We may feel pain in our legs after walking a long time. After exercises too we may feel it. When we are doing some works, we know our muscles and functioning of our body goes in a higher rate. So to provide energy for this, our body need high amount of oxygen. Some times the respire oxygen may not be enough for all these functioning. At this time, some cells may not get enough oxygen and they start using the energy stored in cells. So this makes us pain.