As the new generation spends most of its time in online activities, a new research has revealed that children are becoming weaker, less muscular and unable to do physical tasks that previous generations found simple.

The researchers say the 10-year olds can do fewer sit-ups and are less able to hang from wall bars in a gym, when compared with the kids a decade ago. Arm strength has declined in that age group, as has their ability to grip an object firmly.

The findings, published in the child health journal Acta Paediatrica, have led to increased concerns about the impact on children's health caused by the shift away from outdoor activities.

Academics led by Dr Gavin Sandercock, a children's fitness expert at Essex University, studied how strong a group of 315 Essex 10-year-olds in 2008 were compared with 309 children the same age in 1998. They found that:

• The number of sit-ups 10-year-olds can do declined by 27.1% between 1998 and 2008
• Arm strength fell by 26% and grip strength by 7%
• While one in 20 children in 1998 could not hold their own weight when hanging from wall bars, one in 10 could not do so in 2008.

"This is probably due to changes in activity patterns among 10-year-olds, such as taking part in fewer activities like rope-climbing in PE and tree-climbing for fun," Sandercock said. "Typically, these activities boosted children's strength, making them able to lift and hold their own bodyweight."

The fact that 10% could not do the wall bars test and another 10% refused to try was "really shocking", he added. "That probably shows that climbing and holding their own weight was something they hadn't done before."

Previous research has already shown that children are becoming more unfit, less active and more sedentary and, in many cases, heavier than before.

But the new study also found that children in 2008 had the same body mass index (BMI) as those a decade earlier. Lead author Daniel Cohen, of London Metropolitan University, said this meant that, given their declining strength, the bodies of the recent test group are likely to contain more fat and less muscle then their predecessors. "That's really worrying from a health point of view. It's good news that their BMI hasn't risen, but worrying that pound for pound they're weaker and probably carrying more fat," said Sandercock.

"Climbing trees and ropes used to be standard practice for children, but school authorities and 'health and safety' concerns have contrived to knock the sap out of our children," said Tam Fry of the Child Growth Foundation.

"Falling off a branch used to be a good lesson in picking yourself up and learning to climb better. Now fear of litigation stops the child climbing in the first place."

He added: "Fitness tests may or may not be appropriate, but Sandercock should not be discouraging the use of BMI measurements."

A UK government official said the government had introduced several programmes promoting active lifestyles among the young, and the health survey for England reported back on physical activity levels. She added: "The Department of Health has no current plans to introduce fitness testing for children."