How stress can affect sport performance?

Not all stress is bad for your performance. Stress can affect your performance in two different ways. Stress can help you when it makes you more alert, more motivated to practice, and gain a competitive edge. In the right amount, stress helps you prepare, focus, and perform at your optimal level. Conversely, too much stress, or bad stress, can cause performance anxiety, which hurts your health and does not allow you to play relaxed, confident, and focused in competition.

“You’re always going to be nervous teeing it up in a Major Championship. It’s very natural and it’s a good thing. It means that you want it.”

~Rory McIlroy, first round leader at the 2011 Masters

Every competitive athlete experiences some stress; good and bad. Your stress may be positive and helpful or instill anxiety and apprehension. Pregame jitters can cause some athletes to not sleep well the night before competition. Some athletes can’t eat the morning before a big game. Your pre-competition jitters may make you feel like you have to throw up.

You want to feel excitement or thrill in anticipation for competition (what I call positive pregame jitters). A high level of activation will help you perform your best – up to a point where you may be too jacked up to play well. Too little or too much intensity (or stress) can cause your performance to decline. Your ability to cope effectively with pregame nerves is critical to consistent peak performance.

From: http://www.peaksports.com/sports_psychology_blog/?p=1718