Aspects of joint coordination are preserved during pointing in persons with post-stroke hemiparesis

Understanding the fundamental deficits that underlie abnormal reaching movements in persons with hemiparesis is important to the development of rehabilitation approaches for these persons. The purpose of the present study was to investigate whether, and to what extent, persons with hemiparesis retain the ability to exploit motor abundance to coordinate their joint motions to control the hand's path during reaching--a general feature of joint coordination in age-matched control persons. Eight subjects with mild to moderate right hemiparesis following a stroke and seven age and gender matched control subjects performed pointing movements with each arm individually to targets in the contralateral and ipsilateral workspaces. Ten joint motions and characteristics of hand movement were measured over multiple repetitions. The variance (across trials) of joint combinations was partitioned into two components at every point in the hand's trajectory; joint variance that led to a consistent hand position and joint variance that led to an inconsistent hand position from trial to trial. All participants were able to limit joint configurations that would have led to trial-to-trial variance of the hand's path, while using a range of joint configurations consistent with a stable hand path. These results demonstrate that persons with mild to moderate hemiparesis and no measured sensory or perceptual deficits utilized available motor abundance to stabilize performance variables that were important to successful completion of a reaching task, just as was found for age-matched control persons. A principal components analysis of joint angle variance across several points in the movement cycle demonstrated that the persons with hemiparesis showed different patterns of joint couplings during reaching compared with the age-matched control persons; this was particularly evident for the more impaired individuals. These findings were related to the two features of synergy as proposed by other workers. It is suggested that these results support the idea that, while subjects with mild-to-moderate hemiparesis demonstrate differences in the feature of a synergy related to the specific patterns of joint coupling, they retain the feature of a synergy that is related to error compensation.