If the mountain biker Adrien Niyonshuti's considerable legs push him to Olympic glory on Sunday, it will surely be the most inspirational story of the Games. One of them bears a scar. He knows when he got it, but he cannot remember how.

Niyonshuti was seven when the militia men came to his village in Rwanda. Somehow, he escaped the April 1994 genocide – but six of his brothers did not. In the years since, cycling has been his therapy, his companion, his hope.

"For the April month when I ride bikes, sometimes I get headaches because I remember a lot of things from my family for genocide," Niyonshuti, speaking in broken English, recalls in Rising From Ashes, a documentary film about the genocide survivors who make up Rwanda's first national cycling team.

"Before genocide I was still younger. When the rebels were coming into the house, put the fire, come in the house, take everything. If they see you they just kill you, you know. From my family I lost six of my brothers. And then, I lost my family from my mum. All this family was like, about 60, immediately gone. Even now it's still hard to know."

The purge of Tutsis and moderate Hutus wiped out 800,000 people in 100 days, many slain with machetes. Somehow – he does not understand how – Niyonshuti and his parents got away, hiding in the bush for days without food or water. Now, he finds it difficult to talk about the experience that robbed him of loved ones, including a grandmother.

"I survived because I was with my parents," he says in a phone interview. "When the guards took people, I was with my parents. It was a long stay [in the bush] and we survived like that. It was incredible.

"I'm moving forward in my life and sport. You lose someone in your family and try to forget but it comes back. I think if I keep talking I think it will hurt me and come back in my mind."

The young, traumatised Niyonshuti needed to busy himself in something that would help him forget. He found cycling. He rode a steel-framed bike that belonged to his uncle. It demanded total concentration and created a sense of freedom. Rwanda's hills, roads and footpaths were the perfect arena.

In 2006, he entered Rwanda's first annual cycling festival, created by Jonathan "Jock" Boyer, the first American to finish the Tour de France, and mountain-bike pioneer Tom Ritchey. He recalls in the documentary: "It was difficult because it was my first time to ride a mountain bike. I was so scared to race with them … I crossed the line, I win. I surprised myself."

Mountain biking has typically been a white man's sport. "If we had more opportunities to compete with those white folks, I think it would bring hopes to our lives and our country," Niyonshuti told an interviewer at the time. "We might develop as cyclists."

Five months later, Boyer returned to set up and coach Team Rwanda, unmissable in their bright blue and yellow shirts with the image of a sun rising above hills.

In Rising From Ashes, the American says: "I was impressed with Adrien in his way of pedalling, in his attitude. You could tell he was a real kind person that had a lot more thought in what we were doing than any of the others."

The documentary, narrated by the Oscar winner Forest Whitaker, shows Niyonshuti and other cyclists, who began training on 25-year-old equipment, still coming to terms with what they witnessed as children. Boyer reflects: "After I spent time with the riders it really started to hit me. Most of it's all unspoken, just looking at them.

"The way I could really see their past in their eyes is how much hope the bike gave them. You could see how much they were hanging on for dear life because it was their way out."

In 2009, after success in international competition, Niyonshuti joined a team in South Africa, where again he has been the victim of criminal violence. When four robbers armed with knives and guns stormed into his house, he hid in a bedroom cupboard. "I was very scared," he told Sports Illustrated last year. "When I saw those guys I was thinking about genocide."

Recently he has been training in Switzerland to prepare for the London Olympics, where the mountain biking will be at Hadleigh Farm, Essex. He will be the sole cyclist in a team of about 10.

His story is a testament to the regenerative power of sport but nobody, least of all Niyonshuti, believes it can heal every scar. "There are still a few tears after 18 years. Sometimes it's difficult; other times, I tell myself I must not think about it."