Whole-body pointing as superposition of two multi-joint components

In this study, we address the interactions between the organization of a dynamically stable reaching movement involving the legs and trunk and keeping balance in the field of gravity. This issue is intimately linked to the phenomena of trade-offs between performance-stabilizing synergies at different levels of a control hierarchy, which have not been explored in joint configuration spaces. Young, healthy participants performed natural pointing, movement of the shoulder only and of the endpoint with respect to the shoulder only to match spatial targets, and the two component movements combined without an explicit target for the pointer. Motion kinematics was quantified in a two-dimensional action space. The framework of the uncontrolled manifold hypothesis was used to quantify variance components in the joint configuration space and in the two-component space affecting and not affecting the task variable and their relative magnitudes (synergy index). The values of the synergy index confirmed stabilization of the task-specific effector coordinate across analyses with only minor signs of a trade-off between the two levels of the assumed hierarchy. Both variance components were smaller for the shoulder trajectory without a change in the synergy index. We discuss the differences in the control of movements and force production and possible differences between laboratory tasks and more natural tasks within the framework of the neural control with spatial referent coordinates. The implied postural constraints contributed to lower inter-trial variance in the joint configuration space quantified with respect to the shoulder without affecting the synergic control of the explicit task.