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You Just Had Sex, So How Many Calories Did You Burn?

Thirty years ago, 10 men went to a lab, masturbated, had sex with their wives — pressing a button while they orgasmed — and ran on a treadmill.

They did all this for a study, reported in the Archives of Internal Medicine, that found sexual activities were “the equivalent of light to moderate exercise.” In flagrante, orgasms made the men’s hearts beat fastest, but climax averaged only 10 to 16 seconds.

Trying to measure the health benefits of sex is difficult, as it is for any activity. (I discuss this in-depth in my feature this week about fitness trackers.) The main problem is the wide variation among people, including a range of body types and exercise styles. How can we make sense of the different ways different bodies process different exercises, including ones that take place in bed?

That 1984 study, by researchers at Southern Illinois University and the University of Minnesota, found that heart rates were determined more by the variation among subjects than by the type of sex (e.g. man-on-top vs. woman-on-top), even with a relatively homogeneous group of volunteers. Characteristics such as body type mattered more than position. For that reason, the researchers cautioned against public-health messaging comparing sex to, say, climbing two flights of stairs.

Other factors may also have skewed the numbers: The men were having sex in a lab while following protocols — “The couple then was instructed to begin foreplay” — and wearing lab equipment to measure their vitals. As the authors drily noted, “The mask used to collect the husband’s expired air kept him from kissing or talking, and the ECG electrode wires and BP cuff hoses restricted body movement.”

Not exactly realistic or romantic.

The study nonetheless was about as good a source as Stephen D. Herrmann, an exercise scientist at Augustana College, and his co-authors could find for their Compendium of Physical Activities, a guide to the calorie-burning value of hundreds of activities. The 2011 compendium reports that having sex burns 2.8 times more calories than sitting still. Kissing and hugging is just 1.3 times as valuable. (Table tennis scores a 4.)

Last year, researchers at the University of Quebec in Montreal and the University of Montreal conducted a new sex study. This time, armbands did the measuring, leaving subjects — men and women — to have sex at home and without a mask. Like in the 1984 study, the researchers found that sex provided only a moderate exercise benefit — treadmill walking burned calories faster — but the new research method allowed for more subjects and a more specific measuring of burned calories.

The findings still might not apply to you: The sex might have been different because it was being studied, the sample size was small and not especially diverse, and each person’s results varied greatly.

“Human sexuality is relatively new” as an area of study, study co-author Antony D. Karelis, an exercise scientist, said in an interview. “I believe in the future we will be able to study more and more without any problems.”

Carl Bialik was FiveThirtyEight’s lead writer for news.