Humans learn to walk when they are 1–2 years old. At that point, the human muscles are well enough developed to support a moving body by themselves. 

How long can a human run? If you move a lot and do it regularly, even a distance of several kilometers will pose no problems to you. However, sooner or later you will realize that your muscles are beginning to tire. 

Your muscles need energy and oxygen. These substances are transmitted to your muscles by your bloodstream. 

In the muscles, the energy is used to contract muscle cells. This reaction is not possible without oxygen. 

If there is not enough oxygen available to the muscle, it will start to feel painful. When exercising, you may notice that you will begin to breathe more quickly. This is a sign that your body requires more oxygen. 

To move, you need:

1. Oxygen. You get it by breathing air. During exercise, your breath rate will become more rapid.
2. Energy. You get it from food. During long exercises, you can eat quickly digestible foods, such as bananas. 

Your muscles use oxygen and energy to contract. This makes produces movement.