title: Complexity and variability of the center of pressure time series during quiet standing in patients with knee osteoarthritis.
abstract: BACKGROUND
  While several studies have investigated the traditional linear measures in patients with knee osteoarthritis, no study has yet reported the non-linear structure of postural sway in these patients.
  
  
  METHODS
  We used two non-linear methods, recurrence quantification analysis (percent of determinism-%DET) and central tendency measure, to respectively investigate differences in the complexity and variability of sway dynamics between two groups of knee osteoarthritis patients (n=27) and healthy controls (n=27) under different conditions of postural and cognitive tasks. The experimental conditions included standing on (1) rigid surface with open eyes; (2) rigid surface with closed eyes; (3) foam surface with open eyes; and (4) foam surface with closed eyes. All these conditions were performed isolated (single-task) and while performing concurrent cognitive task (dual-task).
  
  
  RESULTS
  The results showed greater %DET and lesser central tendency measure (both in mediolateral direction) in patients compared with healthy subjects. Moreover, in both groups, the %DET increased and central tendency measure decreased with increasing postural difficulty while %DET decreased and central tendency measure increased when moving from single- to dual-task conditions.
  
  
  CONCLUSIONS
  The complexity loss was observed in patients compared with healthy controls. The observed increase in the variability coupled with a decrease in the complexity could be explained by the exploratory behavior of postural control system to gather information during difficult postural conditions relative to the easy ones. Moreover, the observed increase in the complexity coupled with the decrease in the amount of variability may enhance the flow of information to facilitate the perceptual control of standing balance during dual-task conditions.
authors.affiliation: Musculoskeletal Rehabilitation Research Center, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran. Electronic address: honegahban@yahoo.com.
authors.forename: Hossein
authors.initials: H
authors.lastname: Negahban
authors.affiliation: Biomechanics Lab., Rehabilitation Research Center, Faculty of Rehabilitation, Department of Rehabilitation Basic Sciences, Iran University of Medical Sciences, Tehran, Iran. Electronic address: sanjarima@alum.sharif.edu.
authors.forename: Mohammad Ali
authors.initials: MA
authors.lastname: Sanjari
authors.affiliation: Musculoskeletal Rehabilitation Research Center, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran. Electronic address: karimi_mehrpt@yahoo.com.
authors.forename: Mehrnoosh
authors.initials: M
authors.lastname: Karimi
authors.affiliation: Faculty of Mechanical Engineering, Sharif University of Technology, Tehran, Iran; Department of Information and Industrial Engineering, Hanyang University, Ansan, Gyeonggi-do 426-791, Republic of Korea. Electronic address: parnianpour@sharif.edu.
authors.forename: Mohamad
authors.initials: M
authors.lastname: Parnianpour
mesh_terms: D000368:Aged; D016022:Case-Control Studies; D003071:Cognition / Q000502:physiology; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D020370:Osteoarthritis, Knee / Q000503:physiopathology*; D004856:Postural Balance / Q000502:physiology*; D011187:Posture / Q000502:physiology*; D011312:Pressure
keywords: Central tendency measure; Knee osteoarthritis; Non-linear dynamics; Postural sway; Recurrence quantification analysis