Abnormal coordination of upper extremity during target reaching in persons post stroke

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 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${IS}_{\theta }=({TIV}_{\theta }-{TRV}_{\theta })/({TIV}_{\theta }+{TRV}_{\theta })$$\end{document} and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${IS}_{\tau }=({TIV}_{\tau }-{TRV}_{\tau })/({TIV}_{\tau }+{TRV}_{\tau })$$\end{document} 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 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${TRV}_{\tau }$$\end{document} 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.