Loss of Joint Individuation and Abnormal Synergy Post Stroke in Upper Limb Movements

BackgroundStroke often leads to long-term impairments in upper extremity motor function, including muscle weakness, spasticity, and abnormal joint synergies, which hinder independent joint control and daily activities.ObjectiveThis study examined multi-joint motor impairments and characterized abnormal synergy patterns post-stroke using a robotic exoskeleton.MethodsThe exoskeleton independently controlled shoulder, elbow, and wrist joints while measuring responses across all joints during horizontal plane movements. Fifty-three stroke survivors and 24 age-matched controls performed single-joint movements under constrained (fixed joints) and unconstrained (free joints) conditions. Coupled range of torques and range of motion at non-instructed joints were calculated relative to instructed joint movements and summarized in a 3 × 3 matrix.ResultsStroke survivors showed significantly higher coupling torques and motions at non-instructed joints compared to controls, with the greatest impairments in isolating distal movements, particularly in a proximal-to-distal gradient. Abnormal synergy patterns were systematically identified, revealing that stroke survivors exhibited two common patterns for shoulder and elbow tasks, marked by excessive coupling at neighboring joints. For wrist movement tasks, four distinct patterns emerged, involving excessive coupling at both shoulder and elbow joints.ConclusionThese findings demonstrate characteristic impairments in joint individuation and synergy following a stroke, providing a framework to understand motor deficits and guide rehabilitation strategies aimed at restoring joint-specific control.