Impaired Scaling of Step Length in Parkinsonian Postural Instability

BackgroundPostural stepping is an important strategy for recovery of balance in response to postural perturbations. It is disrupted by Parkinsons disease (PD) and other conditions. The nature of this disruption remains poorly understood. Understanding the motor control nature of this impairment can guide the development of novel interventions. ObjectivesTo identify the motor control abnormalities responsible for parkinsonian impairment of postural stepping. MethodsWe studied four groups of participants: control, aged, PD, and normal-pressure hydrocephalus (NPH). We performed kinematic analysis of postural stepping by recording participants body motion during a modified version of the clinical pull test, which was performed multiple times with different amounts of pulling forcefulness. ResultsSuccessful postural stepping in the control group was accompanied by linear scaling of their first steps length and latency to the bodys initial motion: more forceful pulls caused larger initial body acceleration, which resulted in longer steps that began earlier. PD patients exhibited reduced scaling of step length: they maintained normal reaction time but took steps that were inadequately short. Reduced step length scaling was present, but less severe, in aged individuals, and was more severe in NPH patients. Aged individuals and PD patients exhibited partial compensation for reduced step length scaling: their step length included a component that was independent of initial body acceleration, which was absent in control and NPH groups. Conclusionsthe impairment of postural stepping caused by PD and related conditions is due to inadequate scaling of movement amplitude and is thus a form of hypokinesia.