It’s like this. Whatever activity you take on, your bones adapt to. There are “bony landmarks” or “bony markings” to which your tendons attach your muscles. If you are accustomed to, say, rock climbing, the muscles in your in your fingers, hand and arms are going to pull on those attachments like a lever. This wear and tear on your bones triggers bone building cells (osteoblasts) which leads to more and more bone tissue where you need it most. If this didn’t happen, then you’d be snapping off bone every time you contracted a muscle. Any time you exercise your muscles, you are also building bone, too. If your bones are not prepared for muscle strain, they will hurt pretty bad and you wont be able to beat your opponent, or worse break. Also, there are little organs in your muscles near the tendons called Golgi tendon organs. If they sense that there is too much stress on the muscle, they will relax so the muscle won’t tear. You don’t want this to happen during a wrestling match. The more you work on any area, the more these structures can build up to the resistance and stress. So having overall good bone density is a good thing to shoot for as a youth (because once you’re an old person, it becomes more difficult and you don’t absorb calcium as well).