Every parent knows it’s not great for their child’s health to have them watching hour after hour of television. Now, a new study has quantified just how unhealthy it is.

Canadian researchers have found that every hour of TV that two- to four-year-olds watch contributes to their waist sizes and their ability to perform in sports by the end of grade 4.

Lots of studies have linked excessive TV watching with childhood obesity. But this study is the first to look at how time in front of the TV affects kids’ strength, using a test called the standing long jump, which is a good measure of something called “explosive leg strength.”

The long jump test can be an important indicator of a child’s athletic ability, since explosive leg strength is needed for a number of sports such as running, skating, basketball and football.

Researchers at the Universite de Montreal and its affiliated Saint-Justine Mother and Child University Hospital looked at 1,314 children from the Quebec Longitudinal Study of Child Development database.

When the children were between two-and-a-half and four-and-a-half years old, the kids’ parents were asked about how many hours of TV the kids watched during the week.

The average was 8.8 hours per week at the beginning of the study. That figure increased as the kids got older, so that by the time most kids were four and a half, they were watching almost 15 hours of TV a week.

When the kids were in Grade 4, the researchers tested the kids’ athletic ability again. They found that each weekly hour of TV the kids watched at 29 months of age was linked to a drop of about a third of a centimetre in the distance the child could jump in a standing long jump test, compared to the average.

In terms of waist size, the researchers found that each additional hour of weekly TV logged between age 2.5 and 4.5 was linked with an increase of waist size of about half a millimetre.

So by the time a child who watched 18 hours of television at 4.5 years old reached age 10, or Grade 4, they would have gained an extra 7.6 millimetres around his middle.

Lead author Dr. Caroline Fitzpatrick notes a drop in athletic performance even in the younger years could affect children’s participation in sports for the rest of their lives.

"The pursuit of sports by children depends in part on their perceived athletic competence," Fitzpatrick said in a news release.

"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."

The Canadian Paediatric Society discourages screen-based activities for children under 2 and advises limiting older kids’ use of TV to less than two hours per day, or no more than 14 hours a week.