The skeleton provides support, mobility & protection to human body. Skeleton also acts as a reservoir & store for calcium & other minerals. In body like heart & brain, Bone is also a living tissue and not just inactive mechanical support as many tend to think. The mass and architecture of bones constantly keep on changing to a large extent, depending on the physical activity of the body.
Bone is composite material consisting of crystals bound to protein. The calcium & phosphate in the shape of hydroxyappeite crystals is bound in an orderly manner to a matrix made of protein called collagen. All the components of bone i.e. Minerals & Proteins are in a process of constant replacement. Old tissue is removed & new added regularly. So, we can say that, the skeleton you have, is not the one you had one or five years ago. The skeleton is undergoing a constant process of maintenance & renewal.
As muscles develop with exercise, so do the bones also strengthen if they are loaded i.e. used. The bones of an inactive or paralysed person become weak quickly. They lose mass and calcium & strength. The size & shape of the bone is determined genetically but can be greatly influenced by physical activity. Physical activity for a bone primarily means loading. A bone is loaded when it is carrying weight. Like when you stand, walk or run, the bones of your spine & legs are loaded. When you sit on a chair the spinal bones are still loaded, but the bones of legs are unloaded.
To develop good bones, body needs good nutrition i.e. proteins, minerals & vitamins, along with physical activity. The most important Vitamins for the bones are Vitamin D, which is derived from sunlight. Children getting good nutritious diet & participating in sports are likely to have good quality bones.
The bone mass continues to increase during childhood and 'peak bone mass' is attained between 20-25 years.