Abstract
Understanding abnormal synergy of the upper extremity (UE) in stroke survivors is critical for better identification of motor impairment. Here, we investigated to what extent stroke survivors retain the ability to coordinate multiple joints of the arm during a reaching task. Using an exoskeleton robot, 37 stroke survivors' arm joint angles (θ) and torques (τ) during hand reaching in the horizontal plane was compared to that of 13 healthy controls. Kinematic and kinetic coordination patterns were quantified as variances of the multiple-joint angles and multiple-joint torques across trials, respectively, that were partitioned into task-irrelevant variance (TIV(θ) and TIV(τ)) and task-relevant variance (TRV(θ) and TRV(τ)). TIV(θ) and TRV(θ) (or TIV(τ) and TRV(τ)) led to consistent and inconsistent hand position (or force), respectively. The index of synergy (IS(θ) and IS(τ)) was determined as [Formula: see text] and [Formula: see text] for kinematic and kinetic coordination patterns, respectively. Both kinematic IS(θ) and kinetic IS(τ) in the stroke group were significantly lower than that of the control group, indicating stroke survivors had impaired reaching abilities in utilizing the multiple joints of the UE for successful completion of a reaching task. The reduction of kinematic IS(θ) in the stroke group was mainly attributed to the lower TIV(θ) as compared to the control group, while the reduction of kinetic IS(τ) was mainly due to the higher [Formula: see text] as well as lower TIV(τ). Our results also indicated that stroke may lead to motor deficits in formation of abnormal kinetic synergistic movement of UE, especially during outward movement. The findings in abnormal synergy patterns provides a better understanding of motor impairment, suggesting that impairment-specific treatment could be identified to help improve UE synergies, focusing on outward movements.