This is a

This is Scientific Americans 60 Second Science.

I'm Karen Hopkins.

This will just take a minute.

You're strolling down the street,

or maybe hauling that load of laundry down a flight of stairs,

when all of a sudden, your laces can. be hauling that load of laundry down a flight of stairs.

When all of a sudden, your laces come undone.

If you've ever pondered what precipitates this pedestrian wardrobe malfunction,

you might want to tie your shoes and beat a path to the proceedings of the Royal Society A.

In that journal, researchers have trotted out data that show that a combination of whipping and stomping forces is what causes laces to unravel without warning.

The investigators noted that shoelace knots frequently fail when people are walking, but not

when they're, say, sitting on the edge of a table and swinging their legs.

Laces also stay tight when marching in place with no forward motion.

That led the gumshoes to suspect that stepping and swinging somehow worked together to foil footwear security.

But how?

To untangle this naughty problem, the researchers made a slow-mo video of a student running on a treadmill,

thus recording the literal steps that lead to catastrophic

knot failure. Here's what they slowly saw. When the foot strikes the ground, the force of the

impact causes the knot at the center of the lace to deform and stretch.

And when the foot swings forward, the ends of the shoelace fly forward.

That whipping motion pulls the knot open a bit more, which allows the free end of the lace to slip through a tiny bit.

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