The skeletal frame is made up of an intricate matrix of collagen and calcium phosphate which constantly changes. New bone replaces old bone and reaches its peak between 20 to 30 years of age. From then on, calcium is lost at a higher rate than it is absorbed. Joints are reliant on nutrients to maintain cartilage and flexibility, but lose this through wear and tear, leading to poor function and arthritis in old age. Hips and knees are pivotal points where this can occur.