title: Validity of treadmill- and track-based individual calibration methods for estimating free-living walking speed and VO2 using the Actigraph accelerometer.
abstract: BACKGROUND
  For many patients clinical prescription of walking will be beneficial to health and accelerometers can be used to monitor their walking intensity, frequency and duration over many days. Walking intensity should include establishment of individual specific accelerometer count, walking speed and energy expenditure (VO2) relationships and this can be achieved using a walking protocol on a treadmill or overground. However, differences in gait mechanics during treadmill compared to overground walking may result in inaccurate estimations of free-living walking speed and VO2. The aims of this study were to compare the validity of track- and treadmill-based calibration methods for estimating free-living level walking speed and VO2 and to explain between-method differences in accuracy of estimation.
  
  
  METHODS
  Fifty healthy adults [32 women and 18 men; mean (SD): 40 (13) years] walked at four pre-determined speeds on an outdoor track and a treadmill, and completed three 1-km self-paced level walks while wearing an Actigraph monitor and a mobile oxygen analyser. Speed- and VO2-to-Actigraph count individual calibration equations were computed for each calibration method. Between-method differences in calibration equation parameters, prediction errors, and relationships of walking speed with VO2 and Actigraph counts were assessed.
  
  
  RESULTS
  The treadmill-calibration equation overestimated free-living walking speed (on average, by 0.7 km · h(-1)) and VO2 (by 4.99 ml · kg(-1) · min(-1)), while the track-calibration equation did not. This was because treadmill walking, from which the calibration equation was derived, produced lower Actigraph counts and higher VO2 for a given walking speed compared to walking on a track. The prediction error associated with the use of the treadmill-calibration method increased with free-living walking speed. This issue was not observed when using the track-calibration method.
  
  
  CONCLUSIONS
  The proposed track-based individual accelerometer calibration method can provide accurate and unbiased estimates of free-living walking speed and VO2 from walking. The treadmill-based calibration produces calibration equations that tend to substantially overestimate both VO2 and speed.
authors.affiliation: School of Exercise and Nutrition Sciences, Deakin University, 221 Burwood Highway, Burwood, VIC 3125 Australia ; Institute for Health & Ageing, Australian Catholic University, Melbourne, VIC 3000 Australia.
authors.forename: Anthony
authors.initials: A
authors.lastname: Barnett
authors.affiliation: School of Exercise and Nutrition Sciences, Deakin University, 221 Burwood Highway, Burwood, VIC 3125 Australia ; School of Public Health, The University of Hong Kong, Pokfulam, Hong Kong SAR.
authors.forename: Ester
authors.initials: E
authors.lastname: Cerin
authors.affiliation: Centre for Physical Activity Studies, Central Queensland University, Rockhampton, QLD 4702 Australia.
authors.forename: Corneel
authors.initials: C
authors.lastname: Vandelanotte
authors.affiliation: Baker IDI Heart and Diabetes Institute, 75 Commercial Road, Melbourne, VIC 3004 Australia.
authors.forename: Aya
authors.initials: A
authors.lastname: Matsumoto
authors.affiliation: School of Human Movement and Nutrition Sciences, The University of Queensland, Brisbane, QLD 4072 Australia.
authors.forename: David
authors.initials: D
authors.lastname: Jenkins
keywords: Energy expenditure; Exercise; GPS; Measurement; Physical activity; Wearable devices