After exercise, many people choose to engage in some kind of activity to cool down. Generally, a cool down after an exercise allows the body to return to normal equilibrium, restoring normal blood flow and stretching out muscles. This is an important practice to improve flexibility and to prevent injury and soreness. People accomplish this in many ways. Methods include post-exercise stretching and slow, short-distance jogs.
The most important purpose of a cool down is to return the body to a normal state, as opposed to the excited state the body is in during exercise. This is especially important to the body's vascular system. During a cool down, one's heart rate is allowed to gradually slow down, and one's breathing rate is allowed to return slowly to normal. When one suddenly stops engaging in vigorous activity, there is some risk of dizziness or fainting as blood pools in the muscles. A cool down exercise can prevent this, allowing one to slowly transition from an exercising state to a normal, relaxed state.
The muscles themselves can also benefit greatly from a cool down after exercise, leaving them better prepared for more exercise in the near future. During exercise, various chemicals and waste products tend to gather in the muscles and tendons. A proper cool down exercise can help to remove these chemicals and waste products from the body and ensure they do not cause problems after exercise. When exercise is stopped suddenly, the chemicals can pool in the muscles and cause intense soreness.
One such chemical usually thought to be particularly significant to muscle soreness is lactic acid, which is produced during cellular respiration while exercising. Lactic acid is produced during exercise more quickly than the body can process it. It was believed for quite some time that this led to muscle soreness. Studies have shown, however, that it does not actually have any significant relation to soreness.
There are two significant aspects to a proper cool down after exercise: slowing the intensity of the exercise and stretching. Slowing the intensity of the exercise is a natural progression that allows the body to return to a normal state. Typically, it is best to slow the intensity of the exact same exercises that one has previously been engaging in. This works the same muscles and gives those muscles a chance to slowly return to equilibrium while slowing one's cardiovascular function to a normal rate at the same time.
Stretching also allows muscles to return to a healthy equilibrium while preventing future muscle damage. Stretching after an exercise helps to return the muscle fibers to a healthy resting length. It can also help to improve one's flexibility over time.