A new study conducted by researchers at the University of California, Berkeley explains exactly why shoelaces come undone. Researchers claim the culprit is “a double whammy” of stomping and whipping forces that act like an invisible hand loosening the knot and tugging on the free ends of the laces. For the study, researchers filmed a runner on a treadmill in slow-motion. By attaching accelerometers to the runner’s feet while she walked and ran in different shoes, the team was able to determine that the foot strikes the ground at seven times the force of gravity. In response to that force, the knot stretches and then relaxes. Then, 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. Researchers were surprised by how many forces were interacting on the foot, while walking or running. By the way, Nike has just developed self-lacing running shoes.
Image source: https://commons.wikimedia.org/wiki/File:Shoelaces_02.jpg