Before television commercials for drugs like Viagra and Cialis became major sponsors of the nightly news and every Major Sporting Event, the term "safe sex" was generally associated with the prevention of sexually transmitted diseases like HIV. Nowadays, men with graying temples regularly appear in TV ads, soaking their libidos in claw-foot bathtubs or behind the wheels of vintage muscle cars as they counsel viewers to "ask your doctor if you're healthy enough for sexual activity."
Healthyenough? Has sex suddenly become like high-school sports, where you need a physical from your doctor and a permission slip in order to participate? How much of a physical feat is intercourse, and what kind of strain does it put on our bodies? Should men who aren't taking erectile dysfunction drugs worry, too? As someone who might qualify as the doctor in "ask your doctor," I thought I should find out.
It turns out that sex, that pinnacle of male machismo, is more like Disney's Matterhorn than Mount Olympus. Whatever it feels like, sex is not a particularly strenuous physical endeavor. We know this in part from a 1984 study that involved 10 married couples who were paid to have intercourse in a monitored lab setting. Blood pressure, pulse, and oxygen consumption were recorded, but only for the men; no one paid attention to the women. (Indeed, medical research is only now beginning to emerge from its dark, sexist past.) Foreplay was allowed, but the "results-oriented" nature of the experiment (the husband held an event-marker button to be pushed at the beginning and the end of his orgasm) did tend to play into a male view of sex. As the paper noted, "Some physical expression … was limited by the recording equipment. For example, the mask used to collect the husband's expired air kept him from kissing or talking."
The men's levels of physical exertion during sex were quantified in terms of metabolic equivalents, or METs. (In that system, 1 MET represents the amount of energy expended while a subject sits quietly in one place, not even twiddling his thumbs.) The data showed that foreplay created small increases in cardiac and metabolic expenditures, but the big surge of effort came during the 10 to 16 seconds it took on average to achieve "Surrender Dorothy." When the man was on top, he spent an average of 3.3 METS on coitus; with the woman on top, he needed just 2.5 METS. Meanwhile, stimulation-via-partner and the do-it-yourself method expended only 1.7 METS. Researchers found a fair amount of variability among the different couples. For example, one man expended 2.0 METS and another 5.4, proving that sex is different for everybody.
The results of this seminal 1984 study pushed gigolos from the ranks of professional athletes and placed sexercise on the border between what the U.S. health department categorizes as "light-intensity" (1.1 to 2.9 METS) and "moderate-intensity" (3.0 to 5.9 METS) exertion. This lumps sexual activity in with water aerobics; folk dancing; ice skating at a leisurely pace; snorkeling; bagging grass or leaves; playing hopscotch, foursquare, dodge ball, T-ball, or tetherball; or putting groceries away. Since one man's do-si-do might be another man's promenade, a more standardized and reproducible comparison equates workloads for sexual activity with walking a mile in 20 minutes or climbing 2 flights of stairs in 10 seconds.
As curious as these comparisons may be, they portray intercourse as a kind of sport. Robert DeBusk, a cardiologist at Stanford School of Medicine and an expert in the cardiovascular risks of sex, cautions against seeing the physiological demands of sex as being entirely visible or physical. Emotional arousal—adrenaline—has potent effects on blood pressure and pulse. "If you look at people who have had sex with a customary partner and you find out what the average heart rate is during that experience," DeBusk told me, "and then took the heart rate achieved by the same person with a novel partner, the heart rate is going to be 20-plus beats higher with the novel partner than with the customary partner."
So we know that sexercise gives you a light-to-moderate workout, but will it cause a heart attack? Can a roll in the hay turn into a long dirt nap?