The genetics and the bone structure determines the shape of the leg. However, running can change your bones, because bones experience huge forces during movement. Long-distance running builds leaner muscles, while sprinting adds bulk. Bones from swimmer are not much different from the bones of people who don't exercise.High impacts (hitting the floor from a jump, or striking a tennis ball) are needed to produce big enough muscle and impact forces to make our bones change. This is the reason why running changes our bones, and swimming does not.