Coordination of Activity of the Shoulder Belt and Shoulder Muscles in Humans During Bimanual Synchronous Two-Joint Movements

We examined coordination of EMG activity of flexors and extensors of the shoulder and elbow joints during realization of synchronous bimanual ramp-and-hold movements within a horizontal plane similar to those in the course of rowing. A tested subject moved handles of two levers rotating on the vertical axes and traced a trajectory of the command signal presented on a monitor. Test movements included displacements of the lever handles "to the chest" (TCh) and "from the chest" (FCh) (durations 0.4, 1.0, or 2.0 sec), separated by a phase of fixation of the extremum position after the first phase (duration 6.0 sec); the amplitude of turns of the levers was equal to 30 deg. The movements were realized under conditions of application of external loadings (33-28 and 19-15 N) having FCh or TCh directions. During realization of the movements, EMG activity of the shoulder belt and shoulder muscles was recorded bilaterally; patterns of rectified and integrated EMGs were considered correlates of the central motor commands (CMCs) coming to the corresponding muscles. Analysis of EMGs recorded from 12 muscles (six for each limb) demonstrated that the activity of the latter is coordinated in a rather complex mode. The peculiarities of functional interactions (synergies) of muscles during coordinated displacements of the shoulder links and forearms under conditions of realization of the above-mentioned test movements are described. Significant effects of the velocity factor on the dynamic components of CMCs addressed to the examined muscles were observed. In each muscle, statistically significant differences between the EMG amplitudes during realization of the TCh and FCh movements were found. Differences between the dynamic and static EMG components under the action of external loadings of opposite directions were also significant. It was found that, in the course of realization of the movements, CMCs coming to the elbow flexors were more variable (flexible) compared with CMCs directed toward shoulder extensors. With increase in the duration of active phases of the test movement, the amplitude of these phases (D1 and D2) in EMGs of all examined muscles decreased at both directions of the external loading. The dependence of stationary EMG levels on the direction of action of this loading has been also demonstrated. Static components of EMG activity of all muscles were considerably greater at extending loadings compared to those at flexing ones.