system
Given the following post, respond as if you're commenting on Reddit:
human
Whenever you exercise, are you causing permanent damage to your joints?

I've heard that professional athletes are prone to joint problems years after retiring.
gpt
I am a 4th year medical student applying to orthopedics residencies in September so take this for what it is worth.

You can do some damage with overuse (tendonitis, wearing down cartilage, etc), but the big problem is going to be osteoarthritis, which is basically the breakdown of the cartilage that protects the ends of your bones from each other (i.e. the end of your femur and top of your tibia for your knee). Profession athletes tend to put a lot of stress on their joints, but it seems that the biggest factor is going to be weight, and to a certain extent genetic factors play a role. 

Athletes have joint problems down the road for a number of reasons, and many of them are related to acute injuries rather than the chronic use of the joints. In general, the things you want to avoid injuring are cartilage and ligaments because the blood supply is limited or non-existent, and as a result (this is a simplified explanation) tend to not really be able to rebuild themselves. 

An anterior cruciate ligament (ACL) tear is a good example, as you need to have it surgically replaced. Although many people don't need to have this surgery done, you can get cartilage damage if don't have an intact ACL because the knee is unstable and so the cartilage rubs in a way it was not meant to. 

In short, just by exercising you are probably not doing a ton of damage to your joints. The benefits of exercising far outweigh the costs unless you are running marathons, etc. If you are playing competitive football or soccer where you are prone to joint injuries by the nature of the game, then you very well may run into problems down the line.