When you exercise -- be it something as vigorous as running hill repetitions or as mellow as bowling -- your heart rate increases to some extent. Not only that, but your perceived effort scales neatly with the degree of increase; working at around 50 percent of your maximum heart rate leaves you able to chatter away comfortably with your training partner, whereas hammering away at 90 percent has you sucking wind. Regardless of your chosen activity, different heart-rate "zones" offer different benefits, and coaches recommend different zones for different goals.
According to the Mayo Clinic, a typical resting heart rate is 60 to 100 beats per minute. A number of factors can influence this figure, including temperature, body position, emotional state and fitness. It's not uncommon for athletes to have resting heart rates of 40 or even lower.
At the other end of the spectrum, many adults can achieve a heart rate of 200 for short periods when exercising all-out, although maximum heart rate varies widely. Regardless of your current max, know that it -- like too many other things -- declines with age. A reasonable estimate of your max: 220 minus your age.
According to UK Athletics coach Brian MacKenzie, once your heart rate rises to 60 percent of its maximum, you are working at a level that will develop basic aerobic endurance. On the other hand, a heart rate between 60 and 70 percent of max is considered a "recovery zone" because in this range your body burns a lot of fat but uses very little glycogen, the body's storage form of glucose, thereby allowing for replenishment of glycogen stores depleted by previous, more intense workouts. Suggested activities include vigorous walking, easy running, swimming and easy cycling, but what you do is up to you -- it's the heart rate that matters.
When your heart rate edges above 70 percent of its maximum, you've entered a zone in which your cardiovascular system begins to accrue significant benefits. In plain terms, this means that you're improving your body's ability both to transport the oxygen your breathe in to muscles and those muscles' ability to convert it to useful work. At this level of exertion, you're still burning mostly fat as long as you stay below about 80 percent of your max, but you're also tapping significantly into your glycogen reserves, as the proportion of exercise work that's reliant on glucose rises directly with intensity level.
When you boost your heart rate about 80 percent, you've entered serious-business territory. In this zone, you're burning a great deal of glycogen and have moved from a sole dependence on aerobic metabolism to a heavy dependence on anaerobic metabolism.
Once you're over 90 percent of max, no matter how fit and experienced you are, you're putting out the sort of wattage you won't be able to sustain for very long. For example, if you are a runner, if you do repeat quarter-miles at one-mile race pace with a couple of minutes of rest between runs, you'll spend much of the workout in this "red zone."
Realistically, training at high intensity is only necessary if you are a competitive athlete. If you are simply trying to lose weight or get fit, you can more comfortably float along at between 60 and 80 percent of max heart rate.
L.T. Davidson has been a professional writer and editor since 1994. He has been published in "Triathlete," "Men's Fitness" and "Competitor." A former elite cyclist with a Master of Science in exercise physiology from the University of Miami, Davidson is now in the broadcast news business.