Synergies during reach-to-grasp: a comparative study between healthy and C6-C7 quadriplegic subjects

Analysis of functional capacities modifications after spinal cord injuries may have an interest in clinical rehabilitation programs. In this framework, this study is focused on the analysis of motor synergies i.e. how the upper-limb degrees of freedom (DOFS) are organized when considering C6-C7 quadriplegic subjects performing a reaching task. A previous work has shown that sharing patterns between DOFS are preserved. Using the uncontrolled manifold hypothesis (UCM), we investigate the second aspect of motor synergies i.e. how performance is allowed to show both stability against perturbations and flexibility to solve concurrent tasks. This procedure allows to evaluate the stabilization of a specific and selective performance variable through the variability of joint angles. In the present study, the wrist 3D path was considered and the variability of its trajectory is decomposed into two parts: one which does not affect the value of this performance variable (goal equivalent variance or GEV) and one which does (non goal equivalent variance or NGEV). Results showed that quadriplegic subjects present the same variance decomposition than healthy subjects suggesting that, despite the motor deficiency, flexibility is still allowed during movement execution.