Freddie Roach, the trainer in the corner of boxing superstars like Manny Pacquiao and Julio Cesar Chavez, Jr. as well as MMA stars like Georges St. Pierre and Andrei Arlovski, tells Marcos Villegas of Fight Hub TV that he advises his fighters to abstain from sex for 10 days before a bout.
Is there anything to this or is it just an old wife's tale?
The BBC spoke to a few medical doctors and got a range of opinions:
"The night before has no effect on strength or endurance or any of the physical abilities of the athletes," says physiology expert Dr Ian Shrirer, a former president of the Canadian Academy of Sport Medicine. "A lot of people think it has to do with the psychological effects."
"It's never been studied, how sex affects the psychological attitude the next day. But if it were true that it decreases aggressiveness, then you might expect it to be detrimental for those who aren't aggressive enough, but beneficial for those who are too aggressive."
And then there's Israeli physician Alexander Olshanietzky, who's all in favour of sex - for female athletes, at least.
"We believe that a woman gets better results in sports competition after orgasm," he said. "Generally, it's true of high jumpers and runners. The more orgasms, the more chances of winning a medal.
"Coaches generally tell their athletes to abstain before competition. In the case of women, that's the wrong advice."
But Dr Olshanietzky warned male athletes off going for it the night before a competition, saying that while sex energises women, it just tuckers men out.
Sounds like Freddie Roach might know just as much as the medical authorities on this topic.