The Proceedings of the Royal Society has published a research paper on the roles of impact and inertia in the failure of a shoelace knot. In other words, why your shoelaces come untied. The study is impressive, with pendulum swings and measurements of force and plenty of data. The short version is easier to understand.
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.
This explains why velcro shoes are preferred by older people and mothers of young children. Read a synopsis of the research at Geeks Are Sexy.
(Image credit: Daily-Diamond,Gregg, O'Reilly)