So you're chugging away on a cardio machine, growing increasingly peeved that the "calories burned" display isn't rocketing skyward. You hit that mysterious "MET" button and squint at the readout. How's that? Does anyone have a clue what this number means?

You've come to the right place.

Calculating METs -- or metabolic equivalent units -- can offer proof you're accomplishing something fitness-wise, just as "calories burned" can. Think of METs as shorthand for how much oxygen your body is consuming, at rest, work or play. Bear with us for a moment; we'll make the egghead part as brief and painless as possible: One MET equals 3.5 milliliters of oxygen per kilogram of body weight per minute -- which is what our bodies require at rest. The more effort an activity requires, the higher its MET value. (Walking briskly at, say 3.5 miles per hour, burns 3.8 METs per minute.) That value is the same for most of us, regardless of fitness level.

"If you and I walk a mile at the same pace, we will consume the same amount of oxygen and burn the same number of METs," said William L. Haskell, a professor at Stanford University's School of Medicine, who helped develop the Compendium of Physical Activities Tracking Guide, a fairly exhaustive list of activities and their MET values. The person in worse shape will work harder to make the mile and won't be able to sustain the pace as long, Haskell said. If both people weighed the same, the calorie burn would be equal as well. But if one person weighed 110 pounds and the other 220, the heavier person would expend roughly double the calories of the lighter walker.

Thus, the beauty of METs: We can all use the same scale without the need for elaborate metric system calculations. "People who are trying to lose weight can choose activities with high MET values," Haskell said.

So is this a better way to gauge your workouts than heart-rate zones? "Not necessarily," said Conrad Earnest, director of the Center for Human Performance and Nutrition Research at the Cooper Institute in Dallas. "It's just another method."

METs come in handy in two instances: To track your cardio fitness progress over time, you can work toward sustaining higher MET levels for longer periods, and hitting higher MET values during your intervals. Or, if you have a cardiac condition, your doctor can use a MET capacity test to prescribe a safe exercise zone. (This can be done with heart-rate values as well).

An individual's capacity is the highest MET number he or she can sustain for a few minutes, Earnest said. You can increase this capacity by getting more fit. A healthy 50-year-old man should have a capacity of at least 9.2 METs; a healthy 50-year-old woman should clock in at 8.2 METs or higher, according to a recent study on women's fitness in the New England Journal of Medicine. (Some other age-related targets: For men age 20, 13.5 METs; age 30, 11.4 METs; age 40, 10.3 METs. For women age 20, 12.1 METs; age 30, 10.8 METs; age 40, 9.5 METs.)

Too much to remember? Try this: The Centers for Disease Control and Prevention says that, for most people, activities burning three to six METs qualify as moderate intensity; those using more than six METs are vigorous. Your METs capacity is your upper limit, not a recommended workout level. To boost fitness, people should exercise at 60 to 85 percent of that and approach their capacity only during interval training.

We're online from 11 a.m. to noon today to chat about this and other parts of the fitnosphere: www.washingtonpost.com.

-- John Briley

Health: 08/23/05: Close-up of a young man jogging on a treadmill