Our bones are changing throughout our lives
WHEN we think of bones, a lifeless skeleton usually comes to mind, but our bones grow and change shape throughout our life. Much of this 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.
Because muscles normally attach close to joints, muscular forces are even greater than these impact forces. As a result bones also experience huge impact and muscle force during daily tasks, totalling more than five times body weight even during walking.
RACKET ARM
The shin bone briefly becomes nearly a millimetre shorter as your foot hits the ground when running. 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 shins than people who don’t exercise.
But not all exercise gives us big, strong bones. We seem to need high impacts. This means not all exercise appears to be beneficial for bone. Swimmers and cyclists may have healthy hearts, lungs and muscles, but their bones are similar to those of people who do not exercise.
If we start to load our bones less, they waste away. Astronauts lose up to 1% of their leg bone mass a month when in space.
This shaping of bones by force appears to occur throughout life. 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. Bone seems to be most sensitive to loading while we’re still growing. While some of the benefits disappear once you stop exercising, exercised bones remain wider even several decades after exercise stops.
This suggests that exercise in childhood may give us bigger, stronger bones for life – bones that are less likely to break as we get older.
However, effects of exercise on bone in elderly people have so far been much smaller. This might be because we can’t produce as much force, or that bones are less sensitive to the forces we do produce. Or it could be changes in our muscles and bones mean the amount of squashing, bending and twisting also changes.
Currently exercise trials seem most effective in children and in stopping or slowing bone loss in disuse. However, ongoing work is giving us a much clearer picture of how forces contort our bones during different movements. This will allow us to design more effective exercises for bone in different groups, finally allowing us to translate the dramatic effects of exercise on bone seen in athletes into benefits for the wider population. – The Conversation