagonist muscle groups. Motor tasks with coincident directions of the forces generated by the muscles of the both joints were realized with simpler programs as compared with non-coincident orientation of the forces. In contrast to isotonic muscle contractions, generation of the isometric forces was accompanied with rather weak dynamic components of EMG recorded in all muscles under study. Distinct dynamic components of EMG have been appeared only after increase in the rate of the force generation up to 0.12-0.25 s. Using the spatial sector diagrams of the EMG intensities were defined the preferred directions of efforts where certain muscles groups were activated predominantly. Form of the sector diagrams was dependent in more extent on orientation of the force vector as compared with position of the hand segments. There was examined a hypothesis that CMCs could be superimposed in the tasks of summation of the force vectors. Experiments with comparison of three force vectors, one of which presented sum of two another ones, demonstrated that EMGs recorded during generation of the summary force in many cases could be quite satisfactorily presented by sum of the corresponding EMGs of constituent vectors. The superposition was observed for both coincident and non-coincident directions of the constituent vectors.