We all know we should be aiming to exercise at least three times per week. But if you can't afford to join a gym or you're too tired to set foot on the treadmill, then why not take a tip from the women who have hit upon an alternative way to get fit - sex. According to a new survey from Aspire drinks, 76 per cent of women are more inclined to get busy between the sheets if they think it will burn calories. And almost two thirds of women who already have gym memberships said they were more keen to have sex and carry out household chores like vacuuming if they thought it would burn as many calories as a conventional workout. According to Kerry McCloskey in her book The Ultimate Sex Diet, a half-hour sex session can burn anything from 150 to 350 calories, depending on how active you are. There are some celebrity fans of sexercise, too. Sex and the City star Kim Cattrall says that regular sex helps her to stay in shape, while Mel B says that it helped her to shift her baby weight after her last pregnancy. And as she's recently announced that she's pregnant again, I suppose we'll have to take her word for it... A horizontal workout has other benefits, too. The NHS Direct website says that sexercise can reduce the risk of heart attack and even help people to live longer. Personal trainer and Body Guru Gina Hemmings told AOL Lifestyle: "Sex raises the heart rate, which speeds up the metabolism and helps you to burn more calories. "So if you have three half-hour sessions per week, you can expect to burn about 800 calories in total. "Sex also encourages your body to release endorphins, which will boost your mood, improve your confidence and give you more energy. "Not only that, if you're having regular sex you're also more likely to feel body confident - and you'll be more motivated to keep fit so that you can look your best in the bedroom."