If all this were true then schools and teachers, who should be educating bodies as well as minds, would have to accept some responsibility. But the reality of children's fitness is neither as bad nor as irremediable as the scare stories would have us believe.
At Port Glasgow in Inverclyde where a few huge cranes are all that remains of a thriving shipbuilding industry, PE teachers and education authorities are enticing the potatoes off their couches.
Gourock High School, 9.05 on a chilly autumn morning. First-year boys and girls straggle into the echoing games hall and huddle together in subdued groups. PE teacher Janet Harkness enters, says a few words, and the static scene is transformed.
Children begin running around, their trainers squealing on the parquet floor, changing direction on the teacher's whistle. There are collisions, but the children cope well. One or two carry surplus pounds, but most look fairly fit. No one seems distressed. No one collapses.
The warm-up is followed by rugby coaching - passing the ball. This does cause a few problems. Running forwards while passing back is strange and unfamiliar to many of them, so rugby balls squirt out from children in every conceivable direction, including straight up. Five teams of six are selected and games begin.
A team of girls awaiting their turn bridle at the suggestion they might be unfit. They dance, they go swimming, they do drama - preparing the pantomime is hard work. They enjoy PE and would like more of it. At the end of the session the children troop out of the hall, warm now, noisy, wide awake. "If you think we're unfit," Debbie calls back, "you should see some of the teachers."
"I do think the kids entering secondary school are a bit less fit than they used to be," says Mrs Harkness. "In first year we try to give them a taste of a whole range of physical activities - rugby, hockey, volleyball, dancing, badminton, soccer, basketball, athletics, tennis and gymnastics. But nowadays before any of that, as soon as they come here, all of them do a five-week block of circuits, running and aerobics, aimed specifically at increasing their basic fitness.
"Events like the Inverclyde festival of rugby which introduced the first-years to the sport, are great and the kids love them. But I think we also need more PE specialists going into the primaries. Primary teachers have to do everything these days - languages, PE, music. It's not easy for them."
A few miles along the coast, the recently-renovated Port Glasgow High School, with its freshly-painted maroon walls, white window frames and prominent eaves, looks more like a hotel at a tourist resort than a school in post-industrial Britain. Inside, the PE facilities are impressive, with a large swimming pool, fitness suite, games hall and gym.
"We're very keen on promoting fitness and health here," says acting headteacher Colin Sutherland. "We organise a lot of sports after school and at lunchtime, and we've an outdoor education co-ordinator who takes the kids orienteering.
"This area has a bad health reputation so we're trying to educate the children about fitness. We've a pilot project where one of our PE teachers goes into the five associated primary schools. That's good because you get the specialist input to the children at a younger age."
Over in the games hall the first-year boys are playing basketball. There is enthusiasm and, since they have had a few weeks' practice, more than a little skill. The suggestion that computer games are making children unfit is met with indignation:
"We don't just play computer games. We play sports as well."
"I play football."
"I climb trees."
"I like swimming - there's a great pool here."
"On TV it says walking a mile is as good as running a mile."
"That's right, and you just need to walk half an hour a day."
"It takes me half an hour to walk to school."
"I get lost sometimes and walk round the school for half an hour searching for my class."
Out of eight boys chosen at random, seven play more than one sport outside school hours - various combinations of football, golf, basketball, swimming, ice-skating, cycling and fishing - and only one admits to taking no exercise apart from PE classes.
"People talk about fitness much more than they used to," says Ian Samuel,principal PEteacher, "and there's a notion that everybody is less fit nowadays. But there wasn't any testing in the old days, so they don't really know how fit the kids were.
"In my classes I try to get them moving, get them out of breath. I see them twice a week, and if they have one more session outside school with their heart rate in the 'target zone' - above 140 beats a minute and below 180 - that's the minimum they need. Many get more than that. A lot of the boys come to sports sessions that we organise outside school hours.
"Girls are more difficult. If they are invited to co-ed sports they don't come, so single-sex is better. And as the girls get older, it's harder to get them involved in sports at all. But that's not a new problem.
"I'd like to see more safe routes to schools, so that more of them would walk or cycle. A lot of kids in this country live within a mile of school but still catch a bus or get transport. I go abroad refereeing, and many countries have wonderful cycle paths all over the place.
"But really I'm not aware of any dramatic decline in children's physical activity or fitness levels. Now diet, that's a different story. If you saw the rubbish they eat nowadays you really would be worried about their health. "