In fact, as researchers investigate the effect of exercise on bone health, it has become increasingly clear that various kinds of exercise produces varying results on bone conditioning. Until recently, researchers believed that the impact of running, or any similar high-impact exercise, would transform the bone, "deforming" it just slightly. As a result of running, the bone would give slightly to the force of the physical activity, and the bone cells would stretch in response and adapt to the repeated exercise by adding cells and becoming denser. But this is not the case. In actuality, researchers have now found that the bone cells don't stretch; instead, when the bone bends, it "squeezes bone fluid" from one part of the bone "matrix" to another, and it's this extra fluid that causes the cells to add denser bone in response.
The key is that only certain types of exercise produce this kind of bone-bending, and the subsequent response to add dense bone-building. Says Alexander G. Robling, an assistant professor in the department of anatomy and cell biology at Indiana University School of Medicine, "large forces released in a relatively big burst” are crucial to building healthy bones, which is why activities like weight lifting won't help bone-bend or produce stronger, denser bones. But the exercises that will? Running, brisk walking at a fast pace, and surprisingly, repeated jumping.