Experts at a top university have solved one of the great scientific problems - why your shoelaces come undone.

They found repeated impact of the shoe on the floor serves to loosen the knot.

Leg swing as we walk makes the end of the laces go into a whipping motion, causing them to slip, and this combination eventually leads to the whole thing unravelling.

The feeling of footwear suddenly becoming loose has frustrated humans for thousands of years.

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Researchers were actually probing how knotted structures fall part.

They studied slow motion footage of a runner on a treadmill.

It showed that when the knot in her trainers failed the laces came undone in as few as two strides.

Mechanical engineer Christopher Daily-Diamond from the University of California in Berkeley, said: "This is the first step toward understanding why certain knots are better than others."

The study said there are two ways to tie the common shoelace bow knot.

One holds for longer than the other, though it is not known why, but both fail the same way.

Prof Oliver O'Reilly, whose lab conducted the research, said: "We still don't understand why there is a fundamental mechanical difference between those knots."

Researchers found tightly-tied laces need more cycles of impact and leg swinging to fail than normal in a day.

This is why laces don't always come undone.