By Mark Meadows

LONDON (Reuters) - When Rafa Nadal seeks refuge in a towel or an angry Andy Murray mutters to himself, they are not just amusing the Wimbledon crowd with their quirky habits but honing their "peak performance state."

Even top athletes need their rituals, whether it be Novak Djokovic incessantly bouncing the ball or Maria Sharapova's famous grunts and mini-fist pump as she swivels on the baseline.

American Andy Roddick has a particularly memorable routine, pulling his overly large shirt on to his shoulders and pointing to the ballgirl to bring him his towel after almost every point.

Like a baby and a blanket, the importance of the towel to a tennis player cannot be underestimated with champion Nadal wiping himself down regularly even when little sweat has been produced.

"The idiosyncratic movements are very important. If you remove them they are quite detrimental to performance," Mark Cheetham, lecturer in sports physiology at the UK's University of Derby, told Reuters.

"It's all to do with them coping under pressure. A lot of them aren't aware of the extent and frequency of their movements."

France's Marion Bartoli, who knocked out women's defending champion Serena Williams on Monday, has some of the biggest idiosyncrasies on the women's tour.

She bounces the ball exactly four times before serving and frantically jumps around as she prepares to return serve while also taking an array of practice swishes, a habit which can annoy some opponents.

"It helps me to stay in the moment and not to think too much about the scoreboard or the moment or the occasion or who I am playing against," she told reporters.

"It's really important, especially on grass, you have to be really quick. You cannot be slow on your footwork. You cannot wait for the ball."

Nadal, meanwhile, is not confined to just a towel habit. The Spanish world number one also consistently tugs at the back of his shorts, causing sniggers from the crowd.

"With Rafa pulling on his shorts, it's a comfort thing," added Derby's assistant subject manager for Sports Charles Spring.

"It would annoy him and put him off if he didn't do it. You are looking at people in absolutely peak performance state."