Gretchen Reynolds examines research on the extent to which sex is exercise:
[Professor of exercise science Antony D.] Karelis and his colleagues recruited 21 young heterosexual committed couples … and had them jog on treadmills for 30 minutes, while researchers monitored their energy expenditure and other metrics, in order to provide a comparison for the physical demands of sex. The scientists next gave their volunteers unobtrusive armband activity monitors that gauge exertion in terms of calories and METs, or metabolic equivalent of task, a physiological measure comparing an activity to sitting perfectly still, which is a 1-MET task. Then the scientists sent the couples home, instructing them to complete at least one sex act a week for a month while wearing the armbands, and to fill out questionnaires about how each session made them feel, physically and psychologically, especially compared with running on the treadmill.
When the researchers analyzed all of the resulting data, it was clear, Karelis said, that sex qualified as “moderate exercise,” a 6-MET activity for men and 5.6-MET activity for women. That’s the equivalent, according to various estimates, of playing doubles tennis or walking uphill. The jogging, by comparison, was more strenuous, an 8.5-MET activity for the men in the study and 8.4 for women. (Though some men, according to their activity monitors, used more energy for brief periods during sex than they did jogging.) The sex also burned four calories per minute for men and three per minute for women, during sessions that ranged from 10 to 57 minutes, including foreplay. (The average was 25 minutes.) Men burned about 9 calories per minute jogging and women about 7.