Neuromuscular properties of different spastic human joints vary systematically

We quantified the mechanical abnormalities of the spastic wrist in chronic stroke survivors, and determined whether these findings were representative of those recorded at the elbow and ankle joints. System identification techniques were used to characterize the mechanical abnormalities of these joints and to identify the contribution of intrinsic and reflex stiffness to these abnormalities. Modulation of intrinsic and reflex stiffness with the joint angle was studied by applying PRBS perturbations to the joints at different joint angles over the range of motion. Age-matched healthy subjects were used as control. We found reflex stiffness gain (G_R) was significantly larger in the stroke than healthy control wrist at all positions. G_R was also strongly position dependent. It increased continuously with the wrist angle in both group; however, the slope of changes vs. joint angle was significantly larger in the stroke than control group. In contrast, intrinsic stiffness gain (K) was within normal range at most positions and was significantly larger in the stroke than the control joint only at the extremes range of motion. Modulation of K with the wrist angle was similar in both group; it was flat between mid-flexion and mid-extension but sharply increased at the end of the range of motion. Our results demonstrate that although intrinsic and reflex stiffness both increased significantly in the spastic wrist, the changes were dominated by enhancements in reflex stiffness. These findings also demonstrate that abnormalities in reflex stiffness at the wrist were similar to those observed at the elbow, and abnormalities in intrinsic stiffness were similar to those observed at the ankle. These findings will help us better understand the origins of mechanical abnormalities associated with stroke-induced spasticity.