In response to exercise, both aerobic and strength training, your heart rate increases. As the physiological demands of the activity increase, so too does heart rate. The heart beats faster during exercise to pump more blood to the working muscles. The blood delivers oxygen and nutrients to the tissues and removes problematic metabolic byproducts from the cells. According to the American Heart Association, a person's maximum achievable heart rate through exercise roughly corresponds to the formula 220 minus age, although this formula is not accurate for every individual.
Enter your age to find a target heart rate during exercise. You'll get the most out of your activities by staying within this range of heartbeats/minute.
On the other hand, some people like to know with more precision how their body is doing during exercise. If that's the case for you, then taking your heart rate during exercise and using a target heart rate training zone might be just the ticket. Target heart rate zones range anywhere from 50% to 100% of your maximum heart rate (your maximum heart rate is based on your age). Aerobic exercise is anything less than 85%, and anaerobic exercise is anything above that. A nice starting point for a sedentary individual is somewhere in the range from 50% to 65% (you can always increase as you get more fit) and 65% to 85% for more conditioned individuals.
Coyle's work has included studying the muscular efficiency and physiological factors -- including heart rate -- in Lance Armstrong during his acclaimed cycling career. But Coyle says that for most people, it's not essential to track heart rate during exercise.