RIO DE JANEIRO (AP) — Imagine a horse race that starts on a ramp about three stories high, then winds down a quarter-mile track with as many bumps and twists as a roller-coaster ride.

But instead of riding horses, eight competitors are pedaling frantically on bikes, and the race only takes about 40 seconds.

Welcome to BMX racing.

The fun at the Rio de Janeiro Games starts Thursday with the men's quarterfinals at the Olympic BMX Center.

"You're in this starting gate and it's being let out of the gates and everybody goes," U.S. rider Alise Post said. "After that you're riding the roller coaster. Everything happens fast out there."

But with hills and bumps made of varying heights, and 180-degree turns that riders maneuver on wide, sloping berms.

"It's really a big kind of a roller coaster adrenaline rush is what it is," said Post's teammate, Corben Sharrah.

BMX, which stands for "bicycle motocross," traces its roots to Southern California in the late 1960s and early 1970s. An action sport once considered a niche activity is now in its third go-around at the Olympics.

It was added in 2008 in Beijing as a way to attract younger viewers with its frenetic, anything-can-happen pace.

Even the most seasoned riders still get nervous when they get behind the starting gate.

"You get on top of that hill when you load the gate and there's always some kind of nerve," said Sharrah, who won the U.S. Olympic trials. He earned the fifth seed in the quarterfinals following the seeding runs on Wednesday .

"But it's a good nerve," Sharrah said. "It's kind of that nervous (energy) that feeds what you train for every day."

Once cyclists break the starting gate, they're free to jostle for inside position. Joris Daudet of France, who has the top seed in the quarterfinals, compared a race to a 110-meter hurdle race, but minus the lanes.

"You can go everywhere you want," Daudet said. "It's like a sprint but everyone can change lanes."

The competition is Rio is spiced up by a visually appealing track with bright green turns and finely groomed straights with surfaces that look like a baseball infield.

The track plays fast, most riders said Wednesday.

"It's probably the fastest track we've ever had," said David Graf of Switzerland, who is seeded second in the men's draw.

American Brooke Crain called the track one of the best that she's been able to ride. She will be seeded seventh when the women's draw resumes racing on Friday.

"It's just fast," Crain said. "The jumps aren't terribly big, so it's a good racing track."

It's still a bumpy ride. The bikes don't have suspension. Crashes aren't uncommon.

American cyclist Connor Fields isn't fully recovered from a broken bone in his left hand, an injury he sustained in the spring. But Fields, who is seeded fourth, is using a specially designed brace on the hand for the Olympics.

"What we do is pretty intense to begin with," Fields said. "It takes a lot of mental health. You just have to get over that fear and go for it."

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