Living bones modify their structure at gross, tissue, and molecular levels in response to force patterns acting on them. Fossil bones can therefore reveal the forces which acted on them, the placement of muscles from muscle scars, and the possible positions of limbs from the shape and orientation of joints.
The internal architecture of a bone is the distribution of the material that makes up the bone. Forces will induce patterns of stress and strain which are dependent on the internal architecture of the bone, its material properties, and the magnitude and direction of the forces.
a) A model of the proximal end of a human femur with no forces acting on it. Structural density of the model is uniform.
b) The model after one round of loading, structural density is reduced in areas of low stress and increased where stress is high.
c) The model after thirty rounds of loading, the density distribution of the model quite closely resembles that of a real femur.