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Where does moderate exercise end and excessive exercise begin?

Q) There have been various stories in the national press about the increasing numbers of younger people who need joint replacement surgery because their joints have been worn down by excessive exercise. Yet we're encouraged at every turn to keep fit and active to keep joint pain at bay. Can you tell me where moderate exercise ends and excessive exercise begins? I'm a relatively fit person in my late 40s, run three times a week and go to the gym twice a week. My knees sometimes ache and creak, and I have intermittent periods of neck pain.
Gill McPherson, Doncaster, South Yorkshire (Autumn 2006) 

A) When I was younger and more active I conducted a research study on my fellow athletes. We were endurance runners, regularly covering 40–50 miles in a race. We (my colleague and I) collected urine before and after a 33-mile event, including our own specimens. Athletes are very generous and they all filled the little bottles to the brim. Of course, when we froze them (a necessary step as they were to be analysed in Aberdeen) the liquid expanded, resulting in several burst plastic bottles – not a pleasant task when they were thawed for analysis. The results of the study found no evidence that joints were being damaged (we measured a marker of cartilage degeneration). Later, more extensive studies have shown that an injury can accelerate damage to the joint and running an excessive amount can also contribute. What's an excessive amount? Well the studies I've read are reassuring – over 100 miles a week is excessive. Running the amount you do is fine and unlikely to damage your joints. The cardiovascular benefits you get will be of far more benefit to you.

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