The heart rate increases due to an increase in the amount of oxygen the muscles require when exercising. Muscles require oxygenated blood and when someone works their muscles, the muscles need more of this to maintain enough energy to keep up with the demand.
Someone's heart rate helps them to determine if they are either exercising too hard or not enough. In general, the resting heart rate ranges from 60 to 80 beats per minute, according to the American Heart Association. The target heart rate during exercise is 50 to 80 percent of a person's maximum heart rate. The target heart rate ensures that someone is getting adequate exercise without overworking the body.