Q: Some exercise machines provide readouts in METs. What does this mean? - C.B., Piscataway, NJ

A: MET is short for metabolic equivalent; it's a system that may be used to prescribe exercise intensity. This system is based on the amount of oxygen used to produce energy. The human body, at rest, uses about 3.5 ml of oxygen per kilogram (2.2 pounds) of body weight. This resting metabolic rate is quantified as 1.0 MET. The amount of oxygen consumed is directly proportional to the energy you expend during exercise, so at 2 METs you are using approximately twice as much energy as when resting.

Some examples of MET levels are:
Walking, 30 min/mile 2.5 MET
Walking, 24 min/mile 3.0 MET
Calisthenics 4.5 MET
Cycling, 9.7 mph 5.0 MET
Walking, 15 min/mile 6.5 MET
Running, 8 min/mile 9.0 MET
Swimming, 2.5 min/100 yds 10.0 MET
Running, 6 min/mile 15.0 MET
Swimming, 1.6 min/100yds 20.0 MET
Running, 4 min/mile 30.0 MET

Although the MET system is a useful guideline, it is not exact, since it fails to account for individual differences in physical conditioning. The more fit you are, the lower your MET rating for any given workout will actually be.

Source: fitlinxx.com