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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.

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