Quantitative analysis of the velocity related pathophysiology of spasticity and rigidity in the elbow flexors

Objective: To quantify velocity dependent and position related properties of increased muscle tone measured during a constant velocity stretch. Methods: Elbow flexors were vertically stretched under four different velocities (40, 80, 120, and 160degrees/s) through a 75degrees range of motion in 12 patients with hemiparesis, 16 with parkinsonism, and 12 normal controls. From reactive torque measurement, a linear second order model was adopted to dissociate velocity dependent viscous and velocity independent elastic components. The averaged speed dependent reflex torque (ASRT)-defined as the deviation of measured torque from baseline torque was used to quantify the viscous component of hypertonia. Velocity sensitivity of ASRT (VASRT) and, segmented ASRT (SASRT), derived from the slope of the regression line among ASRT velocity plots and from segmentations of reactive torque, respectively, were used to differentiate the increased muscle tone of spasticity and rigidity. Results: ASRT and VASRT were significantly higher in both spasticity and rigidity than in normal controls. SASRT analysis showed three different position related patterns among spasticity, rigidity, and normal groups: spasticity showed progressively increasing muscle tension relative to position; rigidity showed increased (relative to the norm) but constant muscle tone over the entire stretch range; the normal control group showed a consistently low reactive torque over the entire range. Conclusions: Velocity dependence analysis indicates that rigidity and spasticify have approximately equal velocity dependent properties. For differentiating these two types of hypertonia, position-dependent properties my be employed.