Jay Downing preaches the benefits of heart-rate monitors to all who will listen.

Jay Downing preaches the benefits of heart-rate monitors to all who will listen.

"You will see results," said Downing, a fitness specialist at the Carillon Wellness Center in St. Petersburg, Fla. "Used properly, a heart-rate monitor is probably one of the most useful tools we have for overall fitness."

That's because it's the one sure way to know whether you're working hard enough to improve fitness and performance — or so hard that you run the risk of hurting yourself.

The problem is, Downing said, too few runners, bikers and swimmers actually use this small but mighty piece of exercise equipment.

"A lot of people just don't want to be bothered," he said. "Then there are those people who have them but don't know how to use them."

Heart-rate monitors, electronic devices that measure how many times the most important muscle in the body beats per minute, come in all shapes and sizes.

The most basic versions, with a chest-strap transmitter and wrist receiver, sell for under $60. Buy one with more advanced functions, such as GPS technology and downloadable training programs, and you can pay well over $300.

Heart-rate monitors have been a mainstay for cardiac-rehabilitation patients for decades.

"If you have any type of procedure or issue, including a heart attack or heart disease, you should undergo at least 12 weeks of supervised cardiac rehabilitation," said Dr. Vibhuti Singh, an interventional cardiologist. "The heart-rate monitor is a vital part of that rehabilitation."

Heart patients typically start on an exercise bike or treadmill, pedaling slowly or walking, and then picking up the pace if their condition permits it, while a trained technician closely monitors their heart rate.

"A heart-rate monitor lets us control the intensity of their exercise," said Charlie Mack, a clinical exercise physiologist for Bayfit Cardiac Care. "Depending on their diagnosis and medication, heart patients can see benefits by exercising at 40 percent of their maximum heart rate."

But many of Mack's patients have continued monitoring their heart long after their rehab is completed.

"The bottom line is a heart-rate monitor allows you to approach your exercise program scientifically," he said.

Most of the people Downing works with at Carillon want to trim down or simply improve their overall fitness.

"The first thing I do is identify their fitness goals," Downing said. "Do you want to lose weight or are you trying to take a couple of minutes off your 10K time?"

The heart rate, or how many times the muscle beats per minute, varies during rest and exercise. The resting heart rate is — obviously — how many times a heart beats per minute (BPM) when the muscle is at rest. The maximum heart rate (MHR) is how many times a heart beats per minute at full capacity, which varies by age.

To figure out your MHR, start with the number 220 and subtract your age. For example, if you are 50 years old, your MHR is 170.

"If you want to just maintain heart health and burn some fat, you should exercise at 60-70 percent of your MHR for 30 to 60 minutes at least four times a week," Downing said. "But the longer you exercise, the more fat you will burn.'' For a 50-year-old, 60 percent to 70 percent of the MHR is 102 to 119.

But if you want to set a new personal record in, say, a triathlon, you will have to push it a little harder.

"For aerobic endurance training, you should be training at 70 to 80 percent of your MHR," Downing said. So our 50-year-old would keep his or her heart rate at 119 to 136.

Dr. John Gross, a sports-medicine specialist, works closely with elite competitors. An accomplished athlete, Gross is a frequent speaker at triathlon and running clubs, where he extols the virtues of a heart-rate-monitor-based training program.

"There are a lot of good reasons to monitor your heart rate," he said. "Heart-rate monitors act as the link between your mind and your body.

"It will show you when you are training too hard," he said. "It will show you when you aren't training hard enough."

The devices are particularly useful for seasoned athletes who want to fine-tune their performance with speed work and interval training.

"But you have to be careful," Gross said. "Heart-rate monitors do have their disadvantages."

For one thing, they're not all created equal, and price is not necessarily a guarantee of quality. If possible, bring your heart-rate monitor to a trainer or health professional who can gauge its accuracy for you.

Read the directions carefully to be sure you're wearing the monitor and reading it properly.

Also, dehydration, heat, humidity and lack of sleep are factors that can affect your heart rate.

"You are not always the same person that you were yesterday," Gross explained. "So heart-rate monitors are not the holy grail for runners. But the advantages far outweigh the disadvantages."

Scott Chambers is your typical weekend warrior. During the week, the 38-year-old manages his family's auto-supply business.

"I try to run every day," he said. "On weekends I try to race."

Last December, under Downing's direction, Chambers started training with a Polar heart-rate monitor, an entry-level device that retails for about $55.

Polar heart-rate monitors are especially popular with recreational athletes because they are inexpensive and they link up with most gym equipment, which eliminates the need to wear the wrist unit when on the treadmill.

"I have taken about 30 seconds per mile off my time in just a few short months," he said. "I am training at a higher level. I recover quicker ... my body doesn't go into shock. It was just a great investment."