Strapping on a heart-rate monitor can empower you with the motivation to stick to your fitness routine during 2008.

Trying to get in shape without knowing if you're exerting yourself enough -- or too much -- can jeopardize your long-term commitment to an exercise program. Exercising at your proper cardiovascular intensity can help you become fit more quickly and motivate you to continue, says Karyn Hughes, associate director of education for The Cooper Aerobics Center in Dallas, Texas. But doing too much before your body is ready can be discouraging, and ultimately lead you to quit.

"One of the main reasons to wear a heart-rate monitor is that it tells you what your intensity is -- and whether you need to work a little harder, or back off," she says.

A heart-rate monitor is a small, portable device that displays how many times your heart is beating per minute during a workout. A basic model, such as the Polar FS1 from leading manufacturer Polar USA costs about $70. It monitors your heart rate, sounds an alarm if it's too high, and tracks your exercise time. For about $90, you can buy the Polar FS3 , which also monitors the calories you're burning and has a backlight so you can read the monitor if you're exercising in a dark environment, such as an indoor cycling class.

A Polar Heart-Rate Monitor

Information junkies and serious athletes may consider the Garmin ( GRMN) Forerunner 301 ($215), which stores workout data so you can study the results on graphs. But monitors with numerous features can be frustrating for recreational exercise enthusiasts who are just learning about the technology.

Heart-rate monitors are as easy to put on as a pair of Nikes. Most models rely on a small transmitter that you wear in a thin strap around your chest. The transmitter sends your heart-rate information to a monitor that resembles a wristwatch.

Fitness experts advise using a simple formula for estimating your maximum heart rate: 220 minus your age. The difference between the two numbers equals the maximum number of times that your heart should beat in one minute. Most people should exercise at between 65% and 85% of their maximum heart rate, says New Jersey-based personal trainer Shawn Teske.

That means if you're 42 years old, your maximum heart rate would be 178 beats per minute, and you'd want to achieve a heart rate between about 116 bpm and 151 bpm. A fit person of that age whose heart reaches 100 bpm may want to think about working harder. You'll burn more calories, and become leaner faster, as you increase your heart-rate intensity, says Hughes. She recommends sustaining an elevated heart rate for at least 10 minutes at a time, for a total of 30 minutes per day, five days per week.

Teske, who trains clients at The Body Shop in Marlton, N.J. and The Firm in Mount Laurel, N.J., says he has encouraged the use of heart-rate monitors for more than a decade. He advises his clients to buy one that includes a calorie counter. "A lot of people just do the time when they come to the gym," he says. "They think that just because they've exercised for a half hour that they're worthy of eating cookies. But they're never burning as many calories as they think they are -- and they don't know what they've burned without a heart-rate monitor."

Trainer Shawn Teske Doing a Demonstration for Clients

He says the calorie counter is more accurate than those integrated within fitness machines, which, he says, compute the information based on a standard user profile instead of your actual gender, weight and height. Some equipment, however, such as the Cybex ( CYBI) Arc Trainer , or Brunswick Corporation's ( BC) LifeFitness Engage treadmills, transmits information from Polar-brand heart-rate monitors onto their screens.

Genevieve Belfiglio first used a heart-rate monitor five years ago, after she added cardiovascular fitness to her longstanding weightlifting routine. She wanted to run, but found she could barely make it around the block, she says. But when she strapped on the heart-rate monitor, she learned she wasn't anywhere near her target range. "My mind kept telling me I was working really hard and had to stop, but the monitor told me the hard truth: I wasn't working as hard as I could. My mind was tricking me," says Belfiglio.

She has since developed an interest in cycling and wears the monitor during every ride. "When I'm going up hills, and I think I can't do it, the monitor is a reality check to tell me how much steam I really have left," says Belfiglio, a medical writer in Princeton Junction, N.J.

"The monitor makes exercise non-boring," says Teske, who was also a 2007 state champion in a United States Cycling Federation event. He recommends challenging yourself with interval training: exercise at a steady pace and then move as hard as your can for one minute, noting your heart rate increase. "It takes your mind off the time. "Knowing that you have to get your heart to a certain level helps people and sets the goal for when they come to the gym," he says.
Suzanne Barlyn is a writer in Washington Crossing, Pa.