Keirin is the most extreme sport you've never heard of. Competitors circle a track on fixed-gear bicycles with no brakes, pacing themselves behind a motorbike before making a mad sprint at the end as betting fans cheer like mad. Photographer Jasper Clarke photographed these speed demons during a day of training in October.

The sport started in Japan and dates to 1948, when it was created specifically as a betting sport. Even now it remains just one of four sports upon which betting is legal (the others being horse racing, boat racing, and Formula 1). The Kokura Velodrome hosted the first official race, and the sport has grown steadily. No professional sport in all of Japan has more competitors, and it is popular around the world. Keirin became an Olympic event in 2000, and the races are nothing less than amazing.

Races cover about 2 kilometers, with competitors—who are shaped something like redwood trees—typically riding in a pace line behind a motorbike that gradually gains speed with each lap until the entire field is moving at 50 kilometers per hour (about 31 mph). About 700 meters before the end of the race, the motorbike peels off the track, and hell breaks loose as riders sprint to the end at speeds reaching 70 kmh (about 43 mph) just inches from each other. Crashes are common, and occasionally horrifying.

The sport is highly regulated in Japan, because of the gambling involved. Before each race, cyclists announce their tactics so people know how to bet. For example, a rider may say he will ride seiko, staying out front but not getting aggressive until the very end. Riders' positions within the pace line often play to specific strengths, like sprinting or blocking. The sport requires exceptional endurance, as top-tier racers—who can earn many hundreds of thousands of dollars annually—may compete in as many as a dozen races a day during meets that last four days. Competitors are housed in dormitories and denied access to the Internet or phones, to prevent cheating or other shenanigans that may affect the betting.

Professional racers are licensed, and dedicate a year to rigorous training at a keirin school. Clarke became fascinated by the training earlier this year after seeing a video of Shane Perkins, an Australian professional track cyclist, training at an academy. The intense regimens and steadfast determination to succeed impressed him. "How many chances does anyone get to be really great at something?" Clarke says. "These people had given up any semblance of normality for a year of their life to try and excel in their chosen field."

A student waiting in the wings for his race. Jasper Clarke

The photographer spent a day at the Japan Keirin School in Shuzenji, about two hours south of Tokyo. He describes life there as "Spartan." Students wake at 6:30 am and consume an energy-packed breakfast of miso and fish that can top out to a whopping 1,300 calories. After breakfast, they immediately launch into rigorous training–running drills, hill climbing, lap training and race simulations. They also attend classes to learn about the rules of the sport, its strategy, and training. After an exhausting day, its lights-out at 10 pm. "I was massively impressed with the students dedication, hard work, and their resulting fitness, strength, and resilience," Clarke says.

Clarke shot with a Nikon D3S and Zeiss manual focus lenses, using only the available light. An academy instructor and Clarke's interpreter accompanied him everywhere, and although the students didn't speak English, everyone managed to share a few jokes. The photographer would like to go back to Japan and approach the sport from a gambler’s eye view.

Clarke is an avid cyclist, but only recreationally. He’s ridden a BMX bike since his father bought him one in the 1980s, and says he understands why keirin riders are so dedicated to the sport. "I just love being on two wheels. It’s freedom," he says.