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‘Sex makes you happy. Happy people do not run a 3:47 mile.’ So said American runner Marty Liquori many years ago. He was adding to an old debate: what impact does sex have on athletic performance? Back in 444BC Plato urged athletes to abstain before competition. But in 77AD Pliny the Elder said ‘athletes when sluggish are revitalised by lovemaking.’ Much, much later football managers of the 1970’s and 1980’s turned the ‘nothing after Wednesday if you’re playing on Saturday’ rule into sporting cliché. More recently, Brazilian superstars Ronaldo and Ronaldinho have extolled the virtues of ‘scoring’ the night before a big game. The abstinence argument was based on the perceived effects of pre-event exertion on physical and mental strength: it was felt sex would burn too many calories and dull the edge needed for competition.

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In the last 20 years, research in this area has backed Pliny. In 1995, a US study of treadmill runners found no difference in VO2 max between those who had had sex 12 hours before an event and those who hadn’t. Another study, from Colorado State University, US, found that men who had abstained from sex for six days did no better on a power test than men who had had sex the previous night. So while there is no evidence that sex before an event gives you an advantage, it seems there are no disadvantages, either. The exception, say researchers in Switzerland, is if you get frisky within two hours of a race, in which case you may suffer from a lack of concentration and focus.

As a post-race ritual, however, women should pay particular attention: researchers at Rutgers University, US, discovered that orgasms activate the anterior cingulate cortex and the insula – two areas of the brain associated with an analgesic effect. The takeaway is that an orgasm could be a pretty effective recovery tool. There are potential mental gains for both sexes, says psychosexual therapist Denise Knowles. ‘From a clinical point of view it’s never made sense to keep sport and sex apart,’ she says. ‘They’re both physical and enjoyable pastimes, so why wouldn’t one enhance the other? When we have sex we release feel-good endorphins and when we feel better we have more confidence. That mood boost, allied to the training you’ve done, can make a difference on race day.’

Perhaps this is why a survey by the London Marathon revealed that those runners who’d had sex the night before the race finished an average five minutes quicker on race day than those who’d abstained.

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How does running affect your sexual ability?

Forget popping the little blue pills and pop on your running shoes, instead. It seems there’s a link between treadmill endurance and sexual endurance, according to a study published in the American Journal of Cardiology that monitored both treadmill and sexual performance. The results clearly showed a ratio of each extra minute lasted on the treadmill translating to an extra two to three minutes of sexual activity.

There’s more good news when it comes to reaching the finish line. One University of California study found that after following a moderate aerobic regime four times a week for nine months, male subjects had sex 30 per cent more often, and with 26 per cent more orgasms. A second study, conducted with volunteers of both sexes over a period of 18 years at the Center for Marital and Sexual Studies in Long Beach, California, found regular exercisers consistently reached orgasm easiest and most often.

There’s a note of caution to add, however. To take advantage of your enhanced physical state you must first be broadly in the right psychological frame of mind, says psychologist Dr Petra Boynton. ‘There’s a lot of stuff to be read about how more exercise automatically means more and better sex,’ she says. ‘That’s not necessarily the case. If you’re not having much sex, you need to look at why you’re not – especially if you’re in a relationship. Has something happened in your past that has blocked you? Are you fundamentally low on confidence and embarrassed to let go? If so, getting physically fitter is not going to solve these issues, you need to talk to someone to get to the root of the problem.

‘It’s also always important that you trust your instincts and do what makes you feel good,’ says Boynton. ‘Getting a better body might allow you to try new things, but while swinging from the chandelier may be great for some people, it’s not right for everybody, so it’s important not to believe that as you increase your exercise you suddenly have to start having more sex and in wacky ways.’ In other words, if your improved fitness simply gives you the physical and mental confidence, and ability, to be more intimate with your partner, even if you don’t actually change much about how you have sex, that’s fine.

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How does running affect your sex life?

First, the situation for men. Weight and, in particular, a high body mass index have been shown to have a significant effect on both sperm count and the chances of erectile dysfunction. Research shows the more fat you carry, the greater your chances of suffering from these issues. Happily, increasing your running can improve matters, especially in middle age: a Harvard University study found that men over the age of 50 who run for at least three hours a week have a 30 per cent lower risk of impotence than those who do little or no exercise.

