The general aim of this study was to examine the stiffness and damping characteristics (rate at which oscillations diminish) of limbs of a small population of patients with spastic cerebral palsy (CP) during the Leg Drop Pendulum Test while under general anesthesia and to compare them with patients without CP, both awake and under similar anesthetized conditions. To facilitate a quantitative comparison the authors defined a set of parameters that reflect the stiffness and damping of limbs with spasticity under general anesthesia. To give these parameters more meaning a similar set was constructed for children without disabilities under conditions of normal wakefulness and under anesthesia. The test population of 10 patients with spasticity ranged in age from 4 to 19 years and had a mean age of 10 years 9 months (SD 5 years 10 months); there were four females and six males. The control population consisted of eight patients without disabilities who ranged in age from 7 to 19 years with mean age of 13 years 9 months (SD 3 years 3 months); there were six females and two males. To even out the populations, two limbs from the non-affected sides of two patients with hemiplegic spasticity (both female) were used in the control group. The Leg Drop Pendulum Test was administered to 10 limbs with spastic CP and 10 control limbs under general anesthesia. The two test populations were chosen from patients who were previously scheduled to undergo surgery. Two measures compared limbs of those under anesthetic with those under alert conditions: the damped natural frequency and the damping ratio. The natural frequency is the frequency at which a pendulum oscillates as it comes to rest; the damping ratio is a measure of how quickly the oscillations diminish. In spasticity, active muscle tone disappeared under anesthesia. In both groups the underlying biomechanics of the limb varied considerably. In controls the variation seemed to be growth related.

Data from the pendulum knee test has been used to develop two active models that use external torques to closely match the experimental knee trajectories of subjects with spasticity due to cerebral palsy. These data were collected from three subjects who are identical triplets; two of whom have clinically measurable spasticity. A passive model that accurately describes the knee trajectory of the nonspastic subject serves as the passive plant for two active models. One of these models allows direct application of external torques, and the second provides additional torque as the result of velocity feedback. Both active models and the passive model use separate parameters of stiffness and damping for the agonist and antagonist muscles.

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