Now experts say they have unravelled the mystery of why laces come undone. A pair of forces interact like an invisible hand, first loosening the knot and then tugging until the laces trail on the ground.
Using a slow-motion camera, scientists have revealed how knot-failure happens in seconds, triggered by a complex interaction of forces.
Lead researcher Christopher Daily-Diamond, from the University of California at Berkeley, said: ‘This is the first step toward understanding why certain knots are better than others, which no one has really done.’
OK, so it's annoying your laces keep coming untied, but why should science care? The researchers say understanding the mechanics of knots and how they fail could be useful far beyond simply taking the dog for a jog.
"When you talk about knotted structures, if you can start to understand the shoelace, then you can apply it to other things, like DNA or micro structures, that fail under dynamic forces," said Christopher Daily-Diamond, a graduate student at Berkeley and co-author of a study that will appear Wednesday in the journal Proceedings of the Royal Society A. "This is the first step toward understanding why certain knots are better than others, which no one has really done."
The study began with co-author and graduate student Christine Gregg lacing up a pair of running shoes and jogging on a treadmill while a colleague filmed what happened next.
Here's how shoelace knots come undone, according to the paper. When you run, your foot hits the ground with a force seven times harder than the force of gravity alone. All that impact makes the knot in your laces stretch and then relax while the action of swinging your leg pulls on the end of your laces. In other words, the very action you lace up for also conspires to untie them. As the knot loosens, the swinging leg applies an inertial force on the free ends of the laces, leading to rapid unravelling in as little as two strides.
Of course, we don't all have to re-tie our shoelaces every three steps. The researchers found that different knot-tying techniques, types of laces and levels of knot tightness all factor into how long it takes shoelaces to come undone. A really tight double knot usually gets me through a run, for example.
Sooner or later, things -- or at least the knot in our shoelaces -- fall apart, the researchers seem to be saying. But I have one pair of shoes I slip on and off without ever touching the knot in the laces that's been there for many months now. Have I found, uh, a loophole in the forces that inevitably act to untie my shoes?
Probably not, it seems, because I use those shoes almost solely for biking, which means they don't get that ground-stomping action seen while running.
"You really need both the impulsive force at the base of the knot and you need the pulling forces of the free ends and the loops," Daily-Diamond said. "You can't seem to get knot failure without both."