Hundreds of dancers twirl in unison by a riverside in northeastern China, thrusting hips and shimmying shoulders in an unusual but beloved routine that has taken the country by storm.

China has a vibrant public square dancing culture, with ranks of retirees thronging city plazas at dawn and dusk to groove in time to thumping electronic music.

Despite the occasional noise complaints, authorities generally view the aerobics gatherings as good for maintaining public health in a rapidly ageing society.

And one of the country's most popular dance programmes in recent years has emerged from the fading rust-belt province of Heilongjiang.

Participants in Jiamusi Happy Dancing perform a series of unconventional moves, from strutting with puffed-out chests to pinwheeling their arms.

Founder Yu Jicheng, 73, said the exercises stand out for their "focus on the waist, abdomen... shoulders and hips".

"They're all done on the move, unlike the sedentary routines broadcast on the radio," the retired civil servant told AFP.

Yu designed the routine in 2008 and still teaches daily public classes in Jiamusi, a city of around two million people near the Russian border.

Lean and spry with a ramrod-straight posture, he keeps a close watch over classes of up to 500 participants, contorting himself to demonstrate each move and barking orders at those who execute them incorrectly.

"The aim of all of this is health. Our concept is: you exercise to be healthy and happy, and you do it by relying on yourself," he said after wrapping up an early morning session.

"It relieves fatigue, helps you lose weight and keeps you fit."

China faces a looming demographic crisis as hundreds of millions of people move into their twilight years in the next few decades, putting new pressures on society and the country's fragile healthcare system.

Yu contends that his routine can ease the transition, claiming it helped him lose 30 kilos (66 pounds) since he turned 50 and brought him back within a healthy weight range.

In recent years, other cities in China have also set up Happy Dancing branches.

Regular participant Zhang said eight years of the exercises had left him in far better health. 

"I've definitely felt the effects -- it cleared up my hunched back and my paunch," the 65-year-old told AFP.

"It's great for physical fitness, as it's systematic and scientific in making use of each joint."

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