Cardiologists and athletes often monitor heartbeats as a way to gauge health or fitness. But the most popular formula used to calculate maximum heart rate is inaccurate for women because it’s based on data from men, according to a study published Monday in the journal Circulation.

For nearly four decades, physicians have used a simple math equation — 220 minus a person’s age — to determine the fastest rate an individual’s heart can work, measured in beats per minute. During exercise, people often try to reach between 65 and 85 percent of their peak heart rate, depending on how hard they push themselves.

Doctors use the traditional formula during exercise stress tests to estimate the risk of heart disease. During the test, a patient is monitored while running on a treadmill or riding a stationary bike. If he fails to reach at least 85 percent of his peak heart rate during the test, he has a greater risk of dying prematurely from heart disease, research shows.

For women older than 35, however, the standard equation overestimates the risk of heart disease during a stress test, according to the authors of the new study. They concluded women should use a new gender-specific formula for maximum heart rate: 206 minus 88 percent of a person’s age.

Under the old formula, a 50-year-old woman would have a peak heart rate of 170. The new formula would drop it to 162.

The new equation, derived from an analysis of 5,437 healthy Chicago-area women who were followed over 16 years, would improve the accuracy of stress tests, says lead author Martha Gulati, assistant professor of medicine and preventive medicine at Northwestern University’s Feinberg School of Medicine.

If a woman has trouble getting her heart rate up during exercise, she is more likely to die prematurely, an issue the new equation more accurately addresses, the researchers said.

When doctors use “220 minus age” they are likely to tell women they have a worse prognosis than they actually do, says Gulati, a cardiologist. “We know women are physiologically different than men — women are not small men — and we can’t assume their hearts function in the same way.”

Though the “220 minus age” formula is programmed into treadmills, heart rate monitors and other technologies, experts have long fussed over its shortcomings and proposed new equations. More than a dozen factors can affect heart rate, including stress, illness, the time of the day and medications.

“One problem with (the traditional) formula is quite clear: Maximum heart range can vary a great deal,” says cardiologist Mike Lauer, director of the division of cardiovascular sciences at the National Heart Lung and Blood Institute, who was not involved in the study.

“It’s very good at describing a phenomenon in a population, but it’s not so good with individual patients,” he says. “That makes the findings of this study that much more remarkable. (The researchers) found a new formula that, despite the noise, is still a powerful marker for risk.”

Many athletes use heart rate to enhance training. Chicago’s Kristen Pengelly, 32, says she started using a heart rate monitor to get back in shape after a running injury. She spends most of her workout at 70 to 80 percent of her peak heart rate, which would drop under the new formula.

“I was curious about how hard — or inadequately — I was working out,” says Pengelly, an instructor at DePaul University. “Also, heart disease runs in my family so I’m pretty cognizant of heart health. Wearing one forces me to work harder when I’m working out on my own.”

She says she has no immediate plans to adopt the new formula, but Gulati suspects that eventually both men and women will move away from using “220 minus age.” The science behind the original formula is weak, she says, and it was never meant to be adopted as a mainstream tool.

“My biggest problem with the literature, particularly with stress testing, is that it’s old information,” she says. “Stress testing was developed in the late ’60s. Should we not as women ask that they study us and use data based on us?”