Judo: Slapped, kicked and beaten with bamboo - the training horrors that Japan's women underwent in the run-up to London 2012

In the afterglow of the London Olympics, Ryuji Sonoda enjoyed the sort of gold-plated reputation enjoyed only by a small group of elite athletes in Japan. A former junior judo star, the 39-year-old was known for his fierce dedication to the traditional martial art and its sweat-drenched ethic of discipline and courage. As head coach to Japan's female Olympic squad he was revered, watching over the women last summer like a bear over his cubs. Unknown to all but the women, however, Sonoda practised a particularly painful brand of tough love.

In letters to the Japanese Olympic Committee last December, 15 of the judoka alleged their coach slapped, pushed, kicked and sometimes beat them with bamboo sticks during training sessions.

Athletes deemed slackers were dubbed "pigs" or "ugly" or simply told to "die". The bullying and abuse by Sonoda and his staff had been endured for four years without protest, says former Olympian Kaori Yamaguchi, who advised the women to fight back. "I couldn't believe it when I heard that top-class athletes were being treated this way," she recalls. "It's totally counter-productive. What were the coaches thinking?"

The violence was bad enough, but the atmosphere of intimidation and fear was worse, lawyers for the women said in February. Training camps were run like army barracks. The women's lives were controlled; one was banned from sitting a college exam. Sonoda would pick on the weaker judoka and relentlessly harass them. The message was clear, said one lawyer. "If you don't try harder, this could be you. It created fear." Sonoda had the power to make or break careers because he decided who went to the Olympics and who stayed behind. "If he disliked you, that's it, you didn't go," says Yamaguchi.

Caught between wanting to help the victims and protect the sport she loved, Yamaguchi went to the All Japan Judo Federation in September and quietly asked for an investigation. Japan's performance in the London Games, where it won just one gold in a sport it pioneered, was under review amid accusations that the top-down coaching style was costing medals.

To Yamaguchi's amazement, the all-male federation ignored the women and sent Sonoda back to work as national coach after a verbal warning. A month later, he was still boasting that his "strict" training methods paid off in London.

In desperation, the athletes took their complaint to Japan's Olympic Committee, who finally launched a probe. Sonoda quit in January after admitting the allegations were true (he declined to be interviewed for this article). Alarmed that the story would damage Japan's chances of hosting the 2020 Olympics, the JOC surveyed nearly 4,000 top Japanese athletes and revealed last month that 11 per cent of them admitted suffering violence, sexual harassment or other abuse.

Most observers believe that only scratches the surface. "The figures raise serious questions because they cover only the top athletes," the lawyer Takashi Ito told the Asahi newspaper. "There actually must be many more cases."

The emerging judo scandal coincided with the suicide in January of a 17-year-old Osaka high school student who was being bullied by his middle-aged basketball coach. In an investigation by the school, the coach admitted repeatedly slapping the boy across the face and calling him names to "toughen him up".

The boy's parents described finding a suicide note in which he wrote how he could no longer cope with the abuse. The scandal revived memories of Takashi Saito, a 17-year-old sumo wrestler who died during training in 2007 after repeated blows from beer bottles and a metal baseball bat. His trainer Junichi Yamamoto also pleaded the "tough love" defence but was sentenced to six years for manslaughter.

Japan is not the only country struggling with violence and hazing in sport. Mike Rice, head basketball coach at Rutgers University in the United States, was forced to quit this month after a video surfaced of him viciously abusing his players. But observers say Japan's problem is complicated by the deep cultural reverence for the sensei, or teacher.

"There is a huge gap between teacher and student," says Noriko Mizoguchi, a retired judoka and Olympic silver medalist. "There is very little two-way communication – athletes just do as they're told. If there's a problem, they can't challenge him."

Foreign observers are often struck by the strict coaching of young athletes in Japan, with its endless repetition of basic moves, often overseen by a screaming sensei. Students have no power to choose their coach, or change to another if they don't get on. "They're bound together, and the student must simply endure," says Yamaguchi, who once coached in the UK for a year. "The British students would ask me: 'Why do we do this or that?' That's unheard of in Japan." She believes the bullying is widely accepted among older people but says attitudes among the young are changing.

That remains to be seen. The sports minister, Hakubun Shimomura, recently called the crisis the "gravest" in Japan's sporting history and ordered a national review. The JOC, meanwhile, cut funding last month to the Judo Federation as punishment for the scandal, and ordered them to hire more women as coaches and executives.

The JOC has also set up an anonymous telephone-based system to report violence or misconduct in sport. "The judo scandal is a turning point," Ryosho Tanigama, the head of the prestigious Japan Sports Science University, told incoming students this month. "We have to make corporal punishment in sports a thing of the past."

But some fear the old methods will reassert themselves once the dust has settled. Tellingly, none of the 15 judoka who triggered the scandal has talked to the media. Some reports say they have gone underground, surprised by the lack of sympathy for their plight. "They're not happy with the situation," admits Yamaguchi, who is still in touch with some of the women.

She says it took a lot of guts for the women to break ranks, but if they fail to bring about change, violence will return. "People will say: 'Ah, the old way is best; the result is all, whatever makes athletes strong'. But those values are outdated. You praise even dogs while raising them. So why do that to children?"