Spatial mapping of posture-dependent resistance to passive displacement of the hypertonic arm post-stroke

Abstract

Muscles in the post-stroke arm commonly demonstrate abnormal reflexes that result in increased position- and velocity-dependent resistance to movement. We sought to develop a reliable way to quantify mechanical consequences of abnormal neuromuscular mechanisms throughout the reachable workspace in the hemiparetic arm post-stroke. Survivors of hemiparetic stroke (HS) and neurologically-intact (NI) control subjects were instructed to relax as a robotic device repositioned the hand of their (hemiparetic) arm between several testing locations sampling the arms passive range of motion. During transitions, the robot induced motions at either the shoulder or elbow joint at speeds ranging from very slow (6 {degrees}/s) to fast (90 {degrees}/s). The robot held the hand at the testing location for at least 20 seconds after each transition. We recorded and analyzed hand force and electromyographic activities from selected muscles spanning the shoulder and elbow during and after transitions. Hand forces and electromyographic activities were small at all speeds and sample times in NI control subjects, but varied systematically by transport speed during and shortly after movement in the HS subjects. Velocity-dependent resistance to stretch diminished within two seconds after movement ceased in the hemiparetic arms. Hand forces and EMGs changed very little from 2 seconds after the movement ended onward, exhibiting dependence on limb posture but no systematic dependence on movement speed or direction. Although each HS subject displayed a unique field of hand forces and EMG responses across the workspace after movement ceased, the magnitude of steady-state hand forces was generally greater near the outer boundaries of the workspace than in the center of the workspace for the HS group but not the NI group. In the HS group, electromyographic activities exhibited abnormalities consistent with stroke-related decreases in the stretch reflex thresholds post-stroke. These observations were consistent across repeated testing days. Implications of the findings are discussed.