A little stress means a lot

We're not sure exactly why this, but the stress felt by your bones during exercise is actually necessary to stimulate your bone-building cells, the osteoblast, to make new bone. The bones in the raquet arm of professional tennis players are 15 to 20% denser than the bones in the other arm. Women with a lifelong habit of regular walking have higher bone density and a 30 percent lower fracture rate than their sedentary age-mates.