Why kids look funny when they run

little girl running

TODDLER TROT  Children’s short legs leave them little time to lift off the ground when running, a new study shows. 


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My 2-year-old daughter loves to run, and I love to watch her. She propels her little body forward with gusto, creating a hilarious vision of bobbly legs swinging out, head bouncing high and a jiggly smile. Her wild movements definitely get her from place to place, but they don’t yet resemble the smooth, controlled gait of an adult runner.

It turns out that there’s a reason for her wild running, and it’s not that she’s still learning how to do it, says locomotion researcher Jim Usherwood, coauthor of a study in the SeptemberJournal of Experimental Biology that measured how children and adults run.

Short legs mean a short stride, and that means that toddlers’ legs simply don’t have the time they need to bound up into the air like adult legs do. Kids’ feet stay earthbound for much of their trot. “Even when they think they’re running, they’re barely getting off the ground,” Usherwood says. Instead of being “bad” runners, children are simply working with what they got.

IN MOTION Cameras and force-sensing plates captured the mechanics of running children in the lab. Jim Usherwood
That realization came in part from studying the movements of 18 children between ages 1 and 5 (two of which were Usherwood’s daughters), and five adults as they walked and ran. Usherwood and his colleague Tatjana Hubel, both at the Royal Veterinary College in Hatfield, England, used cameras to track positions of limbs, and force-sensing plates to measure how hard their feet were hitting the ground.  

By virtue of their longer legs and longer strides, adults had time to summon up the muscles needed to bound up into the air. But children’s short legs left them with less time to bound, producing what looks like a herky-jerky toddler trot, the researchers found. “If you walk next to a child with your hand on their head, feel for clunk,” Usherwood says. (I’ll be trying that momentarily.) That gait is the result of a complex balancing act of mechanical power and work demands, all calculated seamlessly in a child’s body.

Of course, adults do this calculation too, as a recent study found. When constrained by robotic exoskeletons that changed their movements, people easily found the laziest way to walk, my colleague Meghan Rosen writes.

So it seems that kids aren’t worse at running than adults; they’re just doing it in a way that works better for their stubby little bodies. 

LEG LIFTS Varying leg lengths of study coauthor Jim Usherwood’s 8-, 5-, and 3-year-old daughters mean the girls run differently. Jim Usherwood

Laura Sanders is the neuroscience writer. She holds a Ph.D. in molecular biology from the University of Southern California.

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