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‘However, don’t overdo it,’ says Federico Febbraro, lead physiotherapist at The London Clinic. ‘Athletes who regularly undertake gruelling endurance events such as marathons can have problems with fertility. Why? ‘Firstly, lowering your body fat too far can affect the quality and quantity of your sperm,’ says Febbraro. ‘Secondly, men who do a lot of mileage running in tight shorts, like compression kit, that squeeze around the reproductive area can sometimes develop varicocele.’ This is an enlargement of the veins around the penis caused by compression of the arteries around the perineal region (between the scrotum and anus). The resulting lack of blood flow to the area affects sufferers’ ability to achieve and sustain erections. Febbraro recommends wearing compression around the groin area either for training or recovery but not both, and not sleeping in recovery tights except after really hard sessions.

And for women? The same doctrine of moderation applies when it comes to female fertility. According to NHS guidelines regular moderate exercise is key for promoting a healthy pre-pregnancy state. The benefits of running – such as less stress, increased circulation, greater strength and flexibility, and detoxification – all help to increase the chances of conception. But if you’re trying to get pregnant, Febbraro recommends that you limit your training to three 30-minute mid-intensity runs per week, plus two half-hour strength-training sessions. Pushing yourself too hard, the evidence shows, can have a negative impact on your chances of conceiving. A Norwegian study of 3,000 women of child-bearing age found only a ‘moderate exercise’ protocol was beneficial to fertility, while those who were either sedentary, trained almost every day, or regularly trained to exhaustion, all experienced problems. Those who did little or no exercise were found to be at greater risk of ailments such as polycystic ovary syndrome, diabetes and hypertension, all of which can reduce chances of conception.

At the other end of the scale, the study (along with numerous others) suggested low levels of body fat (less than 17 per cent) are related to fertility issues. ‘This is a problem that can be quite prevalent among elite female athletes,’ says Febbraro. ‘There are two issues: low body fat can lead to irregular periods or the menstrual cycle stopping altogether because of decreased production of oestrogen and progesterone, two hormones essential to fertility. In addition, studies on athletic women have shown that their cortisol [the stress hormone] levels are constantly elevated and their thyroid hormone levels tend to drop, which leads to greater adrenal stress and can cause poor reproductive function.’ The good news, though, is that the scales can be reset if you make sensible changes to your exercise regime. ‘All the evidence shows that if you cut back to a moderate level of exercise your stress levels will drop and your periods should go back to normal within a couple of months,’ says Febbraro.

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How does running affect your libido?

If you’re worried your training is going to leave you with no energy in the bedroom, think again: research has shown that regular exercise actually boosts your libido. This is partly down to exercise’s effectiveness in combating stress, a major cause of low sex drive. The more stressed you are, the less likely you are to want sex; a study from the University of Gottingen in Germany found that people who have sex less often tend to take on more work to compensate for their frustration. The increased work then leads to more stress and, you guessed it, even less sex. A recent study published in the Journal of Sexual Medicine that looked at levels of post-training testosterone and alpha-amylase (a marker of nervous system activity) in women found a significant rise after 20 minutes of vigorous exercise, leading to test subjects being in an increased state of ‘readiness for sexual activity’.

‘There is a similar boost for men,’ says performance psychologist John McDermott. ‘Studies have shown that men who do short, sharp exercise sessions of 20-60 minutes three to five days a week experience an increase in testosterone production, which contributes to increased sexual desire and confidence.’

And this increased friskiness is not age-dependent. Two studies from Harvard University, US, measured the increase in sexual desire after regular exercise in both college students and people aged 40-60. In both age groups the more physically vigorous subjects reported a higher incidence of sexual activity.

According to McDermott, the psychological element of improving confidence about your physique also plays a role in the relationship between exercise and libido. This view is backed by US research from the University of Arkansas, in which physically fit men and women rated their own sexual desirability higher than less active men and women of the same age.

Eighty per cent of men and 60 per cent of women who exercised two to three times per week rated their sex appeal as ‘above average’.

In our own Runner’s World Sex & Running Survey, 93 % of more than 1,500 respondents said they felt running gives them more body confidence in the bedroom, with 68 per cent also saying they felt more sexually aroused after a run.