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Why monitors -- and heart rate recommendations -- aren't always the best gauge of how hard you should work out.

Fitness buffs have long turned to heart rate monitors -- either ones they own or those found on exercise machines -- as a way of gauging whether they're exercising hard enough. But heart rate monitors aren't as helpful as you might think.

The problem? First off, the numbers used to calculate your maximum heart rate are a little squishy. The traditional calculation has been based solely on age, but factors like fitness level and genetics also play a role in just what heart rate is healthy or even safe for individuals. If you're quite sedentary, for instance, and you try to reach the peak heart rate for your age group, you could get into cardiovascular trouble quickly -- trouble that includes breathlessness and dizziness. In fact, the latest research suggests that the traditional calculation isn't right for, oh, just about half of the population.

New Heart Rate Guidelines for Women

Under the old guidelines, you were told to subtract your age from 220 for an estimate of your maximum safe heart rate and then multiply it by 85% to get your "target" heart rate. But the studies that gave rise to this equation were only done on men. And, after looking at the correlation between heart rate and heart attack in the exercise tests of more than 5,000 women, researchers at Northwestern Medicine in Chicago found the maximum heart rate for a woman really should be determined by subtracting .88 of her age (her age multiplied by 0.88) from 206. 

The new math may sound like gobbledygook, but here's the takeaway: If you're a woman and if you've found the goals hyped by personal trainers and elliptical machines to be too strenuous, you're justified in slowing down a bit. A 40-year-old woman who has been aiming for a maximum rate of 180 beats per minute (with a target rate of 153 beats per minute), for instance, can now strive for a maximum rate of 171 beats per minute (with a target rate of 145 beats per minute).

More important, while treadmills and elliptical machines often let you monitor your heart rate (and even provide tidy charts of "fat burning" and "cardiovascular training" zones), tailoring your workout to your heart rate is not as important as tailoring it to how you feel. That is, you can have an excellent workout at a lower or higher heart rate than what's recommended, depending on what you do and how long you do it -- and as long as it's fun.

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