Increased heart rate causes your body to work harder at moving. Everything you do on a daily basis, even if it is sleeping, burns calories. Your body is never truly at rest as your organs are continually moving and doing their jobs. Same with muscle. The target heart rate for aerobic exercise - which means that your muscles are using oxygen as they work - allows your body to reach a set number to make it easy to know where you need to aim for.
Cool downs are important to allow your heart rate to gradually return to normal and your body temperature to as well. Doing it gradually allows for less health risk to your heart and muscles.
at 9:07 AM on Feb. 6, 2009