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Why Do Your Shoelaces Come Untied When You Run?

A study in slow motion.

As the legend goes, Alexander the Great loosened the Gordian Knot with a slice of his sword.

Had he been a little less eager to storm through Asia Minor, Alexander might have seen the knot unravel spontaneously, as if picked apart by invisible hands. That’s the gist of new research from the University of California—Berkeley, which took on a more pedestrian knotting problem: Just why it is that your shoelaces come undone when you run?

When people run and their feet strike the ground, they create a force that first stretches, then relaxes, the knot on their shoes. The swinging motion of their legs, meanwhile, creates inertial forces that tug on the ends of the laces, an effect researchers exaggerated by adding weights to the ends of the laces.

These forces all conspire to loosen the knot, as shown in this video of a research subject on a treadmill.

The researchers showed weaker knots will always fail, but that even strong knots come undone with enough time under the right forces. Not unlike a brashly conquered empire torn asunder by rival ambitions after the early death of its ruler. Patience, Alexander.