DAN Medical Frequently Asked Questions

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Calculating Physical Activity Intensity

The intensity of any physical activity can be calculated directly — by measuring the amount of oxygen you use for energy metabolism (a factor that's abbreviated as VO2, short for "volume of oxygen") per minute of exercise — or indirectly — by measuring your heart rate and using that value as an index of the strain your exertion is placing on your heart and lungs.The amount of energy you use at any given time is proportional to the amount of oxygen your body requires. At rest, the average healthy person uses roughly 3.5 milliliters of oxygen per kilogram of body weight per minute; this is known as "resting metabolic rate." The energy cost of a physical activity can be expressed as a multiple of resting metabolic rate; this is known as "metabolic equivalent of task," or simply metabolic equivalent, and is abbreviated as MET.

An individual of average fitness can achieve about a 12-fold increase in metabolic rate (which is expressed as "12 METs"), while top athletes can exceed a 20-MET increase.
The table to the right lists examples of activities classified as being of light, moderate or vigorous intensity, based on the amount of energy required to do them.

Sources: "Compendium of physical activities: an update of activity codes and MET intensities"; "Oxygen consumption in underwater swimming"; and "Oxygen uptake studies of divers when fin swimming with maximum effort at depths of 6–176 feet" (see "Further Readings and Sources" for details on these sources).

An individual's peak aerobic capacity is expressed as maximum oxygen uptake while engaged in all-out exercise (which is abbreviated as "VO2 max"). Measuring VO2 max accurately requires following strict protocols in a sports-performance lab — a procedure known as a "maximal exercise test." Conducting such tests is time-consuming and expensive, so they are used only in special situations. It is also possible to make a relative estimate of an activity's intensity by measuring its effects on your heart rate and respiration rate. This can be done in several ways.

Talk test: If an average healthy person can talk but not sing while exercising, that activity is considered to be of moderate intensity. A person engaged in vigorous-intensity activity is not able to say more than a few words without pausing for a breath. If you must gasp for air and cannot talk during what is generally considered to be moderate-intensity exercise, it means that your physical capacity is below average.

Heart rate test: Your heart rate rises in a regular fashion as the intensity of your activity increases (though the maximum heart rate you're able to achieve will decline as you age). You can figure the average maximum heart rate for healthy individuals your age by subtracting your age from 220. For example, the maximum heart rate for a 50-year-old person would be calculated as follows: 220 - 50 = 170 beats per minute (bpm). You can then use your actual heart rate to estimate the relative intensity of various activities you engage in and to indirectly estimate your maximum exercise capacity. Experts often recommend reaching and sustaining a certain heart rate to improve or maintain fitness.

Submaximal exercise test: A submaximal exercise test can be used to figure your maximum exercise capacity without exceeding 85 percent of the estimated maximum heart rate for your age. Conducting such a test calls for gradually increasing your exercise intensity, based on a defined protocol, while your heart rate is being monitored. When you reach the target heart rate, you stop exercising, and your maximum exercise capacity can then be extrapolated using various methods. However, because of variations in the relationship between heart rate and exercise intensity due to age, fitness level and other factors, an indirect estimation of maximum aerobic capacity has limited value. Nevertheless, the test is still a valuable clinical tool to assess an individual's tolerance for exercise and likelihood of having ischemic heart disease.


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