Primary and nursery pupils as young as threeJare working out on exercise bikes and treadmills supplied by a new company.
Gymkids produces smaller, brightly coloured versions of adult exercise machines, which enable children to mimic their parents' fitness regime.
The company, whose founders include two former teachers, is targeting nurseries and primary schools, with different machines recommended for each key stage. But it has been criticised for encouraging an obsession with fitness and weight.
Steve Bloomfield, of the Eating Disorders Association, said: "What's the message that you're giving to a child with this equipment? It's unlikely to be based around self-esteem or feeling good as you are. If prepubescents drop below their appropriate weight, they risk never developing sexually.
Their fertility may be dramatically affected."
And Jeya Henry, professor of nutrition at Oxford Brookes university, said:
"It would be a terrible tragedy if we tell kids to go on these machines, rather than running around school grounds," he said. "As a nation, we're obsessed with trying to maintain body weight in very young children."
Gymkids, which was launched in March, markets different ranges for nursery, key stage 1 and 2 pupils. So far, 120 primaries and nurseries across Britain have bought the equipment. These include air-walkers and rowing machines. Most items cost pound;120.
Several machines include a display panel, which highlights time spent, distance covered and the amount of energy burned up.
Matt Runacus, head of PE at Parkfield primary, in Birmingham, has already purchased several machines. He said: "We use the calorie-counter in a positive way, for cross-curricular activities. In maths, they record how far they can row in three minutes.
"Children who aren't necessarily good footballers or cricketers can compete against themselves."
But Mr Bloomfield said: "Gym exercise and burning calories could become an obsessive or self-competitive concern. And if you burn more calories than you eat, you stand a high chance of becoming anorexic."
Stephen Smith, Gymkids director, said that the machines all involve cardiovascular exercise - none of them builds up muscle. He said: "We don't want children to grow up too quickly, but this is a great way to encourage them to get involved in physical fitness. It's a fun way to combat the obesity epidemic in this country.
"Yes, children should be outside playing, but that doesn't happen. Our products interest them. Four or five-year-olds view it as a form of active play, not an addiction."
He said that inner-city schools can use the equipment to compensate for limited outdoor space. Raju Surelia, who has bought several machines for her Little Learners nursery, in Solihull, agrees. "Children will sit and do a puzzle, then go and get rid of some energy on the treadmill," she said.
But Rebecca Crutchley, a teacher at Lloyd Park nursery, in east London, will not be adding the bikes to her equipment. She said: "Exercise should be an integral part of children's lives, not something that has to be planned for."