title: Calibrating physical activity intensity for hip-worn accelerometry in women age 60 to 91 years: The Women's Health Initiative OPACH Calibration Study.
abstract: OBJECTIVE
  We conducted a laboratory-based calibration study to determine relevant cutpoints for a hip-worn accelerometer among women ≥60 years, considering both type and filtering of counts.
  
  
  METHODS
  Two hundred women wore an ActiGraph GT3X+ accelerometer on their hip while performing eight laboratory-based activities. Oxygen uptake was measured using an Oxycon portable calorimeter. Accelerometer data were analyzed in 15-second epochs for both normal and low frequency extension (LFE) filters. Receiver operating characteristic (ROC) curve analyses were used to calculate cutpoints for sedentary, light (low and high), and moderate to vigorous physical activity (MVPA) using the vertical axis and vector magnitude (VM) counts.
  
  
  RESULTS
  Mean age was 75.5 years (standard deviation 7.7). The Spearman correlation between oxygen uptake and accelerometry ranged from 0.77 to 0.85 for the normal and LFE filters and for both the vertical axis and VM. The area under the ROC curve was generally higher for VM compared to the vertical axis, and higher for cutpoints distinguishing MVPA compared to sedentary and light low activities. The VM better discriminated sedentary from light low activities compared to the vertical axis. The area under the ROC curves were better for the LFE filter compared to the normal filter for the vertical axis counts, but no meaningful differences were found by filter type for VM counts.
  
  
  CONCLUSIONS
  The cutpoints derived for this study among women ≥60 years can be applied to ongoing epidemiologic studies to define a range of physical activity intensities.
authors.affiliation: Department of Epidemiology, Gillings School of Global Public Health, University of North Carolina - Chapel Hill, Chapel Hill, NC, USA.
authors.forename: Kelly R
authors.initials: KR
authors.lastname: Evenson
authors.affiliation: Department of Epidemiology, Gillings School of Global Public Health, University of North Carolina - Chapel Hill, Chapel Hill, NC, USA.
authors.forename: Fang
authors.initials: F
authors.lastname: Wen
authors.affiliation: Department of Biostatistics, Gillings School of Global Public Health, University of North Carolina - Chapel Hill, Chapel Hill, NC, USA ; Carolina Population Center, University of North Carolina - Chapel Hill, Chapel Hill, NC, USA.
authors.forename: Amy H
authors.initials: AH
authors.lastname: Herring
authors.affiliation: Division of Public Health Sciences, Fred Hutchinson Cancer Research Center, Seattle, WA, USA.
authors.forename: Chongzhi
authors.initials: C
authors.lastname: Di
authors.affiliation: Department of Epidemiology and Environmental Health, School of Public Health and Health Professions, University at Buffalo - SUNY, Buffalo, NY, USA.
authors.forename: Michael J
authors.initials: MJ
authors.lastname: LaMonte
authors.affiliation: Division of Public Health Sciences, Fred Hutchinson Cancer Research Center, Seattle, WA, USA.
authors.forename: Lesley Fels
authors.initials: LF
authors.lastname: Tinker
authors.affiliation: Division of Preventive Medicine, Brigham and Women's Hospital, Harvard Medical School, Boston, MA, USA.
authors.forename: I-Min
authors.initials: IM
authors.lastname: Lee
authors.affiliation: Division of Public Health Sciences, Fred Hutchinson Cancer Research Center, Seattle, WA, USA.
authors.forename: Eileen
authors.initials: E
authors.lastname: Rillamas-Sun
authors.affiliation: Division of Epidemiology, Department of Family Medicine and Public Health, University of California, San Diego, La Jolla, CA, USA.
authors.forename: Andrea Z
authors.initials: AZ
authors.lastname: LaCroix
authors.affiliation: University of Illinois at Urbana-Champaign, Champaign, IL, USA.
authors.forename: David M
authors.initials: DM
authors.lastname: Buchner
keywords: Calibration; Indirect calorimetry; Low frequency extension filter; Oxygen uptake; Physical activity; Sedentary behavior; Validity