Just being able to stand up or to walk is one of the most amazing tricks it is possible to learn. When you stand still you are performing a constant act of balancing. You change from one leg to the other, you use pressure on your joints, and your muscles tell your body to go this way and that way. Just to keep our balance as we stand still takes the work of about 300 muscles in our body. In walking, we not only use our balancing trick, but we also make use of two natural forces to help us.
The first is air pressure. Our thigh bone fits into the socket of the hip joint so snugly that it forms a kind of vacuum. The air pressure on our legs helps keep it there securely. This air pressure also makes the leg hang from the body as if it had very little weight. The second natural force we use in walking is the pull of the earth’s gravity. When we raise our leg, the earth pulls it down again.