Children are getting weaker because they are doing less exercise, according to worrying new research from the University of Essex.
The study found today's 10 year olds are taller and heavier than those measured 16 years ago, but there has been a significant decrease in muscle strength.
Scientists are warning that inactivity in childhood can lead to multiple health problems in adult life.
Dr Gavin Sandercock, who led the study, said: "The findings speak for themselves. Year-on-year we keep finding lower and lower fitness levels suggesting children are doing less and less exercise.
"Inactive lifestyles are a health risk but physical fitness is the single best measure of health in childhood, adolescence and on into adulthood.
"Poor fitness and inactivity lead to multiple health problems in their adult life."
Published in the Journal of Science and Medicine in Sport, the research found that although ten-year-old children measured in 2014 were heavier than in 2008 and 1998 their Body Mass Index (BMI) had not changed because they were also taller.
This latest study, led by Dr Sandercock, used data from the Chelmsford Children's Fitness and Activity Survey, which has been monitoring the fitness of ten-year-olds since 1998.
"As today's ten-year-olds are taller and heavier than the children measured six and 16 years ago we expect them to be stronger and more powerful, but this was not the case," added Dr Sandercock.
"Like other affluent areas in the UK, only 6% of Chelmsford ten-year-olds were obese, a figure that has changed little over 16 years.
"However, over the same 16-year period we have seen a 20% decrease in muscle strength and a 30% decrease in muscle-endurance. Despite being bigger, we are finding ten-year-olds are getting weaker."