An hour later, the pint-sized Nagoya housewife is close to tears after narrowly failing to retain her title from a field of 34 bronzed and impressively buff ladies, most of them in their fifties.
"I was pathetic!" the 56-year-old Yamanouchi told AFP backstage at the close of the event earlier this week, sporting a skimpy gold bikini.
"A silver medal means nothing to me," sniffed the sinewy ex-champ, who stands just 1.58m tall and weighs 50kg. "It just means you're the best loser."
Breaking down gender stereotypes
A self-confessed gym rat, Yamanouchi is the poster girl for Japan's growing number of female bodybuilders, helping break down gender stereotypes in a country obsessed with the "kawaii" (cute) fluffiness of its ubiquitous pop culture.
"I want to help change perceptions so that more people can appreciate the beauty of a muscular woman," said the five-time national champion after a punishing two-hour workout a week before the competition.
"When I tell people I'm a bodybuilder, it freaks them out," added Yamanouchi, who became hooked on the sport in her late forties after looking for a way to keep fit.
"My husband didn't like it when I started either, his wife wearing a bikini in public, but he came around."
The number of bodybuilders registered with Japan's national federation has almost doubled over the past six years to around 3 000, with women making up 10% as part of a nationwide fitness boom, officials said.
In ageing Japan, female bodybuilding is dominated by women in their forties and fifties, as many usually only start after their children have grown up.
Japanese bodybuilder Satoko Yamanouchi poses during the Japan bodybuilding championships. (Behrouz Mehri/AFP)
Yamanouchi, who takes around 10 different supplements a day to boost muscle growth and aid recovery, insists she knows where to draw the line, despite her bulging physique.
"I don't want to look like the Hulk," she said, taking a gulp of protein shake.
"I want to look beautiful and keep my femininity. I just don't feel like a regular housewife," added Yamanouchi. "I'm always striving to create the perfect body."
Women's bodybuilding is a serious business.
The oldest competitor at the Japan championships, 64-year-old Mariko Takamatsu, stormed off after failing to make the top 12.
The eventual winner, Megumi Sawada, struck a series of eye-popping poses to the theme tune of Godzilla, stunning Yamanouchi to take the title.
Winning Japanese bodybuilder in the women's category Megumi Sawada and winner in the men's category Masashi Suzuki. (Behrouz Mehri/AFP)
"It's unbelievable I've won," gasped the 56-year-old gym instructor, who used to compete in secret to avoid upsetting her mother.
"I want to create the kind of body that stops people in the street," laughed Sawada, sporting a bob hairstyle and silver nail polish.
"I don't care what people think - you can express feminine beauty with this kind of body."
Horsemeat for breakfast and lunch
Other bodybuilding sub-genres have sprung up in Japan, including "bikini fitness" - a category that has turned Yuri Yasui into a magazine cover girl.
A two-time Japan champion, the statuesque 33-year-old is another who caught the workout bug after initially wanting to lose weight.
"When I started training seriously, my parents were dead against it - even my friends were," said Yasui, a bank employee from Nagoya, a city southwest of Tokyo, who won her first national title less than a year after taking up the sport.
"They didn't want me up there in front of strangers in a bikini flashing my bottom."
Japanese bodybuilder Yuri Yasui works out at a gym in Tokyo. (Behrouz Mehri/AFP)
"At long last women are starting to work out regularly, but Japanese men still don't really accept muscle-bound women," added Yasui after lifting weights at a Tokyo gym.
"It's important to change attitudes. The way to a feminine body - getting that tiny waist and a round bum - is by building muscle."
Yasui eats horsemeat for breakfast and lunch to help keep her body fat low and models her striking figure on an American feminist icon.
"Ever since I was at college, I adored Wonder Woman," said the 1.73m tall Yasui, who even wears a star-spangled bikini on the cover of her official video.
"I wanted that hour-glass body with the tight waist, big breasts and buttocks. You can get it - you just have to work at it."
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