The end of the bone is encapsulated and protected by membrane which secretes a fluid. In addition there are two little cushions that stop the two bones from touching. Movement sets off the production of greater quantities of fluid, so avoiding dehydration and ensuring good operation. The sponges need to be kept engorged with fluid to mantain their sponginess. If they dried out they would disintegrate.
Movement improves the muscles, and they help to keep a good position.
Working against gravity and keeping active will make a tremendous difference to your health. The benefits of movement can be seen in all of the the cardiac and respiratory apparatus - the blood flows more freely through the capillaries and it brings higher haemoglobin levels and more oxygen to the whole body, and the veins keep their elasticity. When your heart beats, it pumps blood around your body. The heart needs oxygen, exercise reduces the possibility of an heart attack, blood pressure and colesterol level normalise. When you move vigorously, a greater amount of blood is flushing through your blood vessels, keeping them clean and maintaining their elasticity, with reduced likelihood of aneurisms.