Several Moving Crew chat participants have asked recently if their hearts were beating too fast during strenuous cardio exercise. Should they worry if their heart rates hovered in the zone beyond 90 percent of maximum?

(First, the inevitable caveat: As well-informed and lucid as your loyal Crew servants are, we are not, how you say, cardiologists. If you are not healthy generally -- or if you have or are at elevated risk for hypertension, diabetes, high cholesterol or coronary heart disease -- your physician is the person best qualified to answer this question. And no matter who you are, if exercise causes dizziness or chest pain, stop what you're doing and contact a doctor.)

Okay, then: In healthy, active people, the news is good. The body has mechanisms to prevent you from hurting or killing yourself by overdoing cardio exercise. If you are moving along at a fairly comfortable pace -- and even punctuate it with hard sprints that make your heart spike briefly -- your heart will beat in a safe range, no matter what the number on the screen says.

"You cannot sustain heart rates above about 90 percent of your maximum heart rate for more than a few minutes," said Barbara E. Ainsworth, a professor in the Department of Exercise and Nutritional Sciences at San Diego State University and a fellow with the American College of Sports Medicine. Fatigue will force you to stop exercising well before you have a cardiac event.

The concerns about apparently off-the-chart heart rates come from the fact that the popular calculations of maximum heart rate, or HRM, are blunt instruments. The widely accepted HRM formula of 220-minus-your-age -- that's the one you see on the cardio machines at the club, which is where most of this mischief starts -- can be off by a dozen or more beats per minute based on genetics: Some people's hearts can pump quicker than others.

Further, running generates a higher HRM than cycling, rowing or swimming, because running engages more muscle mass. So maxing out on a treadmill will yield an HRM up to six beats higher than that on a bicycle and up to 14 beats higher than what you could reach swimming.

A formula (for running) shown to be more accurate than 220-minus-your-age is 217-minus-(0.85 x your age), but that still doesn't account for genetics. No matter how precise the math, you still may see numbers on the treadmill's screen that are scary-high only because your heart rate naturally differs from the norm. The bottom line is that no matter what the number says, your body will make you quit before you can do damage.

You can learn your precise HRM for running via a stress test conducted by cardiologist or sports medicine physician. But why bother?

It's always safe to use the "talk test": If you can hold a conversation, just barely, while you're working out, you're in a zone that will improve your cardiovascular fitness. Once you get strong, add intervals -- brief, harder bursts of effort -- to make your heart-lung apparatus even more efficient. As you improve, try to add intensity or time to your intervals, and trim the recovery periods in between.

But don't worry about the number on the screen. Take it to a numerologist for a reading, or use it as your locker combination. Or bet the numbers in the lottery.

If that number hits, we trust you'll remember it was your good friends at the Moving Crew who made it all possible.

No chat today; back next week. E-mail us at move@washpost.com. (And to the guy who sent us the e-mail about research behind a recent column: please resend your message. We wiped it out by mistake. Really.)

-- John Briley

If you're healthy, your heart can handle the spikes in activity.