Not even a freezing afternoon can tempt Yusaku Hosoe to retreat to the warmth of his home. He and a dozen friends are lured to Kukizaki park by a new interest in keeping fit, their exertions punctuated by peals of laughter that echo across the deserted children's playground nearby.
There are no dumbbells or treadmills at the park in Tsukuba, a university town north of Tokyo, just a few pieces of equipment specially designed to strengthen muscles and improve posture and balance.
If those sound like modest goals, it is because most of these amateur athletes are in their 70s. They are part of a growing band of Japanese retirees who are taking over public parks that were once the preserve of toddlers and their mothers.
The boom in workout classes and exercise equipment for "silver" athletes is being fuelled by Japan's skewed demographics. More than a fifth of the country's 127 million people are aged over 65 – including 40,000 centenarians – while the declining birthrate means 40% of Japanese will be above retirement age by the middle of the century.
In suburban playgrounds that once reverberated to the sound of children at play, you are as likely to find a group of septuagenarians doing pushups, stretching, and feeling their way across knobbly reflexology mats.
Faced with the longest life expectancy in the world, a ballooning public debt and a soaring welfare bill, Japan is hoping that regular, if gentle, exercise will keep more over-65s out of retirement homes and hospitals.
"We built these exercise areas to promote fitness among senior citizens and, we hope, bring down healthcare costs," says Hiroko Asano, of Tsukuba council's senior services division.
"We show beginners how to use the equipment, and then leave them to it. The idea is that they can come any time, alone or with friends, and make it part of their routine."
Tsukuba's two special fitness parks, which opened last month at a cost of 3m yen (£20,000) each, are already proving popular with the town's older residents.
"It's a lot more fun to work out with friends," says Hosoe, a 74-year-old former civil servant. "It's not just about wanting to stay fit into old age, but about having a laugh and helping each other out."
Nationwide, the number of pieces of workout equipment for the elderly has tripled since the late 1990s to well over 15,000, according to the government.
While the proportion of children under 13 who make daily visits to neighbourhood playgrounds fell from 50% in 1971 to 34% in 2007, the number of elderly people using them has doubled.
The parks also perform a crucial social service in a country where an increasing number of people live alone. By 2025 more than a fifth of households will comprise an elderly person living alone or an elderly couple, according to the national institute of population and social security research in Tokyo.
"The reason is the trend towards marrying late or not at all, and the fact that many elderly people will live alone rather than with their children after their spouse dies," the institute's Hachiro Nishioka said.
Few of the people who work out at Kukizaki or any of Japan's dozens of other renovated parks harbour ambitions to emulate 74-year-old Tsutomu Tosaka, Japan's "masters' bodybuilding" champion.
"This isn't a cardiovascular workout," says Masato Saijo, director of a physical fitness association that has installed equipment in several parks where swings and climbing frames have fallen into disrepair. "The idea is to work on important muscles that are at risk of atrophying in old age. After we teach them how to use the equipment they're on their own. It teaches them to be more independent.
"Japanese society tends to view old people as precious objects who need to be kept indoors and spoiled. But the best way to stay healthy and active is to spend more time outside. We are animals after all."
