What's the best way to avoid painful knee osteoarthritis, which has approximately doubled in prevalence in the US over the past 20 years and is likely to increase? In the first study to estimate the actual lifetime load on the knee from various activities, researchers have added heavy housework to the occupational risks predisposing people to osteoarthritis of the knee.
However, as in several previous studies, the new evidence reported in the International Journal of Rheumatology ("Is Lifelong Knee Joint Force from Work, Home, and Sport Related to Knee Osteoarthritis?") seems to exonerate recreational sports from a significant impact on knee osteoarthritis. Thus it provides more evidence that "lifelong physical activity is generally safe," conclude the authors.
Subjects for the study, headed by physical therapist Charles Ratzlaff of Brigham and Womens Hospital in Boston, were recruited from community-dwelling members of Canada's largest 50-plus advocacy group, the Canadian Association of Retired Persons. An Internet-based survey asked respondents whether they had been diagnosed with knee osteoarthritis by a doctor and whether they had knee pain every day. It also asked them to report specifics about their previous jobs, including housework activities such as gardening or caring for children or elderly relatives, as well as details about their recreational history, including duration, frequency, and time spent on any of 64 sports activities. (Respondents could specify others.)
Although numerous other studies have assessed the impact of male-dominated manual labor occupations and sports activities on the risk of knee osteoarthritis, this is only the second study to take serious note of the repeated squatting, kneeling, lifting, and stair climbing that is part of housework as a source of stress on the knee joint. The authors say it is also the first to completely classify the joint-loading effect of activities on the knee over a long period of time.
The team applied the data from the survey responses to a newly created scale that estimates cumulative force on the knee as total lifetime hours spent in a specific activity, body weight, and typical peak joint force for that activity.
The results showed that knee OA is associated with aging, prior knee injury, and obesity among men and either overweight or obesity among women. Men who scored in the highest category (5th quintile) for total lifetime knee force and women who scored anywhere in the top 3 of 5 quintiles were significantly more likely than others to report having knee OA. But estimated force on the knee due to lifetime sports activity was not a significant factor.
The study suffers from all of the disadvantages of faulty recall in survey-based research, the authors acknowledge, especially in the area of sports. (Most respondents had not played active sports for the past several decades.) However, they add, most studies that link knee OA with sports have involved professional athletes in sports that place a high load on the knee, and most other studies also find no risk associated with recreational sports.
Therefore, the authors conclude, efforts to encourage physical activity imply no risk to the knee, and prevention efforts should be focused on weight control, avoidance of injury, and occupations that involve heavy loads on the knee.