Agnès Roby-Brami: The potential of upper-limb exoskeletons for adaptation and learning at the joint level

The disruption of flexible upper-limb coordination and abnormal synergies contribute to the upper-limb impairment following stroke. The possibility to modify the patterns of shoulder-elbow coordination in stroke survivors remains a central issue for neurorehabilitation (Jarrassé et al 2014). In this context, we developed a controller (KSC kinematic synergy controller) for the 4 DoF ABLE exoskeleton (CEA-LIST) which uses viscous force field acting at joint level. This controller is prone to modify the redundant shoulder-elbow coordination during a 3D aiming task in healthy subjects (Crocher et al. 2011) and can directly but partially correct excessive abduction in stroke patients (Crocher et al. 2012). Usual studies of motor adaptation to force fields use planar devices and focus on the kinematics of the end-effector. We used similar adaptation protocol in 30 healthy subjects to investigate the effect of prolonged exercise with the KSC controller on 3D aiming movements (Proietti et al 2017). Post-effects were studied after the removal of the force fields (wash-out and follow up). The kinematics of movements were quantified at both end-point and joint levels and inter-joint coordination through PCA. Adaptation (characterized by a decrease of the perturbation due to KSC followed by a post effect in the opposite direction) occurred only on 21% of the motions. In contrast, the modified coordination persisted in most of the cases (55%) during and after the exposition to KSC, and even during the follow-up. There were important inter-individual differences with small variability within subjects. Temporal and spatial generalizations were observed. These preliminary results suggest that certain features of upper limb movement can be corrected using force fields in an exoskeleton. Further studies are needed to test the generalization to free unconstrained movements in healthy subjects and to evaluate the clinical potential of different parameters of the KSC controller (providing either error-correction or error augmentation at joint level) for the rehabilitation of stroke patients.