Strenuous exercise is scientifically defined as any activity that expends 7 metabolic equivalents (METS) per minute or more, according to the 2008 Physical Activity Guidelines for Adults issued by the Department of Health and Human Services. In relative terms, this is the equivalent of an energy expenditure of 7 on a scale of 1 to 10, with one representing the energy expended while sitting quietly and 10 representing maximum effort.
The Physical Activity Guidelines for Americans were issued by the Department of Health and Human Services as a resource to help Americans combat obesity and heart disease. According to the guidelines, the recommendation for adults is 150 minutes of moderate intensity activity or 75 minutes of vigorous intensity activity — the equivalent of 500 to 1000 MET minutes — per week. Because determining absolute intensity using METS is somewhat difficult, the guidelines consider relative exertion to be an acceptable measure of intensity, as well. In other words, if a person feels as if he is exercising at a moderate level, or 5 to 6 on a scale of 1 to 10, he probably is expending the requisite number of METS per minute (about 3.5 to 6.5.)
Another way to determine exercise intensity is as a percentage of one’s maximum heart rate, defined as the target heart rate. According to the American Heart Association, a person’s maximum heart rate is roughly 220 minus his age, and his target heart rate for physical activity is between 50 percent (moderate intensity) and 85 percent (high-intensity) of that. In other words, a 40-year-old man has a maximum heart rate of 180. Thus, his target heart rate is about 90 for moderate-intensity exercise, and for vigorous exercise it is about 153. These guidelines vary according to each individual’s level of fitness and overall health.