Toward a New Understanding of Spasticity

Work on this project is devoted to developing an deep understanding of the roles of spasticity and motor control in limiting the function of children with cerebral palsy and traumatic brain injury.

Application of Feldman’s Equilibrium Point Hypothesis to Spasticity
Felman’s basic concept is that the CNS provides a virtual trajectory of joint motion, representing space and timing, with the actual movement dynamics being produced by the interaction of limb inertia, limb load, and position feedback.

We are employing the Wartenberg Pendulum Knee test to experimentally measure the movement of the relaxed knee joint operating under a torque produced by gravity. Knee trajectories of individuals without spasticity appear to be pendular, while those of individuals with spasticity have a very altered, but consistent trajectory that reflects the addition of unplanned muscle energy. Previous attempts at employing the Equilibrium Point Hypothesis to explain this alteration of the trajectory have been unsuccessful, unless the stiffness and damping of the joint are arbitrarily varied (no physiological evidence) or the virtual trajectory is manipulated in a complicated manner.

The Equilibrium Point Hypothesis in Isometric Movements
To extend our understanding of how the Equilibrium Point Hypothesis could explain spasticity, we have developed an additional sent of experiments that use isometric movements (i.e. force input without substantial movement). We have studied the response of the metacarpalphalangial joint of the index finger to rapid displacement. Subjects apply isometric forces to the Haptic Master at levels ranging from 0 to 20 Newtons. The Haptic Master displaces the tip of the finger approximately 15 mm is at time less than 50 msec. This short time is less that than the monosynaptic reflex loop, this preventing any reflex from occurring. In these experiments, since there is no intended movement, we hypothesize that the virtual trajectory remains at constant value for each applied isometric force. We have conducted experiments with non-disabled subjects and have found that the Equilibrium Point Hypothesis is supported by our results. Subsequent experiments with children with spasticity will allow us to assess the joint impedances of those with spasticity prior to any possible reflex. Extension of the displacement time beyond the reflex loop time will provide further understanding of how possible reflexes behave in spasticity. Additional studies of other joints are planned.

The Coordination of Multiple Joints
Since functional movement is comprised of coordinated movements of multiple joints, we are developing models of two and three joint systems. Movement data are captured for reaching movements of subjects and include both the Cartesian position of the hand, as well as the angles of each joint. To date, our two segment model has been evaluated in a pilot study, with remarkable success. We are able to input simple virtual trajectories, with the driving torques generated as functions of the differences between the actual and virtual trajectories, and the velocities of the differences between the actual and virtual trajectories of each joint. Further work will use our model to explain the movement patterns of children with spasticity.

Stimulation of the Otolith Organs as a Window into Spasticity
Pioneering work by James Fee at the A.I. DuPont Hospital for Children indicates that repetitive vertical accelerations may result in a temporary reduction in spasticity. While not yet confirmed, it is thought that accelerations sensed by the Otolith organs may result in changes to inhibition seen at the spinal level. We are expanding this work to explore how such vestibular intervention can allow a better understanding of the neural basis of spasticity, and the development of possible clinical interventions based on this idea. We will use the models we have developed to assess the manner in which vestibular stimulation alters the behavior of single and multiple joints.

We have developed a mathematical model that extends the Equilibrium Hypothesis to include relative damping, which allows a very simple inclusion of extra torques that can be represented by a simple function of the velocity of the joint minus the velocity of the virtual trajectory. The virtual trajectory is computed by fitting an exponential curve to the equilibrium points of the subjects’ knee trajectories. Preliminary experiments have shown that knee joint trajectories of spastic subjects can be reproduced by our model with considerable accuracy. Further experiments are underway to support the idea that spasticity is in part a disorder in which the virtual trajectory specified by the CNS is distorted.