Shoe-string theory: Science shows why shoelaces come untied

The researchers found that a shoelace knot unties like this: When running, your foot strikes the ground at seven times the force of gravity. The knot stretches and then relaxes in response to that force. As the knot loosens, the swinging leg applies an inertial force on the free ends of the laces, which rapidly leads to a failure of the knot in as few as two strides after inertia acts on the laces.

“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,” said Gregg, a Berkeley Chancellor’s Fellow. “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.”

In addition to the dynamic interaction of forces on the knot, the footage also revealed a large magnitude of acceleration at the base of the knot. To dig deeper, the researchers then used an impacting pendulum to swing a shoelace knot and test knot mechanics using a variety of different laces.