title: Postural orientation and equilibrium processes associated with increased postural sway in autism spectrum disorder (ASD).
abstract: BACKGROUND
  Increased postural sway has been repeatedly documented in children with autism spectrum disorder (ASD). Characterizing the control processes underlying this deficit, including postural orientation and equilibrium, may provide key insights into neurophysiological mechanisms associated with ASD. Postural orientation refers to children's ability to actively align their trunk and head with respect to their base of support, while postural equilibrium is an active process whereby children coordinate ankle dorsi-/plantar-flexion and hip abduction/adduction movements to stabilize their upper body. Dynamic engagement of each of these control processes is important for maintaining postural stability, though neither postural orientation nor equilibrium has been studied in ASD.
  
  
  METHODS
  Twenty-two children with ASD and 21 age and performance IQ-matched typically developing (TD) controls completed three standing tests. During static stance, participants were instructed to stand as still as possible. During dynamic stances, participants swayed at a comfortable speed and magnitude in either anterior-posterior (AP) or mediolateral (ML) directions. The center of pressure (COP) standard deviation and trajectory length were examined to determine if children with ASD showed increased postural sway. Postural orientation was assessed using a novel virtual time-to-contact (VTC) approach that characterized spatiotemporal dimensions of children's postural sway (i.e., body alignment) relative to their postural limitation boundary, defined as the maximum extent to which each child could sway in each direction. Postural equilibrium was quantified by evaluating the amount of shared or mutual information of COP time series measured along the AP and ML directions.
  
  
  RESULTS
  Consistent with prior studies, children with ASD showed increased postural sway during both static and dynamic stances relative to TD children. In regard to postural orientation processes, children with ASD demonstrated reduced spatial perception of their postural limitation boundary towards target directions and reduced time to correct this error during dynamic postural sways but not during static stance. Regarding postural equilibrium, they showed a compromised ability to decouple ankle dorsi-/plantar-flexion and hip abduction/adduction processes during dynamic stances.
  
  
  CONCLUSIONS
  These results suggest that deficits in both postural orientation and equilibrium processes contribute to reduced postural stability in ASD. Specifically, increased postural sway in ASD appears to reflect patients' impaired perception of their body movement relative to their own postural limitation boundary as well as a reduced ability to decouple distinct ankle and hip movements to align their body during standing. Our findings that deficits in postural orientation and equilibrium are more pronounced during dynamic compared to static stances suggests that the increased demands of everyday activities in which children must dynamically shift their COP involve more severe postural control deficits in ASD relative to static stance conditions that often are studied. Systematic assessment of dynamic postural control processes in ASD may provide important insights into new treatment targets and neurodevelopmental mechanisms.
authors.affiliation: Schiefelbusch Institute for Life Span Studies and Clinical Child Psychology Program, University of Kansas, 1000 Sunnyside Ave., Suite 2004, Lawrence, KS 66045 USA ; Kansas Center for Autism Research and Training (KCART), University of Kansas Medical School, Overland Park, KS 66213 USA.
authors.forename: Zheng
authors.identifier: 0000-0003-2265-0891
authors.initials: Z
authors.lastname: Wang
authors.affiliation: Analytical Imaging and Modeling Center, Children's Medical Center, Dallas, TX 75235 USA.
authors.forename: Rami R
authors.initials: RR
authors.lastname: Hallac
authors.affiliation: Center for Autism and Developmental Disabilities, University of Texas Southwestern Medical Center, Dallas, TX 75390 USA.
authors.forename: Kaitlin C
authors.initials: KC
authors.lastname: Conroy
authors.affiliation: Center for Autism and Developmental Disabilities, University of Texas Southwestern Medical Center, Dallas, TX 75390 USA.
authors.forename: Stormi P
authors.initials: SP
authors.lastname: White
authors.affiliation: Analytical Imaging and Modeling Center, Children's Medical Center, Dallas, TX 75235 USA.
authors.forename: Alex A
authors.initials: AA
authors.lastname: Kane
authors.affiliation: Analytical Imaging and Modeling Center, Children's Medical Center, Dallas, TX 75235 USA.
authors.forename: Amy L
authors.initials: AL
authors.lastname: Collinsworth
authors.affiliation: Department of Psychiatry and Behavioral Neuroscience, University of Cincinnati College of Medicine, Cincinnati, OH 45219 USA.
authors.forename: John A
authors.initials: JA
authors.lastname: Sweeney
authors.affiliation: Schiefelbusch Institute for Life Span Studies and Clinical Child Psychology Program, University of Kansas, 1000 Sunnyside Ave., Suite 2004, Lawrence, KS 66045 USA ; Kansas Center for Autism Research and Training (KCART), University of Kansas Medical School, Overland Park, KS 66213 USA.
authors.forename: Matthew W
authors.initials: MW
authors.lastname: Mosconi
keywords: Autism spectrum disorder; Mutual information; Postural equilibrium; Postural orientation; Static and dynamic stances; Virtual time-to-contact