Somewhere near the top of every single one of the the roughly forty-kajillion internet listicles dedicated to the "surprising," "hidden," and "unexpected" health benefits of sex is the not-all-that-surprising-sounding factoid that bumping fuzzies basically doubles as exercise. In reality, however, there has been very little research done to support this claim.
The few studies that have investigated the physicality of sex have typically looked at things like heart rate and blood pressure – important but arguably basic physiological measurements. They've also been conducted primarily in laboratory settings – which, sure, probably falls into some specific category of kink, but for most people is probably a less-than-ideal environment for sexy time. It's not difficult to imagine, for example, how the wires from an echocardiogram, or the bulk of an oxygen-monitoring facemask, might interfere with one's (doubtless considerable) sexual talents, thereby confounding any attempt at accurate physiological measurement.
The point being that these methodological limitations highlight a gap in the existing body of scientific knowledge raises an important question about how physically strenuous sex really is. How much energy does a young, healthy couple actually expend getting physical between the sheets? Are we talking a pastrami sandwich's worth of calories, or a handful of kale's? And to what extent does sex really count as exercise?
In search of answers, researchers led by Université du Québec à Montréal kinanthropologist Antony Karelis set out on a mission:
...not to examine the physiological responses (i.e. heart rate and blood pressure) of sexual activity per se but to get a better understanding of the energy expenditure in kilocalories, which is a unit that is more commonly used today by health professionals and the general public for health purposes.
And so, in a study recounted in the latest issue of PLoS ONE, Karelis and his colleagues asked twenty-one young, healthy heterosexual couples to engage in amorous congress in the name of science. (Amorous congress here defined as "the onset of foreplay, intercourse and at least one orgasm by either the man or woman and ended at the couple's discretion.") Energy expenditure was monitored using a portable mini SenseWear armband*. The goal: measure the free-living energy expenditure (in calories) during sexual activity, in the absence of drugs, alcohol, or ED medications. (Study participants were also asked to forego any and all paraphilic sexual activities – i.e. nothing deemed too freaky by... well... society, we guess.) The final figures are as follows:
So the overall average comes out to roughly 85 kCal (3.6 kCal/min) – about the same number of dietary calories in your standard chicken egg.
So how does sex stack up against traditional forms of exercise? To find out, Karelis and his colleagues also had their test subjects spend thirty minutes engaged in an endurance exercise session that involved a five-minute warmup (walking); followed by 30 minutes of exercise on a treadmill at ~65 % of maximal heart rate; followed by another five-minutes of walking for a cooldown.
Men expended an average of 276 kCal (or 9.2 kCal/minute), while women expended an average of 213 kCal (7.1 kCal/minute). But the researchers wanted to know more than the total number of calories burned – they also wanted to measure the intensity of this workout relative to sex. To do so, they relied on Metabolic Equivalents (or "METs"), a physiological metric defined by the World Health Organization (WHO) as the ratio of a person's working metabolic rate over their resting metabolic rate:
One MET is defined as the energy cost of sitting quietly and is equivalent to a caloric consumption of 1kcal/kg/hour. It is estimated that compared with sitting quietly, a person's caloric consumption is three to six times higher when being moderately active (3-6 METs) and more than six times higher when being vigorously active (>6 METs).
During the treadmill exercise, men and women exercised at a mean intensity of 8.5 and 8.4 METS, respectively – well within the WHO-defined range of "vigorous" activity.
Mean exercise intensity values were slightly lower while couples were having sex, coming in at 6.0 METS for men, and 5.6 METS for women – but that still puts them in the range of moderate physical activity. And remember, that's the mean measurement of intensity, which suggests some of these test subjects were edging, so to speak, into vigorous territory.
This seemed to hold especially true for the men in the study, whose absolute energy expenditure was shown, at times, to be even higher than that of the mean energy expenditure of the thirty minute exercise. (The greatest energy expenditure achieved by a man in the study was 306.1 kCal, compared to the mean energy expenditure of 276 kCal.) The researchers write that this was not observed in the the women who participated in the study.
The upshot, explain the researchers, with an ample helping of hedging:
These results suggest that sexual activity may potentially be considered, at times, as a significant exercise.
Which literally translates to: "Sex qualifies as significant exercise sometimes. Maybe. Possibly."
The authors admit to several limitations in their study, including the fact that their cohort is composed exclusively of young, sexually active, healthy young men and women which reflects "strict inclusion and exclusion criteria." There are, of course, other limitations. The requirement that test subjects avoid paraphilic sexual activities, for example, while certainly useful for the purposes of data normalization, inevitably winnows out sexual practices that could register as more "vigorous."
Be that as it may, it's reasonable to assume that the researchers' most important conclusion probably applies to pretty much anybody, at any age, of any sexual persuasion. As the researchers note: "both men and women reported that sexual activity was a highly enjoyable and more appreciated than the 30 min exercise session on the treadmill."
The researchers findings appear free of charge in the latest issue of PLoS ONE.