When we think of bones, a lifeless skeleton usually comes to mind, but our bones are a living organ that grows and changes shape throughout our life.
Much of this shaping results from forces which press, pull and twist the skeleton as we move, and the biggest of these forces is caused by our muscles.
Bones experience huge forces during movement. When a triple jumper’s heel hits the ground, the force is around 15 times their body weight – or the weight of a small car.
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.
For example, the racket arm bones of tennis players can be 20% wider and contain 40% more bone mineral than their other arm, while sprint runners have up to a third more bone in theirshin bone than people who don’t exercise.
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 differentfrom 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.
But if we start to load our bones less, they waste away and these effects are no less dramatic. Astronauts lose up to 1% of their leg bone mass per month when in space, while people who suffer a spinal cord injury lose up to half of their shin bone mass.