title: Assessing physical activity in persons with knee osteoarthritis using accelerometers: data from the osteoarthritis initiative.
abstract: OBJECTIVE
  Physical activity measured by accelerometers requires basic assumptions to translate the output into meaningful measures. We used accelerometer data from the Osteoarthritis Initiative to investigate in the context of knee osteoarthritis (OA) the following data processing assumptions derived from the general US adult population: nonwear (a period the monitor was removed), based on zero activity exceeding 60 minutes; and a valid day of monitoring, based on wear time evidence exceeding 10 hours.
  
  
  METHODS
  We examined the influence of nonwear thresholds ranging from 20 to 300 minutes of zero activity on mean daily activity minutes (counts>0), mean daily activity counts, and mean daily moderate to vigorous physical activity minutes. The effect of selecting minimums of 8, 10, or 12 wear hours to signify a valid day of monitoring on data retention was examined.
  
  
  RESULTS
  Our sample of 3,536 days of accelerometer data from 519 persons with knee OA showed that mean daily activity minutes increased with the nonwear threshold until stabilizing at 463 minutes per day, corresponding to the 90-minute nonwear threshold. Similar patterns were observed for mean daily activity counts. Varying the nonwear threshold had no effect on mean daily moderate to vigorous physical activity minutes. Choosing the 90-minute nonwear threshold and a minimum of 10 wear hours to constitute a valid day provided 94% data retention.
  
  
  CONCLUSIONS
  Data supported applying the 90-minute nonwear threshold to the knee OA population instead of the 60-minute threshold for the general population, while retaining the 10-hour valid day threshold.
authors.affiliation: Northwestern University Feinberg School of Medicine, Institute for Healthcare Studies, Chicago, Illinois 60611, USA. j-song1@northwestern.edu
authors.forename: Jing
authors.initials: J
authors.lastname: Song
authors.forename: Pamela
authors.initials: P
authors.lastname: Semanik
authors.forename: Leena
authors.initials: L
authors.lastname: Sharma
authors.forename: Rowland W
authors.initials: RW
authors.lastname: Chang
authors.forename: Marc C
authors.initials: MC
authors.lastname: Hochberg
authors.forename: W Jerry
authors.initials: WJ
authors.lastname: Mysiw
authors.forename: Joan M
authors.initials: JM
authors.lastname: Bathon
authors.forename: Charles B
authors.initials: CB
authors.lastname: Eaton
authors.forename: Rebecca
authors.initials: R
authors.lastname: Jackson
authors.forename: C Kent
authors.initials: CK
authors.lastname: Kwoh
authors.forename: Michael
authors.initials: M
authors.lastname: Nevitt
authors.forename: Dorothy D
authors.initials: DD
authors.lastname: Dunlop
mesh_terms: D000054:Acceleration; D000368:Aged; D018450:Disease Progression; D005080:Exercise Test / Q000295:instrumentation*; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D008297:Male; D008875:Middle Aged; D009043:Motor Activity / Q000502:physiology*; D020370:Osteoarthritis, Knee / Q000175:diagnosis / Q000503:physiopathology*; D011446:Prospective Studies; D015203:Reproducibility of Results; D012720:Severity of Illness Index
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