Linear spring-damper model of the hypertonic elbow: reliability and validity.

Hypertonia of the elbow joint complex is common in individuals with stroke and is related to the magnitude of the torque response (described by position dependent parameters) during constant velocity extensions. The objective of this study was to model position and velocity dependent characteristics of hypertonia. For both the more and less affected arms in 17 persons with chronic stroke, we measured the torque response to constant velocity stretches (30-180 degrees/s). The responses were combined in position-velocity space and parameters of stiffness, damping, and offset angle were determined from a linear spring-damper model of the torque profile. The model was assessed at three levels: (1) ability to describe the combined torque profile variance, (2) reliability of parameters, and (3) validity of parameters (i.e. clinical correlation). Model parameters fit the torque profiles of both arm groups well and exhibited day-to-day reliability. Stiffness (r=0.820), damping (r=0.816), and 'viscoelasticity' (r=0.909), a composite parameter index developed posthoc, were highly correlated to a manual assessment of hypertonia (Modified Ashworth Scale). Mechanically determined parameters of hypertonia graded along a continuum may have better discriminatory power than manual assessments and thus, may be better at tracking recovery and evaluating interventions.