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, totalling more than five times body weight even during walking.
These forces squash, twist and bend bones. The shin bone briefly becomes nearly a millimetre 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 their shin bone than people who don’t exercise.
