Children taking part in a study to measure how much exercise they do fooled researchers by attaching their pedometers to their pet dogs.
About 200 children in east London were given pedometers to automatically count how many steps they walked and ran.
Mile End Center for Sports and Exercise Medicine was surprised by the activity levels recorded in some obese children.
Professor Nicola Maffulli said: "Then we realised they were attaching the pedometers to their dogs' collars."
The pilot study in Whitechapel required 11 and 12-year-olds to clip a pedometer to their waists, with researchers at the center collecting the readings by satellite.
"But after a week we found there were some kids who were extremely active but still obese," said Professor Maffulli.
It was "not unheard of" for participants in previous studies to manipulate the readings of pedometers, he added.
Once adjusted to take into account the help from pets, the study indicated that boys in the borough walk or run 12,620 steps a day, below the recommended level of 15,000 steps.
It also found that girls take 10,150 steps, falling short of the recommended 12,000 steps.
It indicated that more than a third of 11 and 12-year-olds in the borough of Tower Hamlets are overweight or obese - 11% higher than the national average.