The skeleton provides a firm support for the muscles when they are working. However, the skeleton also has joints in it which allows it to change shape. If the skeleton did not have joints the muscles would not be able to move the skeleton no matter how powerful they were.

There are over six hundred muscles in the body. If they all moved at once they would pull on the bones and hold the body still. This does not happen because the action of the muscles is controlled by the brain and nerves.

Electrical signals travel from the brain along the nerves to order the muscles to move and to order them to rest. For example when you wish to raise your lower arm a muscle at the front of the upper arm is instructed to move and a muscle at the back of the arm is instructed to rest. If they both pulled together you would not be able to move your lower arm.