Is bone alive? How your skeleton rebuilds itself where it is loaded

Yes, bone is alive. It looks like dead stone on a museum skeleton, but inside you it is living tissue: cells embedded in a mineral-hardened framework, constantly tearing old bone down and building new bone in its place. About a tenth of an adult's skeletal mass is replaced every year, and in the first year of life almost all of it is.

Two kinds of cells do the work. Osteoblasts secrete new bone, and osteoclasts break bone down. Remodelling is not just an occasional repair job. It is an ongoing process that is always happening in a healthy body. It mends the tiny cracks that everyday stress causes, helps regulate calcium in the body, and shapes the skeleton as you grow.

The surprising part is that this rebuilding is steered by use. In the 19th century a German surgeon named Julius Wolff, after a long career of looking at bones, proposed what is now called Wolff's law: bone adapts to the loads placed on it. Load a bone more and it remodels to become stronger. Load it less and it becomes less dense and weaker, because the stimulus that keeps remodelling going is missing.

You can see the evidence in athletes. The racquet-holding arm bones of tennis players are stronger than those of the other arm, and weightlifters often show increases in bone density in response to their training.

So a skeleton is less like a steel frame and more like a garden that is constantly being tended, and your activity decides where the gardeners work hardest.