Deficits in recovery of postural stability after stepping are limb- and phase-specific in children with unilateral cerebral palsy

Background: Recovery of postural stability after taking a step is necessary to maintain functional mobility. Although children with cerebral palsy (CP) are at an increased risk of falling, their ability to recover stability following a step is unknown. Research question: This study aimed to validate a method to assess step recovery in children with unilateral CP and determine if recovery of postural stability differs between limbs and across phases of step recovery. Methods: Children with unilateral CP and matched typically developing controls (5-11 y; n=20/group) stood still on force platforms (quiet stance) and completed a forward step with their more-affected limb. Step recovery was divided into dynamic and static phases using vertical ground reaction force. Postural stability was assessed using center of pressure (COP) sample entropy, distance, and velocity of the more-affected (stepping) and less-affected (trailing) limbs in the anteroposterior and mediolateral directions. Results: Vertical ground reaction force during the static phase of step recovery and during quiet stance were not different for either limb and were strongly related (intraclass correlations >0.97, p<0.001), indicating concurrent validity of the step recovery method. Children with CP exhibited impaired recovery of postural stability after stepping, with deficits most pronounced in the mediolateral direction during the dynamic phase in the stepping limb as exhibited by lower COP sample entropy and higher COP distance and velocity (Cohen's d (d) range = 1.46-2.19, all p < 0.001). Deficits were present in the anteroposterior and mediolateral directions in the trailing limb, but they were limited to COP sample entropy and distance and the magnitude of the group differences was less pronounced (d range = 0.35-1.34). Significance: Step recovery is a valid method to detect deficits in postural stability in children with unilateral CP, and the dynamic phase may be a sensitive target for interventions.