Running and the knees

My non-runner friends are often worried about my knees. They are very certain that running is very very bad for my knees and I will have to use a cane after my 40th birthday. Here is a study from Australia that seems to support this view:

Is physical activity a risk factor for primary knee or hip replacement due to osteoarthritis? A prospective cohort study.

Wang Y, Simpson JA, Wluka AE, Teichtahl AJ, English DR, Giles GG, Graves S, Cicuttini FM.

Wang and colleagues collected physical activity data from 40000 people and looked if they got a knee replacement in the future. Their results show that people, who are physically active are more likely to get knee replacements, but not hip.

I unfortunately don't have access to the whole paper, but I have a few concerns about the study based on the abstract. First of all, general physical activity level was calculated. Not supported (biking, swimming) vs non-supported sports (running, soccer etc.). This would probably affect the number of joint replacements. The effect of running could be positive ("constant stress strengthens the joint" hypothesis) or negative ("you are killing your knees, stop now!" hypothesis).

Another quirk of the paper is that the knee replacement risk falls with increased activity. People who are active 1-2 days a week are in higher danger than people who are active 3 or more days. This could very well be related to random variation, but if the data holds (and holds for an impact sport, like running), I think it supports the "stress strengthens the knee" hypothesis. If you think about it, people who don't run are in less risk, because they don't put any impact on their knees. People who run constantly are in less risk, because their knees are used to it. People in the middle, the beginners or weekend warriors who just decide go out and sprint on a whim are those that stress unprepared knees.

In any case, I am glad they didn't find any connection to hip replacements. I recently heard how a hip replacement operation is done and let me tell you, it is not pretty. Knee replacement is probably not too different, but no one told me about that yet! (and don't you dare!)