Altered multijoint reflex coordination is indicative of motor impairment level following stroke

Following stroke, individuals often are unable to activate their elbow and shoulder muscles independently. There is growing evidence that altered reflex pathways may contribute to these abnormal patterns of activation or muscle synergies. Most studies investigating reflex function following stroke have examined only individual joints at rest. Thus, the purpose of this study was to quantify multijoint reflex contributions to the stereotyped muscle synergies commonly observed following stroke. We hypothesized that the patterns of reflex coordination mirror the abnormal muscle coactivity patterns previously reported for voluntary activation. 10 chronic stroke and 8 age-matched control subjects participated. Reflexes were elicited by perturbing the arm with a 3 degree of freedom robot while subjects exerted voluntary forces at the elbow and shoulder. The force conditions tested were selected to assess the influence of gravity and the influence of joint torque generation without gravity on reflex coordination. Reflex magnitude was quantified by the average rectified electromyogram, recorded from 8 muscles that span the elbow and shoulder. Patterns of reflex coordination were quantified using independent components analysis. Results show significant reflex coupling between elbow flexor and shoulder abductor-extensor muscles in stroke patients during isolated elbow and shoulder torque generation and during active arm support against gravity. Identified patterns of stretch reflex coordination were consistent with the stereotyped voluntary flexion synergy, suggesting reflex pathways contribute to abnormal muscle coordination following stroke.