The Muscles
There are approximately 640 muscles and they produce motion in all directions. No single muscle works in isolation. Various groups of muscles work in co-ordination with other groups. For example, when one group of muscles contract, the opposite group relax or lengthen to allow movement to take place.
The maintenance of posture, such as sitting, involves the muscles controlling the position of the trunk over the pelvis by “switching on and off “continually. When standing and walking, the muscles acting on the spine work actively to ensure that balance is maintained between a large mass over a relatively small area.