Each hour of TV that a two to four-year-old watches elevates the risk of a larger waistline and reduction of muscle fitness, says a study. According to experts, children should not watch more than two hours of TV a day.

"We already knew that there is an association between pre-school television exposure and the body fat of fourth grade children, but this is the first study to describe more precisely what that association represents," said Linda Pagani, who conducted the research with Caroline Fitzpatrick, both from the University of Montreal and its St-Justine Mother and Child University Hospital.

"Parents were asked about their child's TV habits. Trained examiners took waist measurements and administered the standing long jump test to measure child muscular fitness," said Pagani.

"We found, for example that each weekly hour of TV at 29 months of age corresponds to a decrease of about a third of a centimetre in the distance a child is able to jump," said Pagani, the International Journal of Behavioural Nutrition and Physical Activity reported.

"The pursuit of sports by children depends in part on their perceived athletic competence," Fitzpatrick said. "Behavioural dispositions can become entrenched during childhood as it is a critical period for the development of habits and preferred activities.

"Accordingly, the ability to perform well during childhood may promote participation in sporting activities in adulthood," Fitzpatrick said, according to a university statement.

Along with their parents, 1,314 children from the Quebec Longitudinal Study of Child Development database participated in this study. When the children were 2.5 to 4.5 years of age, their parents reported how many hours of TV during the week and weekend they watched, said a university statement.

The average was 8.8 hours per week at the onset of the study, a figure that increased on average by 6 hours over the next two years to reach 14.8 hours per week by the age of 4.5 years.

Thus, 15 percent of the children participating in the study were already watching over 18 hours per week according to their parent's reports at that time.