Postural Control Mechanisms Underlying Reduced Stability in Autism Spectrum Disorder (ASD)

Background: Postural stability has been repeatedly shown to be disrupted in autism spectrum disorder (ASD). Prior studies have quantified the amount of sway during standing, but examining postural stability at the individual level requires analysis of sway relative to each individual’s postural limitation boundary, or the maximum extent to which they are able to lean in each direction without losing their balance. In the present study, we examined postural sway relative to each individual’s postural limitation boundary in individuals with ASD and healthy controls using a novel virtual time-to-contact (VTC) approach.

Objectives: To examine center of pressure (COP) variability, the shared information of the COP time series for anterior-posterior (COP_AP) and mediolateral (COP_ML) directions, and spatiotemporal VTC in individuals with ASD and healthy controls during static and dynamic standing postures.

Methods: Twenty-two children with ASD (12.72 ± 3.64 yrs) and 21 healthy controls matched on age, gender and IQ completed tests of static and dynamic stances. Prior to testing, participants were instructed to stand with their feet shoulder width apart on a force platform. Their foot position was traced on the platform so that their starting position at the beginning of each trial was consistent. Participants’ postural limitation boundary was determined by having them lean their body in each of four different directions (anterior, posterior, left and right) as far as they could without losing balance, and then fitting an ellipse to the COP maxima for each direction. COP measurements were derived from the force platform.

During static stance trials, participants were instructed to stand as still as possible. During dynamic stance trials, participants were instructed to continuously sway their body either front-back or left-right at a comfortable speed and magnitude. Participants completed three 30-sec trials for each stance. Each participant’s VTC was derived by comparing the spatiotemporal relation between their postural sway relative to their own postural limitation boundary during each trial. The amount of shared information of COP_AP and COP_ML also was quantified.

Results: Individuals with ASD showed increased COP standard deviation and trajectory length during all conditions. Individuals with ASD showed increased shared information suggesting less independence of COP_AP and COP_ML sway mechanisms, especially during dynamic stances. In static stance, individuals with ASD were able to compensate for their increased postural sway variability by reducing their temporal VTC duration. In contrast, they were not able to show the same type of compensation during dynamic stances.

Conclusions: Individuals with ASD showed increased postural sway variability relative to controls, but the quality of this deficit varied across different postural conditions. In static stance, ASD patients’ ability to compensate for internal perturbations was relatively intact. During dynamic stances, individuals with ASD failed to acquire spatial VTC information for postural stability. Their ability to utilize temporal VTC adjustment to compensate for increased sway also was compromised during dynamic but not static stance. These findings indicate that ASD patients are impaired when attempting to use spatiotemporal information to correct their sway during naturalistic stances similar to those they use during everyday activities.