Neuromechanical constraints post -stroke: Associations between isometric and functional lower limb tasks

Abstract

Neurological injury following stroke often impairs motor outflow, resulting in multi-joint, multi-dimensional gait disorders. The neuromechanical factors that underlie these gait abnormalities were explored for the goal of improving rehabilitation therapies. Given that movements are derived from joint torques, the first aim quantified limitations in the isometric torque workspace of the paretic hip joint in postures similar to the swing phase of gait. Specifically, abnormal torque coupling within the hip joint and between the hip and knee joints were observed. Multiple linear regression analysis was then used to describe the differential contributions of the abnormal isometric torque behaviors to gait speed and abnormal frontal plane kinematics. These models revealed that abnormal across-joint torque coupling significantly associated with reduced gait speed and increased compensatory pelvic obliquity. To determine if the restrictions to torque output were reflective of limited control options, the degrees of freedom that remain adaptable by the post-stroke nervous system during gait were identified. Externally imposed kinematic constraints to the paretic ankle joint revealed that pelvic obliquity was the only compensatory movement used to maintain toe clearance during swing. Consequently, the hip and knee degrees of freedom restrained by the abnormal torque synergies did not adapt to the imposed constraints, suggesting functional limitations imposed by torque coupling. To understand the neurological foundation of the impaired torque behaviors, muscle synergies (patterns of fixed muscle activations linearly combined to create diverse patterns) were extracted. Similar control structures were observed in the stroke and control groups; however, the elements contained therein were altered. Specifically, stroke subjects were limited in the muscle synergies expressed by the intact motor system, as well as in the manner in which they were recruited. Furthermore, muscle synergies extracted from the isometric task could estimate a large amount of variance in the gait data, thus illustrating a manner by which the abnormal torque behavior may influence gait as indicated by the multiple linear regression analyses. By providing evidence of torque synergies as a major contributor to gait abnormalities post-stroke, therapies may be developed with clear understanding of how those impairments relate to compensatory gait behaviors.