I just caught part of a Discovery Channel's "Sports Science" ( episode #70), on debunking sport myths. One myth was whether "the shout" (kiai) actually makes a difference, and another was whether sex before events hurts an athlete's performance.
The kiai test used a world champion break artist. With a shout he generated about 2000 lbs. of force to shatter a stack of bricks. Without the shout, he only produced about 1500 lbs. and the bottom brick didn't break, rebounding the energy back up into him. I thought that single test, dramatic as it was, cannot be definitive because it was only a single sample. It did seem to substantiate the fact that a kiai helps release adrenaline as well as focus the mind. The martial artist clearly felt inhibited trying to match his previous break while holding back on part of his technique.
For comparison they should have also tested someone who felt confident breaking without a kiai, then having that person add a shout. What I'd really like to know is whether a focused exhalation without a kiai would be equal to using one. Again, I think that would require multiple repetitions and multiple test subjects to get reasonably objective data. Anecdotally throughout history, though, the grunt or shout has been understood as a byproduct of maximum exertion, so I think there is merit to the process. Withholding it may well inhibit performance, but does emphasizing it increase performance or is it a distraction?
The sex myth involved testing a world champion boxer in several categories, including leg strength, lower and upper body cardio endurance and blood testosterone, both before and after a night of sex with his wife. In the follow-up he tested higher in almost every category (cardio results were equivalent) including testosterone levels in his blood, the one result he couldn't fake. The concensus on this was the myth against sex was in fact without merit and quite possibly exactly wrong.
Something I remember from a documentary on Babe Ruth wasn't that sex itself tired out the athlete, but rather the effort in chasing women and carousing beforehand that did the damage. In other words, for guys in a steady relationship, it shouldn't be an issue. The only rationale I can see might be to make a man ornery, or as Eddie Murphy put it in "48 Hours" (and quoting Richard Pryor, perchance): "Lack of p***y makes a man brave!"