For years, people striving to keep fit have been told to hit their target heart-rate zone. Trouble is, the formula is only an estimate based on your theoretical maximal heart rate. Your true maximal heart rate may be as much as 15 beats per minute higher or lower. Consequently, you may be exercising at an intensity that is either too easy or too strenuous (and potentially dangerous).

"You should never rely solely on the numbers," says Patrick Hagerman, EdD, clinical assistant professor of exercise and sport science at the University of Tulsa in Oklahoma. Factors that may affect your maximal heart rate include certain drugs, such as beta-blockers, and eating too few carbohydrates. Other factors: stress, heat, dehydration, and the type of exercise itself.

If you think that's all too complicated to factor in before a jog, you're right. Fortunately, there's an easier way; it's called rating of perceived exertion. You rate the intensity using a scale of 0 to 10. A zero equates to sitting on the couch eating corn chips. A 10 is an all-out, gasping-for-air, praying-for-deliverance sprint.

"You have to listen to your whole body and rate the totality of the sensations," explains Robert Robergs, PhD, director of the exercise physiology laboratories at the University of New Mexico in Albuquerque. If a walk feels somewhat hard and a light sweat is dotting your brow, give it a 4. For an optimal workout, stay between 4 and 6. At the 6 level, says Hagerman, you should be "somewhat out of breath. Conversations should be labored, not chatty." A lot of physical discomfort is a sign you're overdoing it, no matter what your heart-rate monitor says. Ease off a bit.

Tip: To determine your target heart-rate zone, find your maximal heart rate (220 minus your age) and then exercise within 60-80 percent of that number. If you're 40 years old, your maximal heart rate would be 180 beats per minute with a target of 108-144.

Originally published in Better Homes and Gardens magazine, April 2005.