Identification of reflex and intrinsic mechanical properties in stroke and spinal cord injury

We used system identification techniques to characterize the mechanical abnormalities associated with spasticity and to identify the contribution of intrinsic and reflex stiffness to these abnormalities. Modulation of intrinsic and reflex stiffness of elbow flexor muscles in hemiparetic stroke, spinal cord injured (SCI), and normal subjects was studied by applying perturbations to the elbow at different initial joint angles with subjects´ muscles relaxed. We found that intrinsic stiffness was larger in SCI than normal subjects, whereas it was within normal range in stroke arms. Reflex stiffness was similar in spastic SCI and stroke subjects and was significantly larger than in normal subjects. However, the reflex relative contribution to overall joint stiffness was smaller in SCI than stroke group due to greater increase in intrinsic stiffness in SCI subjects. Both reflex and intrinsic stiffness were strongly position dependent; they both increased with increasing elbow extension but reflex stiffness declined at full extension. The mechanical properties of the contralateral arms in stroke subjects were not normal; reflex stiffness was larger and intrinsic stiffness was lower. These findings demonstrate that although increased reflex stiffness is a common mechanical abnormality in SCI and stroke the major mechanical abnormalities of the spastic elbow are enhanced intrinsic contribution in the SCI subjects and increased reflex contribution in stroke subjects. Moreover, the findings suggest that the contralateral side of stroke subjects is also affected and cannot be used as reliable control.