"Shoelace Knot Failure" - at last, a scientific explanation for why the common shoelace bowtie knot always comes undone... "When running, your foot strikes the ground at seven times the force of gravity. The knot stretches and then relaxes in response to that force. 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 after inertia acts on the laces." Courtesy of Christine Gregg, a mechanical engineering graduate student at U.C. Berkeley.

http://news.berkeley.edu/2017/04/11/shoe-string-theory-science-shows-why-shoelaces-come-untied/