Natural history of neuromuscular properties after stroke: a longitudinal study

Background: A rigorous description of the time course of changes in neuromuscular properties after stroke may help us to understand the mechanisms underlying major motor impairments, and it will also help us track the efficacy of rehabilitation treatments. Such time course data have not been collected to date, primarily because of the lack of accurate tools for separating muscular and neural functional measures. Objective: To characterise the time course of changes in elbow neuromuscular properties in hemiparetic stroke survivors over a 1 year period. Methods: Using a system identification technique based on mechanical perturbations of elbow angle, we estimated intrinsic mechanical properties of muscles and stretch reflex parameters at 1, 2, 3, 6 and 12 months after stroke, at different mean elbow joint angles. Results: There were substantial and progressive changes in intrinsic and reflex stiffness in paretic elbow muscles, at all five selected time points, and over a range of mean joint angles. Two temporal patterns of change in these neuromuscular properties were identified. In the first, intrinsic and reflex stiffness increased continuously after the stroke while in the second, intrinsic stiffness decreased continuously over this 12 month interval. Conclusions: These different recovery patterns may reflect the emergence of two simultaneous but potentially opposing mechanisms; brain recovery and changes in peripheral neuromuscular properties. One consequence is that global joint stiffness measures may be misleading as opposing contributions from intrinsic and reflex neuromuscular subcomponents may confound our interpretation of the mean joint stiffness estimates.