Dynamic Constraint of Spasticity on Muscle Lengthening Velocity During Walking

Introduction The influence of spasticity may be represented as a dynamical constraint on functional performance. Spasticity has been defined as “a motor disorder characterized by a velocity dependent increase in stretch reflexes ...”. This pathologic reflex behavior is associated with increased resistance to motion at increased velocities thereby limiting the velocity and frequency of voluntary movement. The velocity threshold at which spastic myoelectric response is initiated, i.e. spastic threshold velocity (STV), has been identified as a quantifiable correlate with spastic severity. Our previous analyses demonstrated the STV is highly correlated with knee angular velocity during fast walking and Gross Motor Function Measure scores in children with spastic cerebral palsy (CP). However, in that study the knee angular velocities were consistently faster than the measured STV. In fact the linear regression demonstrated a slope that indicated peak knee velocities during walking were twice as fast as the STV. This is potentially because joint velocities rather than muscle behavior was quantified. It was hypothesized that the threshold velocity for spasticity recorded in terms of muscle lengthening velocity must agree with the peak rate of muscle lengthening during fast walking.