Bone is one of the strongest biological materials in existence, and is the main supporting tissue of the body.

Even though bones have a very hard and strong outer layer, the middle is soft.

• The material in the middle of a bone is called marrow.
  • Marrow is spread throughout our bones.
  • Marrow is mainly concentrated in the skull, ribs, sternum, vertebrae^* and pelvic bone.
  • Marrow produces the three main types of blood cells.

Bone is a living tissue that can repair and reshape itself. When bones begin growing they are completely solid. It is at a later stage when they begin to develop a soft center.

When running, a human loads the bones at the knee joint with a force^* in excess of five times the weight^* of the entire body. Despite its great strength, bone is a very lightweight material. It is composed of protein and mineral.