Have you ever wondered what causes your shoelaces to loosen even when you tie them as firmly as possible? It is because while running, the force of a foot striking the ground stretches and then relaxes the knot, a new study has shown.
As the knot loosens, a second force caused by the swinging leg acts on the ends of the laces, like an invisible hand, which rapidly leads to a failure of the knot in as few as two strides after inertia acts on the laces.
The findings, published in the journal Proceedings of the Royal Society, may help understand things like DNA that fail under dynamic forces, the researchers 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,” said Christopher Daily-Diamond, a graduate student at the University of California-Berkeley.
Using a slow-motion camera and a series of experiments, the researchers assessed a pair of running shoes that were laced-up and were on a treadmill.
They found that shoelace knot failure happens in a matter of seconds, triggered by a complex interaction of forces, as when running, the foot strikes the ground at seven times the force of gravity.
In addition, the study showed that some laces might be better than others for tying knots, but the fundamental mechanics causing them to fail is the same.
“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,” said Christine Gregg, who is a graduate student at the University of California-Berkeley.