Unscrambling the mystery of METs

So you’re chugging away on a cardio machine, peeved that the “calories burned” display isn’t rocketing skyward. You hit the mysterious “MET” button and squint at the readout. How’s that? 6.2?

Does anyone have a clue what this number means?

Calculating METs -- or metabolic equivalent units -- can offer proof you’re accomplishing something fitness-wise.

Think of METs as shorthand for how much oxygen your body is consuming. 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 at, say, 3.5 miles per hour burns 3.8 METs per minute.) That value is the same for most, 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. Haskell helped develop the Compendium of Physical Activities Tracking Guide, a list of activities and their MET values.

Here’s how METs differ from calories. If two people weigh the same amount, their calories burned walking a mile would be equal. But a 220-pound person would expend double the calories of a 110-pound walker.

Thus, the beauty of METs: We can all use the same scale, without elaborate calculations.

So is this a better way to gauge your workouts than by using 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 fitter.

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. 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 activities burning 3.0 to 6.0 METs qualify as moderate intensity; those using more than 6.0 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% of that and approach their capacity only during interval training peaks.

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Tracking exertion

Here are some MET measurements for common activities according to the Compendium of Physical Activities Tracking Guide.

*--* Activity METs Mild Walking at 2 mph 2.5 Golfing (with cart) 2.5 Stretching, hatha yoga 2.5 Moderate Walking 3.3-4.5 Leisurely cycling 3.5 Doubles tennis 5.0 Low-impact aerobics 5.0 Punching bag 6.0 Vigorous Swimming freestyle laps 7.0 Push-ups, sit-ups, pull-ups 8.0 Step aerobics 8.5 Running(10 min./mile) 10.2 Skipping rope 12.0

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