In trying to find an article this week, I couldn't find a good one, so instead I thought I'd write an anecdote. For the mast several months, my left ankle has been giving me chronic pain. It started mildly but slowly got worse, so I stopped exercising a bit to keep pressure off it. After six months of this, I tried a mild exercise workout on a stationary bicycle...and could only manage for maybe 5-10 minutes before my ankle began hurting. Then, recently, my left knee began giving me problems. I can't even turn it much without it hurting, making my left leg effectively a big stick.
So rather than not exercising, I asked my doctor what I could do, and her solution? Water aerobics. Running, jumping and swimming in water is a no impact way to get a workout in. I can't do it as often as working on a bike due to time constraints, I can at least still get some kind of workout in, which is better than nothing. The moral of the story here is that, even if you have chronic pain or an injury of some kind, there can usually be some way of getting around that and still exercising.