title: Errors in postural preparation lead to increased choice reaction times for step initiation in older adults.
abstract: BACKGROUND
  This study asked whether older adults were more likely than younger adults to err in the initial direction of their anticipatory postural adjustment (APA) prior to a step (indicating a motor program error), whether initial motor program errors accounted for reaction time differences for step initiation, and whether initial motor program errors were linked to inhibitory failure.
  
  
  METHODS
  In a stepping task with choice reaction time and simple reaction time conditions, we measured forces under the feet to quantify APA onset and step latency and we used body kinematics to quantify forward movement of center of mass and length of first step.
  
  
  RESULTS
  Trials with APA errors were almost three times as common for older adults as for younger adults, and they were nine times more likely in choice reaction time trials than in simple reaction time trials. In trials with APA errors, step latency was delayed, correlation between APA onset and step latency was diminished, and forward motion of the center of mass prior to the step was increased. Participants with more APA errors tended to have worse Stroop interference scores, regardless of age.
  
  
  CONCLUSIONS
  The results support the hypothesis that findings of slow choice reaction time step initiation in older adults are attributable to inclusion of trials with incorrect initial motor preparation and that these errors are caused by deficits in response inhibition. By extension, the results also suggest that mixing of trials with correct and incorrect initial motor preparation might explain apparent choice reaction time slowing with age in upper limb tasks.
authors.affiliation: Department of Neurology, Oregon Health & Science University, Mail Code NSI, 505NW 185th Avenue, Beaverton, OR 97006, USA. rajal.cohen@gmail.com
authors.forename: Rajal G
authors.initials: RG
authors.lastname: Cohen
authors.forename: John G
authors.initials: JG
authors.lastname: Nutt
authors.forename: Fay B
authors.initials: FB
authors.lastname: Horak
mesh_terms: D000058:Accidental Falls; D000328:Adult; D000367:Age Factors; D000368:Aged; D002755:Choice Behavior / Q000502:physiology*; D003071:Cognition; D006801:Humans; D008875:Middle Aged; D011187:Posture / Q000502:physiology*; D011930:Reaction Time / Q000502:physiology*