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. The attitude that gets athletes to keep on exercising when they hurt can make them champions, but it also can set them up for permanent injury. A person who exercises regularly and stops when he feels pain can expect to strengthen his bones and joints.