Reflex and intrinsic mechanical changes in spastic limbs of MS patients

Reflex and intrinsic properties of the human knee joint-muscles were studied in vivo in seven spastic multiple sclerosis (MS) patients and ten normal subjects. A powerful joint driving device was used to perturb the knee joint in precisely controlled patterns to manifest both reflex and intrinsic properties. A digital signal processor controlled the device and used a small-amplitude and band-limited white-noise sequence as the command trajectory. The subject was asked to maintain a steady level of background muscle contraction during each perturbation trial. A nonlinear delay differential equation model was used to characterize the reflex and intrinsic properties of the knee joint in terms of the dynamic stretch reflex gains for stretching and shortening muscles, static stretch reflex gain, joint stiffness, viscosity, and leg inertia. Our findings were that spastic MS patients showed significantly lower joint viscosity and static stretch reflex gain than their counterparts in normal controls. On the other hand, the dynamic stretch reflex gains and joint stiffness of the spastic MS patients were higher than those of normal controls.