Separating the control of moving and holding still in post-stroke arm paresis

Abstract

Moving and holding still have been posited to be under separate control regimes for both eye and arm movements. The paretic arm after stroke is notable for abnormalities both at rest and during movement, thus it provides an opportunity to address the relationships between control of reaching, stopping, and stabilizing. In this study, we asked whether independence of these behaviors is preserved in arm paresis. To address this question, we quantified resting postural abnormalities in stroke patients by measuring their biases in force production as they held their hand still in various locations in a planar workspace, and then assessed the influence of these resting force biases on active reaching in the same workspace. We found that patients had marked resting postural force biases at each location, even for the case when arm support was provided. However, these biases did not manifest during any phase of arm-supported planar reaching movements: not during initial acceleration, not to mid-trajectory perturbations, and not during deceleration to a stop. Resting force biases only appeared to switch on after a movement had fully stopped. These findings in stroke suggest that moving and holding still are functionally separable modes of control. At the same time, we found that patients’ resting postural force biases mirrored characteristics of abnormal synergies active during movement: they markedly decreased when arm support was provided; they were higher in more distal positions which require breaking out of flexion; and they scaled with the Fugl-Meyer score for the upper extremity (a measure of intrusion of abnormal synergies during active movement). These three shared features suggest a common mechanism for resting postural biases and abnormal synergies, which appears to be a contradiction given the functional separation of moving and holding still observed in the same patients. To resolve this apparent paradox, we propose a model that predicts a breakdown in the functional separation between reaching and holding still when patients move in the absence of weight support. Thus, the model posits that synergies are the behavioral manifestation of a spillover of posture into movement. Mapping these functional systems onto anatomical and physiological details of lesioned substrate after stroke may provide implementation-level insight into how normal arm motor control is assembled.