When the muscles are resting, they get enough blood, but when the muscles are working they don't get enough blood. This lack of blood causes a lot of pain because the muscles need a lot of oxygen when they are working. Just like when a person jogs, they breathe faster in order to take in more oxygen. The muscles are crying out for more oxygen when the person feels the pain. The reason that these blood vessels don't get enough blood is because of the blockages that develop in the vessels that carry blood to the muscles.
Most people know that blockages develop in the blood vessels that supply the heart with blood.
The same thing happens to the vessels that supply the legs with blood. These blockages narrow the opening of the part of the vessel in which they develop. Therefore, when the muscle needs a larger amount of blood for the activity of walking, the blockages prevent that from happening. The amount of the blockage that is built up can increase over time. The person might notice that the walking pain starts at shorter distances.