It’s long been known that too much sitting can be bad for adults’ health. Now, a new study suggests that it’s also bad for children.

In one of the few studies of its kind involving children, researchers from the University of British Columbia found that sitting for long periods seemed to have a negative effect on girls’ blood vessel function.

This is how the researchers did the study: Girls between the ages of 9 and 12 were divided into two groups. One group of girls sat uninterrupted for three hours as they watched movies and played on tablet computers. The other group did the same, but got up once an hour to ride a stationary bicycle for 10 minutes before sitting down again. A few days later, the two groups traded places.

The effects on the girls’ arteries were stark: Among those who didn’t move for three hours, arterial dilation — the normal, healthy widening of blood vessels — fell by one-third. The girls who took breaks from sitting and used the stationary bicycles had no decline in artery function.

Fortunately, the girls who did nothing but sit bounced back quickly. When they were later put in the group that rode the bicycles once an hour, their artery function returned to normal.

So what’s the takeaway for parents? Clearly, children should not be sitting continuously for long periods, researchers said. While they don’t know the long-term effects of prolonged sitting in children, researchers noted that even the modest bike riding done by the study participants offset the effects of prolonged sitting.

For parents, though, it might mean something else: Telling your kids to sit down and be quiet might not be the best thing after all.