Reasons for Bone Adaptation

Bone is a bendable living tissue filled with a thick, fluid-like substance known as marrow. The inside of bone is spongy and contains a lot of blood vessels. The outer structure of bone is dense, strong and rigid so it can support the weight of your body and give it form.

Stages of Bone Growth

• There are three distinct stages of bone growth and development: osteogenisis, modeling and bone renewal. Osteogenesis is when bone forms on soft tissue during embryonic development and early stages of growth. It also occurs during healing from a fracture or break. Modeling occurs when bone forms on already existing bone tissue during the early stages of growth up to adolescence. Bone renewal, or bone remodeling, occurs from growth to death, and is the only means for changing bone in an adult skeleton. Bone remodeling has two distinct phases, formation and resorption. Formation, or ossification, is when the soft cartilage turns into hard bone during infant and child body development. Bone resorption is when specialized cells known as osteoclasts break down the hard outer shell of the bone to release the phosphate, magnesium and calcium that it contains into the bloodstream. Resorption can also happen when there is a lack of stimulus for bone maintenance.

Wolff's Law

• German anatomist Julius Wolff's classical theory proposes that "bone forms and change formation depending on the force of muscular tension and the stress of gravity that is placed on it." What this means is that bone tissue adapts to the stress that is placed upon it by increasing or decreasing in size as needed. The muscles of the body are connected to the bone. When a muscle flexes, it pulls on the bone, placing tensile stress upon it. This stress stimulates the bone to grow or harden. The size or mass of a muscle also places weight on the bone to which it is attached, so when a muscle grows larger, the bone increases its mass or density so that it can support the muscle's weight.

Exercise

• Bone is adapted during stress and mechanical stimulus, such as exercise. Its density is highest in areas of the body where there is the most tensile stress, such as where the muscles are the strongest and the largest. Bone structure can be adapted to changes in weight load. Its cells may be regulated by local stress stimuli, which is stress that occurs only in one area on the body, such as when you lift a free weight to work on defining your bicep muscles. During exercise, both gravitational forces and muscle loading are responsible for bone adaptation. Walking, running, and climbing stairs take advantage of gravitational forces for bone growth. Resistance training exercises, such as lifting free weights, take advantage of muscle loading for bone growth.