March 8, 2001 -- It's the mantra of any aerobics instructor or personal trainer -- know your maximal heart rate, work to 80% of that number, and check it regularly during your workout. Figuring out that magic number involves a simple enough formula: 220 - your age = the maximum number of heartbeats per minute that your heart can tolerate.
But a new study challenges that widely used formula, saying that despite its widespread use, the formula's validity, especially in older adults, has not been well-studied.
"Our findings suggest that the currently used equation underestimates [maximal heart rate] in older adults. This would have the effect of underestimating the true level of physical stress imposed during exercise testing and the appropriate intensity of prescribed exercise programs," write lead author Hirofumi Tanaka, a researcher in the Human Cardiovascular Research Laboratory at the University of Colorado at Boulder, and colleagues.
The article appears a recent issue of the Journal of the American College of Cardiology.
In the study, Tanaka and his colleagues analyzed data from 351 studies involving more than 18,000 people. He also laboratory-tested 514 men and women aged 18-81, all of whom were healthy and free of coronary artery disease. Each person was assigned to one of two groups: those who had trained in endurance exercise for at least two years and those who performed no regular physical exercise. Each was given a treadmill test during which their maximal heart rate, breathing efficiency, and perceived exertion were monitored.
Researchers found that maximal heart rates were strongly related to age in both men and women. This led to the development of a new formula to estimate heart rate: 208 - (0.7 × age).
According to the authors, maximal heart rate is predicted, to a large extent, by age alone and is independent of gender and physical status.
"The information is especially important for older people, since it seems we have been underestimating their capabilities," says Steven Manoukian, MD, a cardiologist with Emory University School of Medicine in Atlanta.
He compares the old and new formulas, using various ages. For a 40-year-old, 180 is the maximal heart rate in both formulas. For a 60-year-old, the new formula increases maximal heart rate by 5% -- from 160 to 166. For an 80-year-old, there's a 10% difference, from 140 to 152 -- "considerably different," says Manoukian. For a 20-year-old, the numbers decrease by 5%, from 220 to 200.
If the new formula is adopted by the American Heart Association, it will change the advice doctors give their patients, he says. The predicted maximum heart rate reflects what people should achieve when they do their physical activity.
"Heart rate is a reasonably good indicator of the target rate at which any level of athlete should be exercising, but it tends to be variable based on your physical shape," says Manoukian. "If you're in bad shape, your heart rate jumps up quickly; if you're in good shape, it tends to be harder to achieve."
As a rule of thumb, he says, for someone first starting an exercise program, "we would advise working to 50% of your maximum heart rate, then gradually up to 75 or 85%. With healthy adults, we advise to keep challenging themselves to exhaustion."
Even more importantly, the new formula would affect exercise stress tests that cardiologists give to people with heart problems.
"If you came into my office with chest pain, I would put you on a treadmill and monitor your ECG -- I would like to get your heart rate up to 85% of your predicted maximum heart rate," says Manoukian. "Only at that level can a cardiologist provide any significant information about heart function. If I'm inaccurately calculating what maximal is for you, I may not be getting the maximal test."
Also, those with heart disease -- especially if they're over 50 and 60 -- may also be able to exercise at higher heart rates than previously thought, Manoukian tells WebMD.
For most healthy adults without heart problems, the American Heart Association's FIT (frequency, intensity, and time of exercise) recommendations are: frequency of five times week, intensity that is "somewhat hard," and 30-60 minutes of exercise at a time, he says.
"I often tell people to exercise to the point where it feels challenging, that you could have a conversation but exercise might make you somewhat breathless," says Manoukian. "If you're not breathless, you're not exerting enough. You should perceive the exercise to be somewhat difficult."
The American Heart Association has no plans to change its current policy endorsing the 220 - age formula, says Gerald Fletcher, MD, a cardiologist at the Mayo Clinic in Jacksonville, Fla., and a member of the AHA's Council on Clinical Cardiology. "There are many ways to look at maximal heart rate, and the current formula still gives a reasonable prediction," he tells WebMD. "This might in the future help us look at things differently, but right now we're going to stay with it."
While he admits that the current formula probably does underestimate an adult's capability, "with older people, we like to look beyond heart rate to perceived exertion," Fletcher says. "We also find it's easier to train people not to overdo it, to exercise 'til they get tired or exhausted, and not to emphasize meticulous counting of heart rate."
In conducting treadmill tests, cardiologists don't rely too much on the formula anymore, Fletcher says. "We really don't use it as much as we used to," he tells WebMD. "But if you need a formula, the 220 - age is what we're still using."