As far back as the 1950s, couples have been asked to strap on monitors, blood-pressure cuffs, oxygen masks and other paraphernalia and copulate, to scientifically quantify the impacts of sex. The focus is often on whether sex can kill you by precipitating a heart attack.
Happily, these studies generally show that heart rates rise during intercourse, but tolerably. In a 2008 study, middle-aged subjects' heart rates jumped at the point of orgasm by only 21 beats per minute in men and 19 in women, about the same response as if they'd just done a few jumping jacks. The risk for sex-related cardiac arrest is, in fact, vanishingly small, statistics show, though it may be greater when the act is extramarital.
The issue of sex as exercise, however, has remained largely unexplored. "There are these myths," including that sex burns at least 100 calories per session, said Antony D Karelis, a professor of exercise science at the University of Quebec at Montreal who undertook a study, published in PLOS One in October, to look at how much energy is actually exerted during sex. "But nobody had tested" those assumptions.
To do so, Karelis and his colleagues recruited 21 young heterosexual committed couples from the local area 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 analysed 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.)