title: Lifetime trajectory of physical activity according to energy expenditure and joint force.
abstract: OBJECTIVE
  To develop and demonstrate the feasibility of a method for estimating lifetime hip and knee cumulative joint force using survey data on physical activity, and to construct and describe lifetime trajectories of energy expenditure and hip and knee joint force.
  
  
  METHODS
  Exposure data on lifetime physical activity, including type (occupational, household, and recreation) and dose (frequency, intensity, and duration), were collected from a Canada-wide population study of adults ages ≥45 years. Subjects were ranked in 2 ways: in terms of physical activity-related energy expenditure and in terms of a cumulative peak force index (CPFI) for the hip and knee, which is a measure of lifetime exposure and is a time/joint force product involving years of force and subject bodyweight. A relative joint loading index was calculated as the ratio of joint force (CPFI score) to energy expenditure.
  
  
  RESULTS
  A total of 4,269 subjects completed the baseline measurements. Lifetime energy expenditure and hip and knee CPFI scores were higher for occupational and household activity than sport. The mean lifetime energy expenditure from total physical activity in the study sample was 119.1 metabolic equivalent-hours/week. Women had slightly higher total lifetime energy expenditure and CPFI scores than men. The relative joint loading index was highest for male household and sport activity and lowest for female occupational activity.
  
  
  CONCLUSIONS
  Lifetime cumulative hip/knee joint force trajectories were successfully constructed from survey data and followed expected trends. Comparing energy expenditure with joint force revealed variation by age, sex, and activity type, indicating these measures may help distinguish the numerous benefits of physical activity from possible risks.
authors.affiliation: University of British Columbia, Arthritis Research Centre of Canada, Vancouver, Canada. cratzlaf@interchange.ubc.ca
authors.forename: Charles R
authors.initials: CR
authors.lastname: Ratzlaff
authors.forename: Paul
authors.initials: P
authors.lastname: Doerfling
authors.forename: Gavin
authors.initials: G
authors.lastname: Steininger
authors.forename: Mieke
authors.initials: M
authors.lastname: Koehoorn
authors.forename: Jolanda
authors.initials: J
authors.lastname: Cibere
authors.forename: Matt
authors.initials: M
authors.lastname: Liang
authors.forename: David R
authors.initials: DR
authors.lastname: Wilson
authors.forename: John
authors.initials: J
authors.lastname: Esdaile
authors.forename: Jacek
authors.initials: J
authors.lastname: Kopec
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002170:Canada / Q000453:epidemiology; D015331:Cohort Studies; D004734:Energy Metabolism / Q000502:physiology*; D005240:Feasibility Studies; D005260:Female; D006621:Hip Joint / Q000502:physiology*; D006801:Humans; D007719:Knee Joint / Q000502:physiology*; D008019:Life Style; D008297:Male; D008875:Middle Aged; D009043:Motor Activity / Q000502:physiology*; D016474:Weight-Bearing / Q000502:physiology; D055815:Young Adult