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You put on your shoes, tie them as firmly as possible, but soon after the laces come undone.

Now scientists think they know what causes one of life's knotty problems.

They found the force of a foot striking the ground stretches and then relaxes the knot, while a second force caused by the leg swinging acts on the ends of the laces, like an invisible hand.

The researchers say an understanding of shoelaces can be applied to other structures, such as DNA.

Using a slow-motion camera and a series of experiments, mechanical engineers at University California Berkeley found "shoelace knot failure" happens in a matter of seconds, triggered by a complex interaction of forces.

Lead researcher Christopher Daily-Diamond said: "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 microstructures, that fail under dynamic forces.

"This is the first step toward understanding why certain knots are better than others, which no one has really done."

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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.

They found that when running, your foot strikes the ground at seven times the force of gravity.

Responding to that force, the knot stretches and then relaxes.

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.

Ms Gregg said: "To untie my knots, I pull on the free end of a bow tie and it comes undone.

"The shoelace knot comes untied due to the same sort of motion.

"The forces that cause this are not from a person pulling on the free end but from the inertial forces of the leg swinging back and forth while the knot is loosened from the shoe repeatedly striking the ground."

'Avalanche effect'

Scientists conducted tests with a variety of different laces.

But while some laces might be better than others for tying knots, they all suffered from the same fundamental cause of knot failure, the study, which was published in the journal Proceedings of the Royal Society A, found.

Ms Gregg added: "The interesting thing about this mechanism is that your laces can be fine for a really long time and it's not until you get one little bit of motion to cause loosening that starts this avalanche effect leading to knot failure."