'I wept as we filmed them fight'

Gaea girls take their sport seriously: these Japanese women are professional wrestlers with punishing training regimes and spartan lives. Their training camp is a corrugated-iron shed in the countryside, where they sleep in a tiny dormitory. It's tough and requires total dedication and self-discipline; you and I would run a mile.

Gaea girls take their sport seriously: these Japanese women are professional wrestlers with punishing training regimes and spartan lives. Their training camp is a corrugated-iron shed in the countryside, where they sleep in a tiny dormitory. It's tough and requires total dedication and self-discipline; you and I would run a mile.

A world away, in her pleasant, airy flat in Islington, London, the documentary filmmaker Kim Longinotto is forcing herself to be healthy and drink water, before giving in and making a pot of tea. Real tea. She attempts a joke about anarchists and proper tea being theft but blows the punchline, laughs anyway and sits down on the floor.

Her last film was the award-winning Divorce Iranian Style, which followed the proceedings in a Tehran family court where, although the atmosphere was often heated and angry, no one came to blows. Gaea Girls is a completely different ringside experience: women willingly submit to terrible humiliation and physical abuse.

Gentle and unassuming, Longinotto is the very last person I would have expected to turn her camera on such a subject. When I tell her, she seems a little surprised herself: "It is a disturbing film," she agrees. "Very stark and bare. I love it, but I'm also frightened of it."

It wasn't supposed to be this way: "I believed that wrestling was all show," she explains, "and that we were going to make a film about wonderfully dedicated women athletes." She and her collaborator, Jano Williams, were shocked to find the girls fighting for real: "In a way, the more disturbing it got, the more hooked we became."

The focus of the film shifts to follow one trainee, Takeuchi, as she prepares for her "test", the culmination of a year's training, which she must pass if she is to become a professional Gaea wrestler. As the beatings she receives grow in brutality and danger, we are asked to question our feelings towards not just the abusive teaching methods, but the institutional hierarchy which supports them. It's a system Longinotto found uncomfortably familiar: "It reminded me of public school," she tells me. "The rules and the sadism. They are trying to break the girls' spirit, and that's what boarding schools do to make you conform. And you leave and you're completely fucked up."

At school she desperately tried to fit in: "Nobody liked me," she says dispassionately. It was the sharp end of an unhappy childhood, and at 17 she left her parent's flat on Kensington High Street to live rough.

"It was a silly thing to do," she insists. "It was about being totally naive and not realising how horrible it was going to be. The people I met on the streets had been chucked out or abused, but I always knew that I could go back." She hitched and worked her way around Europe, ending up in Istanbul, extremely ill. "It was almost like going to the depths of degradation to cleanse myself of my awful upbringing; I wanted to be rid of it, rid of them."

She did go home long enough to apply for university, after which she flirted with the idea of a PhD on the modern novel but caught the film-making bug on a course at Bristol and pursued her new passion at the National Film and Television School.

"I loved the National," she beams. "You had total freedom and you could work at your own pace." For her first-year film she returned to her boarding school: "It was so good to go back and see that it wasn't me - they were really weird. It was my revenge, and they hated it - my headmistress called me a class traitor; I was so flattered. I needed to do it, and it probably saved me years of therapy."

Longinotto certainly seems pretty sorted, her natural modesty translating into a very understated documentary style: she follows people living their lives, is sensitive to their emotions, and is always prepared to let the film go to the story rather than vice versa; she never appears in the frame. "I like to go in and not disrupt anything at all. The crew don't talk to each other, you don't make any noise, you're very passive. You don't ever tell anyone to do anything, not even to walk into the room again if you missed it."

This yearning for quiet and harmony is also traced back to her childhood: "I've got a horror of fuss and noise," she explains, her familiar nervous giggle barely suppressed. "And I think it comes back to when I was a kid and my dad was a photographer. There were all these lights and I'd have to have my hair done - such a fuss for a stupid photograph - and I remember being really upset by the whole thing. I had to do commercials as well, and you'd do everything millions of times, so maybe that's why I never want to do things in that way."

Longinotto's recollections are slightly unnerving: it's as if her desire for equanimity has flattened the emotion out of her memories. She provides another example: after a walk with her boyfriend and a friend she received a message that her mother had died. "I came into the living-room and said, 'Now, I don't want you to be upset, but my mum just died. Shall I make tea?' And they looked at me, horrified. I'm not a cold person," she rushes to assure me; "there was just nothing between us, and it was much more upsetting to realise that than not to see her."

Back in Japan, Gaea's No 1 wrestler and mentor to the team, Nagayo, refers to the girls as her "children"; she sees herself as their surrogate mother, every blow she inflicts as painful to her as it is to them. Asked about her methods, she talks of her own bitter relationship with her violent father, revelling in her now superior strength. "I bet you're lonely now, father, because I'm better than you," she smiles.

"When we met Nagayo we really felt her charisma," Longinotto recalls. "She'd come into the gym and we'd sort of stand to attention; she was the boss and we respected her. After that interview we talked for ages about what she said, because in a way, in her own logic, she's right. She wants to provoke the killer instinct in the girls, and it is going to be harder for them in the ring if they are lax or careless; they could get killed. But what really struck me is that she still hates her father, and in a way she's talking about herself, because the girls are going to hate her and won't need her."

Gaea Girls' most shocking sequence comes during Takeuchi's test, when Nagayo jumps in and hits and kicks her around the ring: "I think that Takeuchi thought Nagayo might kill her," Longinotto says. "There was this awful sense of electricity; we didn't know what was going to happen, and it felt very wild; there was something truly unhinged about it." She wept as she filmed, and Williams was so distressed, she had to leave. "I'm really glad that we made the film, because everyone there - including a crew from Japanese television - thought we were crazy to react in that way. Then we watched the film the other crew made. It was fascinating because the violence just wasn't there. It was almost as if they didn't see it."

Showing at Filmhouse 2, today 5.30pm and at Cameo 2, Thursday 24 August 7pm