Evolution of reflexive and muscular mechanical properties in stroke-induced spasticity

We studied the natural history of reflexive and mechanical properties in hemiparetic spastic stroke subjects. System identification techniques were used to characterize the mechanical abnormalities of the elbow joint and to identify the contribution of intrinsic and reflex stiffness to these abnormalities over one year post-injury. Modulation of intrinsic and reflex stiffness of the elbow joint was studied by applying PRBS perturbations to the elbow at different joint angles at five intervals following stroke. We found that both reflex and intrinsic stiffness were larger in the stroke than in the control arms. They were also strongly position dependent; they both increased with increasing elbow extension but reflex stiffness declined at full extension in some subjects. This position dependency was consistent during stroke recovery. Both intrinsic and reflex abnormally increased over time after stroke. These findings help better understanding of the origins of mechanical abnormalities associated with spasticity and document the time course of these abnormalities during stroke recovery.