A bold claim! And somewhat ironic, I suppose, as readers of Saving My Knees know I damaged my knees cycling and was unable to ride my bike again for many months, even as I plowed forward with my recovery. So in my case, I got better largely off the bike.
Also, time for the disclaimer dance: I don't think cycling is the best activity for everyone suffering from chronic knee pain. It's dangerous to generalize in the world of knee pain, because different knees often respond best to different things.
For example, one thing needed to ride a bike: good range of motion. If, every time you push your bad knee through the pedal stroke, you experience a painful clicking, or some other type of discomfort that recurs with each revolution, cycling most likely isn't a good activity for you, at least right now. That's partly because good cycling form is about spinning, or making lots of revolutions, every minute (Lance Armstrong, if I recall correctly, used to time trial at 110 rpms -- try doing that on a stationary bike and watch your legs fly!) So you need a decent range of motion that your knees can move through without hurting or "catching" too much, because they will be repeating that motion, again and again and again.
Okay, now that the big caveats are out of the way -- why am I such a big fan of cycling for bad knees?
(1) You want a high-repetition, low-load activity. That's exactly what easy cycling is.
Notice the word "easy." No hills, no sudden accelerations! A moderate rate of spin will get you 80 rpm on a bike -- that means you're flexing your knees 800 times in 10 minutes, or 2,400 times in half an hour. If your cartilage needs thousands of repetitions to spur positive adaptation (i.e. strengthening), this is an ideal exercise, it seems.
(2) Cycling is not a "bone accretive" activity.
If you need stronger, denser leg bones, one activity that won't be prescribed for you is cycling. Riding a bike is a low impact form of exercise that uses a smooth, fluid stroke. It doesn't build up bone mass. (Which leads to the paradox that the cyclist with the heavily muscled thighs who just blew by you on your daily walk may have lousy bone density.)
That can be a good thing, if you've got bone spurs in your knees already from arthritis, which is common with chronic knee pain. Bone spurs can interfere with the normal movement of a knee joint and can be an impediment to the healing of cartilage (as shown in this study: Ding C. et al., “Natural History of Knee Cartilage Defects and Factors Affecting Change”). Cycling should help you strengthen the soft tissue without promoting the formation of unwanted bone.
(3) It may cause your knee cartilage to thicken.
When the knee cartilage of triathletes was studied, an interesting discovery was made. The patellar cartilage was slightly thicker than normal (Muhlbauer R. et al., "Comparison of Knee Joint Cartilage Thickness in Triathletes and Physically Inactive Volunteers Based on Magnetic Resonance Imaging and Three-Dimensional Analysis"). Assuming this was a solid finding that will be reinforced by other studies, what might it tell us?
Well, triathlons generally consist of three activities: swimming, running and cycling. I doubt that swimming, in particular the overhead crawl stroke that you'd expect a triathlete to be doing, promotes the formation of more cartilage under the knee cap. Nor would I expect running to achieve this effect, with its harsh impacts absorbed mainly by cartilage at the end of the leg bones.
Cycling appears most likely to be responsible. Just look at the angle of the knee during the force-exerting part of the pedal stroke. Hopefully future studies will clarify whether cycling does encourage cartilage growth.
(4) Strong quads!
Anyone who reads this blog regularly may be thinking, "But wait a minute -- aren't you the guy who thinks focusing on strong quads is a mistake for people who have chronic knee pain?"
Yup. That's right. But these are two different things.
Having strong quads is great and surely does protect against developing knee pain. Acquiring strong quads through exercise, when your knees tolerate that exercise well, is also great. What doesn't make sense to me: Focusing on strengthening your quads when your knees are damaged and weak and the quad-strengthening exercises only cause more pain.
One nice thing about a cycling program, if you're patient, is that the gentle cycling that you start out doing, to heal your joints and strengthen your knees, can gradually be intensified to the kind of workout (which I do now) that will really develop your quads. Trust me, I know. :)