Movements of the Forearm and Shoulder Performed against the Gravitation Force: Target Positioning under Kinesthetic Control

In humans, we tested targeted movements of the forearm and shoulder performed in the vertical direction (in a parallel manner with respect to the sagittal plane). Movements were realized, first, with the possibility for visual control of the coincidence of the angle of the limb link axis vs the vertical and the target angle value (using an optic system and video recording), and, second, in the absence of the above control. Movements including flexion (i.e., movement against the gravitation force) – extension of the limb link with an individually selected convenient velocity were initiated and terminated according to the presentation of permissive sound signals; simultaneously, EMGs were recorded from a few muscles flexing and extending the elbow and shoulder joints. We analyzed systematic errors of target positioning of the forearm and shoulder in movements realized exclusively under kinesthetic control. In the case of isolated flexion of the forearm for a 90 deg target angle, such errors in all members of the examined group (n = 11) were positive. These errors were, on average, 8.1 ± 0.7 deg without loading and reached 11.2 ± 0.9 deg with introduction of a 10 to 30 N additional loading on the forearm. Isolated movements of the shoulder for a 70 deg target angle (performed without loading, with full extension of the forearm and its voluntary fixation) were accompanied by positive errors of 18.3 ± 1.1 deg, on average. Both the movements and positioning were performed due to changes in the levels of activity of the flexor muscles, with minimum involvement of the antagonists. The nonlinear properties of the receptor apparatus responsible for the formation of a kinesthetic estimate of the joint angle (first of all, of muscle spindles) are a fundamental reason for positive errors of target positioning of the limb links realized under kinesthetic control in the absence of the visual one.