Study: Inactive Kids Make for Injury-Prone Adults

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Tweens and young teens go through tremendous growth spurts, during which time they amass 36 percent of the skeleton they’ll have for life. But in order to build strong bones within this crucial growth window, kids must be physically active. New research confirms that if they’re not out skateboarding, shooting hoops, or even walking to and from school, their bones will be weak and they’ll risk suffering fractures or developing osteoporosis as adults.

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Researchers from the University of British Columbia strapped activity trackers on 309 kids ages 9 to 20 for seven days straight to gauge their physical activity patterns. Then they measured how their leg bones developed over the next four years. Based on various assessments of bone size, density, and architecture, the kids who got 30 or fewer minutes of moderate-to-vigorous physical activity a day had up to 13 percent weaker bones than those who averaged the recommended 60 minutes a day.

Sadly, only 43 percent of the boys and just 9 percent of the girls met the 60-minute daily target at the beginning of the study. And as these kids grew older, their physical activity levels dipped another 31 percent.

Not getting enough exercise between the ages of 10 and 16 cheats the body out of its best opportunity to build bone strength, says Heather McKay, study co-author and professor of orthopedics and family practice at UBC. “You gain as much bone within the two years of your peak body growth as you will lose over the next 50 years,” she says. “The skeleton’s growth in that window is especially responsive to lifestyle choices, especially physical activity.”

This is because what builds muscle also builds bone, McKay says. And while any type of physical activity helps strengthen the skeleton, weight-bearing exercises like running, jumping, dancing, and playing team sports are most effective. “When you do weight-bearing exercise, your muscles generate forces on your bones,” she explains. “This signals to the bones to begin forming more osteocytes, which build more bone to better handle those forces.”

On the flip side, if you don’t get enough exercise, like most of the kids in this study didn’t, your bone strength dwindles. “It’s a drag carrying around all that bulk if it’s not being used,” McKay says. “So if you’re always sitting, your skeleton won’t think it needs so much mass, and bone will begin to resorb.” Therefore, she says, a teen who sits all day at school then goes home and plays video games, sits down for dinner, and watches TV afterward might not develop enough bone strength to stave off issues down the road. “But if he does all that and also goes to basketball practice or does short bouts of jumping or running that add up to an hour, he’ll be protected against bone loss,” McKay says.

Looking back on your own life, if you were a sedentary kid but then started exercising in early adulthood, you’re not totally screwed. “You can retain the bone you already have and make small gains in bone strength later on,” McKay says. “But we reach peak bone strength around age 30. After that, you’ll start losing mass, although physical activity and a healthy diet can help you retain more.” However, if you didn’t move much then and you still don’t now, your risk of serious fractures, especially in your retirement years, is much greater. “Fractures can be devastating events,” McKay says. “Twenty percent of those who have hip fracture die, and 50 percent never make it home from the hospital.”

To avoid these fates, the solution is simple: start moving. And to ensure your kids don’t wind up with weak bones, grab a ball, pick up a racket, or lace up your sneakers and get active together. 

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