title: Reliability and validity of the combined heart rate and movement sensor Actiheart.
abstract: Accurate quantification of physical activity energy expenditure is a key part of the effort to understand disorders of energy metabolism. The Actiheart, a combined heart rate (HR) and movement sensor, is designed to assess physical activity in populations.
  
  
  OBJECTIVE
  To examine aspects of Actiheart reliability and validity in mechanical settings and during walking and running.
  
  
  METHODS
  In eight Actiheart units, technical reliability (coefficients of variation, CV) and validity for movement were assessed with sinusoid accelerations (0.1-20 m/s(2)) and for HR by simulated R-wave impulses (25-250 bpm). Agreement between Actiheart and ECG was determined during rest and treadmill locomotion (3.2-12.1 km/h). Walking and running intensity (in J/min/kg) was assessed with indirect calorimetry in 11 men and nine women (26-50 y, 20-29 kg/m(2)) and modelled from movement, HR, and movement + HR by multiple linear regression, adjusting for sex.
  
  
  RESULTS
  Median intrainstrument CV was 0.5 and 0.03% for movement and HR, respectively. Corresponding interinstrument CV values were 5.7 and 0.03% with some evidence of heteroscedasticity for movement. The linear relationship between movement and acceleration was strong (R(2) = 0.99, P < 0.001). Simulated R-waves were detected within 1 bpm from 30 to 250 bpm. The 95% limits of agreement between Actiheart and ECG were -4.2 to 4.3 bpm. Correlations with intensity were generally high (R(2) > 0.84, P < 0.001) but significantly highest when combining HR and movement (SEE < 1 MET).
  
  
  CONCLUSIONS
  The Actiheart is technically reliable and valid. Walking and running intensity may be estimated accurately but further studies are needed to assess validity in other activities and during free-living.
  
  
  BACKGROUND
  The study received financial support from the Wellcome Trust and SB was supported by a scholarship from Unilever, UK.
authors.affiliation: MRC Epidemiology Unit, Institute of Public Health, University of Cambridge, CB1 9NL,UK. soren.brage@mrc-epid.cam.ac.uk
authors.forename: S
authors.initials: S
authors.lastname: Brage
authors.forename: N
authors.initials: N
authors.lastname: Brage
authors.forename: P W
authors.initials: PW
authors.lastname: Franks
authors.forename: U
authors.initials: U
authors.lastname: Ekelund
authors.forename: N J
authors.initials: NJ
authors.lastname: Wareham
mesh_terms: D000328:Adult; D004562:Electrocardiography / Q000379:methods; D004734:Energy Metabolism / Q000502:physiology; D005080:Exercise Test / Q000379:methods; D005260:Female; D006339:Heart Rate / Q000502:physiology*; D006801:Humans; D016014:Linear Models; D008297:Male; D008875:Middle Aged; D018670:Monitoring, Ambulatory / Q000379:methods*; D009043:Motor Activity / Q000502:physiology*; D009068:Movement / Q000502:physiology*; D011237:Predictive Value of Tests; D015203:Reproducibility of Results; D012420:Running / Q000502:physiology; D012680:Sensitivity and Specificity; D016138:Walking / Q000502:physiology