Q: Are METs better for prescribing and monitoring exercise intensity than target heart rate (THR) or ratings of perceived exertion (RPE)?

A: All physical activities can be classified on the basis of their oxygen requirements. The amount of oxygen consumed is directly proportional to the energy expended during the activity. At rest, the body consumes approximately 3.5 ml of oxygen per kilogram of body weight per minute.

The resting level of oxygen consumption is one MET. Thus, an eight-MET level equals eight times the oxygen used at rest. Although the MET method can be used for prescribing exercise intensity, it has two serious limitations.

First, various environmental factors such as heat, humidity, cold, wind, altitude, pollution, differences in terrain, etc., can change the way a person’s cardiovascular system responds to a given MET level. As a result, a person’s cardiovascular system may be working harder at the same MET level.

Also, as an individual’s level of fitness improves, different MET levels are required to ensure that the training stimulus is sufficient to produce continued fitness improvements. For these reasons, compared to MET equivalents, both target heart rate (THR) and rating of perceived exertion (RPE) are more commonly used to prescribe and monitor exercise intensity.