WATCHING television reduces how far children can jump - and how sporty they will be in adulthood.
Researchers have found that each extra hour of TV watched per week by two-year-olds increased their waist size by half a millimetre and reduced the distance they could leap from a standing start.
That in turn could determine how active they are as adults.
The standing long jump test measures "explosive muscle strength" which is key to most sporting success.
If children do not develop this form of muscular fitness, they will lack the athletic competence to participate in sport as youngsters, and may be put off for life.
Scientists, who followed 1300 children from age two-and-a-half to four-and-a-half, found that for every hour of extra TV a child watched each week, their waist measurement increased by 0.5mm and their jumping ability decreased by a third of a centimetre.
The average child watched 8.8 hours of television a week at the younger age and 14.8 hours when they were older.
But one in six watched more than 18 hours a week, their parents told the Quebec Longitudinal Study of Child Development in Canada.
Experts recommend children over the age of two should watch no more than two hours of television a day.
Dr Caroline Fitzpatrick, of the University of Montreal, who led the study published in the International Journal of Behavioural Nutrition and Physical Activity, said the "pursuit of sports by children depends in part on their perceived athletic competence".
"Behavioural dispositions can become entrenched during childhood as it is a critical period for the development of habits," she said.
"Accordingly, the ability to perform well during childhood may promote participation in sporting activities in adulthood."
The leg strength measured by the standing long jump is crucial for sports such as football, skating and basketball, and loss of this strength is a less-known effect of watching TV.
Better known is that it leads to weight gain, as also revealed in the study which showed a child who watched 18 hours of television at 4.5 years of age would have an extra 7.6mm of waist by the age of 10.
The researchers stress that they have demonstrated an association and further work is necessary to prove that it is causal. But they say the findings should encourage authorities to act to reduce its ill effects.
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