Constraints on disordered locomotion A dynamical systems perspective on spastic cerebral palsy

Abstract

Upper motor neuron disorders (UMN) result in abnormal movement patterns that are due to spasticity, weakness, cocontraction of muscles, inappropriate timing of muscle activations in relation to maximal mechanical advantage, and changes in the mechanical properties of muscles and connective tissues. Since many of the patterns observed in gait are reminiscent of immature patterns in non-disabled toddlers, prolonged abnormal patterns are often viewed as a failure of normal neural maturation. Since the 1960's the emphasis of therapeutic interventions has been to ‘normalize’ the movement pattern through a variety of neurotherapeutic techniques. The efforts have met with limited success. Recently there have been a number of publications that have emphasized the possibility that the observed patterns may develop or be functional adaptations to a neural system that is not ‘normal’, and that the observed movement patterns in pathological populations may be normal and optimal for that system (Fetters, 1991; Holt, 1993; Latash and Anson, in press; Winter et al., 1990). Some authors have emphasized that alternative patterns develop and are facilitated by redundancy in the motor system (Latash and Anson, in press). Our emphasis has been to pursue the notion that pattern development and functional adaptations in locomotion are driven by the underlying dynamics of the task and the dynamic resources available to the individual. We use a model developed within the dynamical systems framework, the force-driven, pendulum-spring model of locomotion, to parse out the energy forms that are needed for continued oscillations, and relate the energy forms to the dynamics that are available to a disordered system, specifically, spastic cerebral palsy (CP). We then attempt to relate the available dynamics to the observed gait patterns. In this way, we seek to understand ordered and disordered gaits as alternative functional solutions to the dynamical requirements for continued cycling of a periodically forced oscillator. The implications of the theoretical model and experimental findings are discussed with respect to therapeutic intervention, and the dynamical systems approach.