title: Relationship of multiscale entropy to task difficulty and sway velocity in healthy young adults.
abstract: OBJECTIVE
  Multiscale entropy (MSE) is a nonlinear measure of postural control that quantifies how complex the postural sway is by assigning a complexity index to the center of pressure (COP) oscillations. While complexity has been shown to be task dependent, the relationship between sway complexity and level of task challenge is currently unclear. This study tested whether MSE can detect short-term changes in postural control in response to increased standing balance task difficulty in healthy young adults and compared this response to that of a traditional measure of postural steadiness, root mean square of velocity (VRMS).
  
  
  METHODS
  COP data from 20 s of quiet stance were analyzed when 30 healthy young adults stood on the following surfaces: on floor and foam with eyes open and closed and on the compliant side of a Both Sides Up (BOSU) ball with eyes open. Complexity index (CompI) was derived from MSE curves.
  
  
  RESULTS
  Repeated measures analysis of variance across standing conditions showed a statistically significant effect of condition (p < 0.001) in both the anterior-posterior and medio-lateral directions for both CompI and VRMS. In the medio-lateral direction there was a gradual increase in CompI and VRMS with increased standing challenge. In the anterior-posterior direction, VRMS showed a gradual increase whereas CompI showed significant differences between the BOSU and all other conditions. CompI was moderately and significantly correlated with VRMS.
  
  
  CONCLUSIONS
  Both nonlinear and traditional measures of postural control were sensitive to the task and increased with increasing difficulty of standing balance tasks in healthy young adults.
authors.affiliation: a Department of Physical Therapy , New York University , New York , NY , USA and.
authors.forename: Anat V
authors.initials: AV
authors.lastname: Lubetzky
authors.affiliation: b Department of Rehabilitation Medicine , University of Washington , Seattle , WA , USA.
authors.forename: Robert
authors.initials: R
authors.lastname: Price
authors.affiliation: b Department of Rehabilitation Medicine , University of Washington , Seattle , WA , USA.
authors.forename: Marcia A
authors.initials: MA
authors.lastname: Ciol
authors.affiliation: b Department of Rehabilitation Medicine , University of Washington , Seattle , WA , USA.
authors.forename: Valerie E
authors.initials: VE
authors.lastname: Kelly
authors.affiliation: b Department of Rehabilitation Medicine , University of Washington , Seattle , WA , USA.
authors.forename: Sarah W
authors.initials: SW
authors.lastname: McCoy
mesh_terms: D000328:Adult; D019277:Entropy; D056228:Feedback, Sensory / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D017711:Nonlinear Dynamics; D004856:Postural Balance / Q000502:physiology*; D011187:Posture; D018709:Statistics, Nonparametric; D055815:Young Adult
keywords: Center of pressure; complexity index; nonlinear analysis; postural control