The body calls glucose (which has been stored by the body as glycogen from the food we eat) to provide the energy needed to contract the muscles. It also uses adenosine triphosphate or ATPbut the body has only small reserves. After quickly using these reserves, the body needs extra oxygen to make more ATP. More blood is pumped to the working muscles to provide extra oxygen. Without enough oxygen, lactic acid is formed instead. Lactic acid is usually removed from the body within 30 to 60 minutes after completing a session. During your workout, small tears form in the muscles, helping them to get bigger and stronger when repaired. The pain is a signal that there are changes that occur in the muscles, it usually lasts a few days.
Your body may need up to 15 times more oxygen when you exercise, so you start breathing faster and harder. Your breathing rate increases until the muscles around the lungs simply can not move faster. This maximum oxygen utilization capacity is called VO2 max. The more a person has a high VO2 max, the more fit the person is.
Like any muscle, the diaphragm can get tired by repeating large breathing movements. Some claim that diaphragm fatigue can cause spasms, causing the dreaded side-point. (Others argue that the sided spot is due to spasm of ligaments around the diaphragm, while others believe that spasms come from the nerves that run from the upper back to the abdomen and are caused by by bad posture!). Deep breathing and stretching can relieve this discomfort in the middle of a workout.
When you train, the heart rate increases to circulate more oxygen (through the blood) at a faster rate. The more you train, the more effective the heart becomes, so that you can work harder and longer. In the long run, regular training lowers the resting heart rate in fit people. Exercise also stimulates the growth of new blood vessels, which also lowers blood pressure in athletes.