In an Olympic Games precinct known for tradition and history, BMX is breaking down the barriers with its thrill-seeking races on a purpose-built track.

Most people had never seen a BMX race before sitting in the stands in London. And most were left shocked when at times all but one rider hit the rock-hard surface, with crashes wiping out the majority of the field.

Brazilian rider Squel Stein had an ugly crash on the second jump of her semi-final. She left the course on a stretcher and was taken to hospital.

If ever there was an Olympic sport that needed an ambulance permanently parked nearby, BMX is it. There was blood, skin grazes and bruised egos as they crashed in front of a massive crowd.

But is it normal to have so many bingles?

"Yeah, it's normally this intense," Reynolds said after she was knocked out of the women's event.

"There are probably a few more crashes than normal, but everyone is just going for it.

"At an event like this, no one holds back. There's one race, one medal and everyone's chasing it. But hey, if we didn't have crashes it would be pretty boring."

Some questioned the sport's Olympic eligibility before the competition started.

Commentator and the voice of cycling Phil Liggett said BMX didn't belong in the Olympics.

"I am sure [Olympics founder] Pierre de Coubertin would have laughed his head off if he found out it was an Olympic discipline," Liggett said.

Most riders live life on the edge and enjoy the dangerous thrill. They navigate the short course of jumps and turns in less than 40 seconds. When one rider goes down, someone else usually follows. When they go over jumps, riders elbow each other to jostle for position.

Reynolds used to ride her bike off the end of a pier and jump into the water.

Now she does it on the world stage and while she was devastated she missed a chance to race for a medal, the 21-year-old wants to stick with the sport she loves.

"But people are now starting to realise what the sport's about and give us some more respect," Reynolds said.

"The track is a bit narrow at some points and it's fierce out there. The competition is so hard and high, no one is holding back."