"Would you feel like running round the block when you were pregnant?" Wollongong University's Julie Steele asked.
"There are these programmes to help them lose weight, but it is the everyday life activities that these kids are struggling with," she told The Sydney Morning Herald.
One-third of Australian children are either overweight or obese - double the rate in 1985.
And Australian kids are piling on the pounds even faster than those in the US. However, childhood obesity is growing at a more alarming rate in New Zealand, Canada and the former East Germany.
Steele and her team studied a group of 43 eight-year-olds who each weighed in at around 40 kg. They took twice as long as average-weight kids to get out of a lounge chair. Some even needed assistance.
"They have flatter feet, collapsed arches," explained Professor Steele. "We think they are just more uncomfortable all the time."
The results of the study were published in the International Journal of Paediatric Obesity.
Steele added that the right approach was not ordering them out to play but helping them deal with daily difficulties like standing up and moving around.