A recent study from Finland showed that former athletes are more likely to suffer from arthritis, while regular exercisers are less likely to do so. The ends of your bones are soft. If they rubbed against each other at the joint, the bones would be ground away and disappear. To prevent this from happening, the ends of bones are covered with a thick, tough white gristle called cartilage.
Cartilage is dynamic tissue.
It is constantly being removed and replaced. Exercise speeds the rate that cartilage is replaced and makes it stronger. Exercise strengthens cartilage, provided that you don't damage it. Once you break cartilage, it can never be replaced. A high school football player who breaks the cartilage in his knee can expect to develop arthritis of his knee later in life.