They twist in the winter's chill defying gravity, whip through icy courses feeling the pull of three G's squeezing their face, and take to the air like birds without wings.

Winter Olympics athletes aren't necessarily crazy, but it would be a nice prerequisite before signing up to represent the Stars and Stripes.

The boys and girls of summer have it easy, sprinting on cushioned tracks, bouncing leather basketballs or taking a cool plunge into the pool. A change of season means accepting the often-brutal consequences of matching man with sleds and skis, and unforgiving mountains that do not discriminate between the world's best and a bunny-slope novice.

And so, a sane person may ask, "What's your motivation?""Adrenaline, fear, everything," said Sanford's Garrett Hines, an Olympic bobsledder who will compete Saturday. "You can take your best roller coaster and multiply it by 10. It's one step below free falling from a plane or going to the moon. It's that exciting."

Unfortunately, there are no free rides.

U.S. bobsledder Joe Sisson recently sustained serious head injuries during a training run at a Swiss resort. Although his race partner was thrown clear and unhurt, Sisson remained stuck in the sled, which continued to slide upside down.

Picabo Street -- racing a month after winning Olympic gold in the women's Super G in Nagano, Japan, in 1998 -- lost her balance and skidded into the protective netting on the side of the course while skiing 60 mph. Street snapped her left femur, tearing the anterior cruciate ligament and shredding the meniscus in her right knee. It was the third major knee injury for Street, who retired from racing after competing in the women's downhill Tuesday.

Katie Monahan fell coming off a big jump and tore cartilage off the bone in her right knee during a spring super-G training run in Zermatt, Switzerland, in 1999. It took three surgeries and two years before Monahan could heal physically and gather the courage to face the mountain again.

"I literally had full shakes," said Monahan, a member of the U.S. Alpine team. "I'm a nervous person to begin with, and I hadn't been in the starting gate in two years. On top of the expectations to do well, there was a huge fear factor, because the last thing you want to do when you've rehabbed and come all the way back [is] to make it to the race season and then you hurt yourself on the first day.

"The rush I got from getting to the bottom and being in one piece, and scoring in the top 30, it was amazing. That's why I came back. Just for that."

That's why they all come back.

Nothing beats the adrenaline rush, and that's why they will use all of the resources of modern medicine to stitch their bodies back together again, and hope that psychologists can help heal the emotional scars.

Blanking out negative karma is the foremost rule to remember while preparing for competition. Otherwise, an athlete is that much closer to a misstep that leads to scars -- physical or otherwise.

"As soon as you are overcome by fear or thinking, 'What if I crash?' those are the times when you crash," said Caroline Lalive, a competitor in the women's downhill for the United States. "You can't go into skiing 80 mph down a course apprehensive. As soon as you're passive, that's the most dangerous time."

Research has shown that athletes under stress are more likely to be injured because of a loss of concentration that prevents them from seeing things around them and slows down their reactions.

It does not get any more stressful than sliding down a luge track at speeds reaching 100 mph or having the eyes of the world look up at you while you brace for a downhill run on the Grizzly Trail in the Winter Olympics.

U.S. athletes can lean on Sean McCann -- the head of the sport psychology department at the U.S. Olympic Training Center in Colorado Springs -- for an emotional crutch.

"It's really helpful to have someone you can talk about fear with," Monahan said. "You don't want to admit it to yourself, but you have to work though the process. You have to get through it and overcome it. How can you get that confidence that makes you unafraid when you're afraid?"

Australia's Jacqui Cooper could have used someone to smooth her psyche this week. While training for the women's aerials competition Monday, Cooper crashed and severely injured her left knee. She was the gold-medal favorite in that event.