Sex as Cardio
Sex has many of the same effects on the body as an intense workout. During sex, your heart races, your metabolic rate increases, you burn calories, and stretch and strengthen muscles. Sex has also been shown to boost the immune system and provide stress relief, just like a good cardio session.
The "average" sex session lasts about 30 minutes and burns between 50 and 100 calories. It doesn't sound like much, but sex is a physical activity like any other where you get out of it what you put into it, so if you are vigorous between the sheets, you'll burn more. That number also relies on many factors like body weight, duration and position to determine an exact burn.
Sex as Strength Training
As far as sex counting towards strength training, you do use a lot of muscles, but which muscles and to what degree is determined by your nookie style. If you prefer to be on top, the legs and arms get a lot more action than if you are a passive partner. The thrusting motion of sex is great to firm the stomach and glutes, which can improve posture. Sex, for women, has the added benefit of engaging the pelvic floor and abs. Kegels, an exercise important for strengthening the pelvic floor, can also make sex more pleasurable for you and your partner, so ladies, take full advantage of that.
This all sound like a big ol' yes toward sex counting as a workout, but before you cancel your gym membership, there are a few things to consider. The proper definition for strength training is working all your major muscle groups through their full range of motion against resistance. While you may feel a burn in certain areas, sex isn't thorough enough to count as real strength training. In terms of cardio, if you're really into it, you can accelerate your heart rate enough to burn some serious calories, but chances are you'll be focusing more on your partner than making sure your heart rate stays in the aerobic stage.
The Bottom Line
Here's the thing: sex is a great way to get your blood pumping and burn calories, and every once in a while, for the sake of your relationship, you can skip the gym for a private session with your partner. For the sake of your health, and how you look during sex, however, make a habit of engaging in both regularly. You're better off getting sweaty in the gym with your partner first, and then reaping the sexual benefits of your workout afterward.
Kelly Turner is a Seattle-based ACE-certified personal trainer and professional fitness writer. She began writing after becoming frustrated with the confusing and conflicting fitness information in the media and the quick-fix, gimmick-centered focus of the fitness industry itself. Her no-nonsense, practical advice has been featured on DietsInReview.com, FitnessMagazine.com, Yahoo! Shine, and she has a regular fitness column in The Seattle Times. Kelly has her own blog at www.kellyturnerfitness.com or follow her on Twitter @KellyTurnerFit.