Concurrent synchronization and covariation in upper extremity joint angles during a reciprocal aiming task

Abstract The ecological-dynamical approach holds that movement is self-organized, implying a circular causality between the kinematics of the end-effector and the coordination of degrees of freedom (DOF) into synergies. Yet, how this circular causality manifests itself in redundant effector systems remains underexplored, because proper models for multi-DOF systems are lacking. Therefore, we investigated concurrently the coordination dynamics of the end-effector and the upper extremity joint angles –as DOF– during a reciprocal aiming task with varying target widths and distances. Results revealed that different joint angles were differently related to the end-effector. A continuous relative phase analysis showed that the three joint angles with the largest range of motion synchronized with the end-effector, suggesting a strong dynamical coupling between the levels. At the same time, all joint angles exhibited variability that was not shared with the end-effector. A detrended fluctuation analysis revealed signatures of fractality as well as drift in this residual variability at the DOF level, suggesting distinct dynamical regimes. Moreover, the drift observed in the joint angles occurred predominantly along the uncontrolled manifold, leaving the end-effector unaffected. We propose that the DOF of a redundant system are not only coordinated into synergies to stabilize the end-effector, but also to keep the system flexible.