Three-dimensional reaching tasks: effect of reaching height and width on upper limb kinematics and muscle activity

In previous studies, upper limb coordination was usually analyzed during two-dimensional (2D) arm movements. Based on joint kinematics and muscle activity, it has been demonstrated that the shoulder joint controls the multi-joint movement. This study focused on three-dimensional (3D) reaching tasks and examined if the coordination strategies previously described in 2D can be transferred to 3D movements and if reaching to different locations in space has an effect on kinematic and upper limb muscle strategies. Ten healthy subjects reached to nine different targets in 3D space placed at arm length. Kinematic data of the shoulder and elbow and electrical activity of 10 upper limb muscles were registered. Differences in kinematics and EMG were compared between different reaching conditions. Activity of shoulder muscles increased earlier than elbow muscles inducing shoulder elevation prior to elbow extension. Reaching at different widths only influenced shoulder kinematics, whereas reaching at different heights influenced both shoulder and elbow joints. Modulation of reaching height induced an immediate adaptation of elbow flexion followed by an adaptation of shoulder elevation. As previously described in 2D, the shoulder joint leads the movement during 3D reaching tasks. Changing the 3D nature of a reaching task influenced the interaction between shoulder and elbow joint, with reaching height primarily affecting the elbow coordination strategy.