Enhancing Healthy Synergies in Stroke Rehabilitation with an Upper-Limb Exoskeleton: An Analysis with Geometric Configuration Similarity

Abstract

Post-stroke upper-limb impairments often involve compensatory movement patterns. Robotic exoskeletons can deliver task-oriented training, but standardized metrics to quantify how they reshape inter-joint coordination are still lacking. In this work, we extended the Geometric Configuration Similarity (GCS) framework from hand to upper-limb kinematic synergies and used it to assess whether exoskeletonassisted rehabilitation enhances healthy synergies in post-stroke patients. Seven patients performed reaching movements with the NEUROExos Shoulder Elbow Module - $\boldsymbol{\alpha}$ (NESM- $\boldsymbol{\alpha})$ exoskeleton providing only gravity compensation (transparent mode) or assistance using impedance control. The first kinematic synergy from each condition was compared with the synergy extracted from a therapist-defined target trajectory, recorded by mobilizing the patient's arm using the exoskeleton in transparent mode. We defined two GCS-based metrics: (i) an average-posture similarity index $\left(\right.$ GCS $\left._{\text {avg }}\right)$ and (ii) the mean value of a time-varying $\boldsymbol{G} \boldsymbol{C} \boldsymbol{S}(\boldsymbol{t})$ computed from synergy-driven joint configurations $\left(\right.$ GCS $\left._{\text {mean }}\right)$. Both metrics significantly increased from Transparent to Assisted movements (GCS ${ }_{\text {mean }}$: $78.47 \pm 3.17$ to $88.93 \pm 4.28, p=0.0007;$ GCS $_{\text {avg }}: 82.58$ [IQR 79.98-89.79] to 95.26 [IQR 93.32-96.94], $p=0.0078)$, indicating that the exoskeleton assistance brought patients' coordination closer to the target synergy. Subject-specific analyses showed that GCS can reveal distinct mechanisms of improvement, such as showing how the exoskeleton assistance enabled otherwise unreachable movements and reduced shoulder abduction-based compensations. These results support GCS as a clinically interpretable tool to quantify the training of healthy synergies and to guide the personalization of exoskeleton-assisted rehabilitation.