Why Can Shoelaces Suddenly Come Undone While You Walk?

Shoelaces often do not loosen gradually enough to be noticed. A knot can look secure and then collapse within a few steps. A 2017 study in Proceedings of the Royal Society A used high-speed video, treadmill trials and a pendulum apparatus to show that failure depends on two repeated actions working together.

When the foot strikes the ground, impact makes the knot compress and deform cyclically, progressively weakening its frictional constraints. As the leg swings, inertia makes the free ends whip back and forth, pulling on lace segments inside the knot. Swinging without impact, or impact while the free ends are restrained, is much less effective. Once the knot loosens beyond a certain point, a slightly longer free end experiences a stronger pull. This creates positive feedback: the farther it slips, the more readily it is pulled again, until the knot can fail completely within only a few strides.

This does not mean every knot is equally unreliable. The researchers compared two familiar bow knots that look similar but have different topology. The version based on a square knot was generally stronger, while the skewed form related to a granny knot failed more readily; even the stronger form, however, could come undone under sufficient repeated loading. Gravity is not simply dragging the knot apart. Impact first weakens its constraints, then inertia pulls the free ends. A tiny everyday failure thus reveals how friction, cyclic loading and instability work together.

https://royalsocietypublishing.org/doi/10.1098/rspa.2016.0770
https://vcresearch.berkeley.edu/news/science-shows-why-shoelaces-come-untied