Berkeley researchers believe your shoelaces coming untied is just a matter of time. Their study found that unwanted untying comes from two forces combining to replicate the effect of the wearer intentionally pulling the laces undone.
The research was originally done to explore why two different versions of the common “bow tie” knot have differing likelihoods of staying tied. While that may seem trivial, it’s a potentially useful exercise in exploring structural integrity, which can apply to engineering projects where the consequences of failure are much more serious.
The difference in question is whether, between the two times the laces are crossed over in tying the bow tie knot, there’s switch between which side crosses over on top. If there is, the knot is considerably stronger and lasts longer before coming undone.
While exploring that topic through slowmotion video of a knot untying as a runner used a treadmill, the researchers found the cause and effect was remarkably similar to manual untying. It’s caused by the combination of the foot striking the ground and the foot swinging.
The foot hits the ground at around seven times the force of gravity. That impact causes the knot to stretch and then relax as the force’s effect fades, which in turn loosens the knot. When the foot is swinging during a stride, inertia pulls the ends of the laces apart, eventually with enough force for the loosened knot to unravel.
The researchers concluded that while it doesn’t always happen before you take your shoes off, the unwanted untying would always happen in the end if you ran for long enough. They also figured out that once the impact of striking the ground loosens the knot to the point of no return, it can take as little as two extra strides for the inertia to pull the laces undone.
One possibility is that the effects of the impact can vary from “strike to strike”, which might be why a pair of laces that feel perfectly tight can come undone in a matter of seconds.