Differential kinematic control and co-ordination among redundant joints during whole arm reaching movements

Normative upper limb movements are produced by multiple redundant joints. While the reaching task is specified at the endpoint, such task objectives become implicit at the level of joints. A fundamental question is whether planning and control of joints is solely in the service of the endpoint or whether they also include joint trajectories. Using Spearmans correlation and zero crossings, we found differential kinematic signatures of control between shoulder and elbow joints in contrast to the wrist joint. However, the extent of control among joints was substantially diminished compared to the endpoint. Further, when such control measures were compared to the subspaces of inter-trial joint exploration, we found that online control at proximal joints, such as the shoulder and elbow, were significantly associated in regulating the task space, while control at the wrist (distal) joint was associated in regulating joint redundancy in null space. These results suggest that null space is not entirely uncontrolled as per the uncontrolled manifold hypothesis but selectively controlled by some distal joints. Additionally, across different directions, either the shoulder or the elbow contributed dominantly towards the movement of the endpoint while the other joint was lagging and that this strategy reflected in our kinematic measures of online and trajectory control. Taken together, this study shows how the selective implementation of a leading joint in task space and a lagging joint in null space can enable the control of multi-jointed movements and attenuate the problem of joint redundancy.