This is what exercise does to your bones

In fact, because muscles normally attach close to joints, muscular forces are even greater than these impact forces (in the same way that you have to push harder to lift someone on a see-saw the closer you get to the middle).

As a result, bones also experience huge impact and muscle force during daily tasks, totaling more than five times body weight even during walking.

These forces squash, twist and bend bones. The shin bone briefly becomes nearly a millimeter shorter as your foot hits the ground when running. The bone senses these small changes, and can grow dramatically – in the months after starting exercise – in order to reduce the risk of breaking.

But not all exercise gives us big, strong bones. We seem to need high impacts (hitting the floor from a jump, or striking a tennis ball) to produce big enough muscle and impact forces to make our bones change. As a result, not all exercise appears to be beneficial for bone. Swimmers and cyclists may have healthy hearts, lungs and muscles but their bones are not much different from people who do not exercise.

Bone’s response to these forces varies along its length. Near the joints, bones get bigger and more dense, whereas bone shafts tend to get bigger and thicker with little change in bone density. Bones also change in shape. The shin bone shaft starts as a circular tube, but gets wider from front to back as we grow and start to move until it forms a tear-drop shape.

This shaping of bones by forces appears to occur throughout life. Even at 15 months old, children who started to walk early have up to 40% more bone in their shin than children who have yet to start walking; effects that last until at least their late teens.

Bone seems to be most sensitive to loading while we’re still growing. Once we reach our final height, bone appears less able to increase its width, particularly near the joints. While some of the benefits gradually disappear once you stop exercising, exercised bones remain wider evenseveral decades after exercise stops.

This suggests that exercise in childhood may give us bigger, stronger bones for life. This is important as bigger, stronger bones are less likely to break as we get older. Certainly, exercise trials can be very effective in making children’s bones stronger, and also in reducing bone loss frombed rest or even partly reversing bone loss in spinal cord injury.

http://www.businessinsider.com/what-exercise-does-to-your-bones-2016-4