Yokohama, Japan: Walking or running in a full suit of samurai armour is not the easiest thing in the world. Swimming in it is even harder, but that’s exactly what some in Japan are doing. For fun.

“It’s heavy, and it’s hot in here. ... Fan me hard,” Mutsuo Koga, a 27-year-old doctor, told fellow disciples of traditional Japanese swimming at a recent meet.

“I’m worried about whether I’ll be able to get myself back out of the water. It’s been three years since I last swam,” said Koga as he readied to take the plunge in a public swimming pool.

Traditional swimming was developed during Japan’s feudal 15th and 16th centuries, when feared samurai swordsmen roamed the country enforcing their masters’ will.

Part survival technique — there were times when a warrior just had to swim for it, boots and all — and part aesthetic performance, traditional swimming now has its place in the pantheon of Japanese martial arts.

And like all martial arts, its adherents say it has real-life applications.

“The primary purpose of this kind of swimming is to acquire practical skills for swimming in a real environment,” said Tadao Koga, Mutsuo’s father and the grand master of the Kobori school, one of twelve recognised by the Japan Swimming Federation.

“If you can swim fast using Western strokes, that doesn’t mean you can survive in a natural environment,” said Koga, 67.

Traditional swimmers have to master a kind of treading water, which will allow them to withstand powerful waves near a seashore.

They also have to learn the “hayanuki” stroke, vital for swimming against a current or up a river, thrusting your body high into the air as your arms crash into the foam.

Whatever happens, you have to keep your head above the water and your wits about you — that obstacle looming in front of you could be a rock, or it could be an axe-wielding enemy.

Oh, and you have to do it while wearing 15 kilograms of armour.

The appeal of the sport is that “swimmers compete on form and beauty, rather than the speed of swimming”, said Yoko Suzuki, 25, a champion of women’s traditional swimming.

Masahiko Yaginuma, chairman of the Japanese Traditional Swimming Committee of the JSF, said the form was widely taught in schools in the early 20th century but fell out of favour and is now only found in a few institutions.

“Nowadays women in their 60s or older are the main group of new learners,” because they see traditional swimming as an extension of the other noble arts a cultured Japanese woman learns — like flower arranging and tea ceremony, he said.

Briton Antony Cundy, a Tokyo-based advertising executive who has spent years being tutored in the Kobori school of swimming, said there were real benefits to taking up the sport.

“It’s a fun way of getting fit and enjoying Japanese culture and history,” said Cundy.

“It’s astonishing you don’t see many foreigners enjoying it.”

Around a minute after he slid heavily into the water, Mutsuo Koga slowly and unsteadily hauled himself out to roaring applause from the assembled crowd.

He had managed about 20 metres using a stroke designed to make him glide across the surface of the water, his helmeted head making no abrupt moves.

“Your chest was supposed to be above the water,” Tadao chastised, as his son stood panting on the poolside.

“I was not very good,” gasped Mutsuo. “My body didn’t float at all.”