title: Risk of severe knee and hip osteoarthritis in relation to level of physical exercise: a prospective cohort study of long-distance skiers in Sweden.
abstract: BACKGROUND
  To complete long-distance ski races, regular physical exercise is required. This includes not only cross-country skiing but also endurance exercise during the snow-free seasons. The aim of this study was to determine whether the level of physical exercise is associated with future risk of severe osteoarthritis independent of previous diseases and injuries.
  
  
  RESULTS
  We used a cohort that consisted of 48 574 men and 5 409 women who participated in the 90 km ski race Vasaloppet at least once between 1989 and 1998. Number of performed races and finishing time were used as estimates of exercise level. By matching to the National Patient Register we identified participants with severe osteoarthritis, defined as arthroplasty of knee or hip due to osteoarthritis. With an average follow-up of 10 years, we identified 528 men and 42 women with incident osteoarthritis. The crude rate was 1.1/1000 person-years for men and 0.8/1000 person-years for women. Compared with racing once, participation in ≥ 5 races was associated with a 70% higher rate of osteoarthritis (multivariable-adjusted hazard ratio (HR) 1.72, 95% confidence interval (CI) 1.33 to 2.22). The association was dose-dependent with an adjusted HR of 1.09, 95% CI 1.05 to 1.13 for each completed race. A faster finishing time, in comparison with a slow finishing time, was also associated with an increased rate (adjusted HR 1.51, 95% CI 1.14 to 2.01). Contrasting those with 5 or more ski races and a fast finish time to those who only participated once with a slow finish time, the adjusted HR of osteoarthritis was 2.73, 95% CI 1.78 to 4.18.
  
  
  CONCLUSIONS
  Participants with multiple and fast races have an increased risk of subsequent arthroplasty of knee and hip due to osteoarthritis, suggesting that intensive exercise may increase the risk.
authors.affiliation: Department of Surgical Sciences, Section of Orthopaedics, Uppsala University, Uppsala, Sweden. karl.michaelsson@surgsci.uu.se
authors.forename: Karl
authors.initials: K
authors.lastname: Michaëlsson
authors.forename: Liisa
authors.initials: L
authors.lastname: Byberg
authors.forename: Anders
authors.initials: A
authors.lastname: Ahlbom
authors.forename: Håkan
authors.initials: H
authors.lastname: Melhus
authors.forename: Bahman Y
authors.initials: BY
authors.lastname: Farahmand
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D016001:Confidence Intervals; D015444:Exercise; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D015207:Osteoarthritis, Hip / Q000453:epidemiology* / Q000503:physiopathology*; D020370:Osteoarthritis, Knee / Q000453:epidemiology* / Q000503:physiopathology*; D011446:Prospective Studies; D012307:Risk Factors; D012865:Skiing / Q000502:physiology*; D016019:Survival Analysis; D013548:Sweden / Q000453:epidemiology; D013997:Time Factors; D055815:Young Adult