Background information-briefly stated
Bone is a connective tissue. When stressed it deforms under the load. Osteoblasts-bone growth cells- migrate to the surface of the bone to counteract the strain caused by this external weight. Because of this activity, bones grow stronger due to the bending, compressive, torsional loads, and the muscular contractions at the tendinous insertion points.
At the point of the strain, immediate modeling of the bone begins. Proteins form a matrix between the bone cells. This causes the bone to become denser due to the calcification process occurring during the growth response to the load.
The new growth occurs on the outside of the bone to allow the manufacture of new cells to continue in the limited space within the bone itself. This outer layer is commonly known as the periosteum.
Adaptations take place at different rates in the axial skeleton (skull/cranium, vertebral, ribs, and sternum) and the appendicular skeleton (shoulder, hips, pelvis, and the long bones of the upper and lower body-essentially the arms and legs). This is due to the differences in the bone types- trabecular (spongy) and cortical (compact) bone.
The stimulus for new bone formations.
Minimal essential strain (MES) refers to the threshold amount of stress applied to the structure which is necessary to elicit the growth of new bone material. A force exceeding MES is required to signal the osteoblasts to move toward the periosteum and begin this transformation. MES is thought to be 1/10 of the breaking force needed to fracture the bone. Training effects have a positive relationship to bone density just as sedentary living habits play a role in the loss of bone density.