Time course of changes in neuromuscular properties following stroke.

To characterize the natural history of stroke effects on neuromuscular properties in elbow muscles, we tracked changes in elbow mechanical properties in hemiparetic stroke survivors after stroke. Using a parallel cascade system identification technique, we estimated intrinsic and reflex mechanical properties at 1, 2, 3, 6 and 12 months post stroke. At each time point, we examined neuromuscular changes during variations in mean elbow joint angle. Modulation of intrinsic and reflex properties was assessed using small amplitude pseudorandom positional perturbations at different mean elbow angles, over the entire range of motion. We identified two patterns of stroke effects on neuromuscular properties. In Group 1, intrinsic stiffness increased continuously after the stroke. In Group 2, it decreased continuously over this interval. Analogous results were recorded for reflex stiffness. These different recovery patterns may reflect the simultaneous emergence of two opposing mechanisms; i.e. brain recovery and secondary effects on neuromuscular properties. It follows that the progress of recovery may not reflect a single mechanism, and could depend on which mechanism is dominant at each time point.