Spasticity is a highly complex phenomenon, which has not been defined in precise and quantifiable terms. Although the muscle stretch reflex is thought to play an important role in spasticity generation, the pathophysiologic basis of spasticity is not completely understood. A valid measure of spasticity is one that is chosen within the context of a theory describing the physiological mechanisms underlying the control of posture and movement in healthy individuals and possible impairments of these mechanisms leading to motor disorders. This research’s goal was to determine the role of stretch reflex threshold in the regulation of impaired motor control through the exploration of the following research questions:

1. Can experimental measures be produced leading to the development of a model of spasticity that can be interpreted within the framework of a general theory of motor control?
2. Can the underlying motor control framework provide unique parameters capable of describing both normal and altered/abnormal movement?

3. Can the model be robust enough to explain active as well as passive movement?
The research method outlined in this dissertation takes the novel approach of incorporating the equilibrium point hypothesis into a trajectory-based analysis of pendulum knee motion. The Equilibrium Point Hypothesis (EPH) of motor control theorizes that the central nervous system (CNS) provides a virtual trajectory of joint motion, representing space and time. A forward dynamic model has been developed that can reproduce kinematic data through the using optimized model parameters. The incorporation of the equilibrium point hypothesis in forward model was not only recognition that examination of the entire trajectory of the limb, rather than just the first amplitude of swing, was necessary, but also, that movement can be characterized by the simple extraction of three parameters: a relative damping coefficient, relative stiffness coefficient and mathematical function which can act as an approximation CNS the virtual trajectory described in the EPH.

This research produced a model of passive motion with the ability to produce parameter values that not only differentiate subjects with spasticity from subjects with no clinical signs of spasticity but that can separate subjects based on severity of spastic condition. Research which began as an endeavor to model the passive motion of the pendulum knee test, led to the development of a unifying model of motor control that is robust enough to describe both active and passive movements.