Neck rigidity in Parkinson's disease patients is related to incomplete suppression of reflexive head stabilization.

Muscle rigidity in PD (Parkinson's disease) patients represents an involuntary increase in muscle tone that stands out upon passive rotation of a joint. The pathophysiology of rigidity is still not well understood. We measured head-trunk torque in PD patients and normal controls during transient passive head rotations by means of servomotors under the instruction to the subjects to relax the neck muscles. We observed that rotation onset was followed by an initial rapid rise in resistive torque, similarly in both subject groups. It then leveled off or declined in controls. With PD patients, in contrast, the rise continued roughly proportional to head eccentricity almost until the end of the rotation. These observations led us to the hypothesis that the initial rise in torque represents reflexive head stabilization that normal subjects in the course of the rotational stimulus are able to suppress, whereas PD patients are less effective in doing so. The hypothesis was implemented into a dynamic control model of active and passive head rotation. Model simulations successfully reproduced the torque responses of normal subjects and PD patients in the present and previous studies.