Researchers at the University of California, Berkeley, have unraveled one of life’s enduring mysteries: why do shoelaces come undone? The researchers – Oliver M O’Reilly, a professor of mechanical engineering, and Christine E Gregg and Christopher A Daily-Diamond, students who are pursuing a PhD in mechanical engineering – reported that the force of your foot striking the ground and the motion of your leg combine to help loosen and ultimately untie the knot. When running, the foot hits the ground at about seven times the force of gravity. That impact is transmitted to the knot, which stretches and relaxes in response. As the knot loosens, swinging legs apply an inertial force on the free ends of the laces and – voilà! – pretty soon your laces are flopping around, looking like overcooked spaghetti. The researchers identified “strong” knots, those commonly associated with square knots, and “weak” ones tied in the style of a “granny” knot. For a quick way to know which is which, look at the loops of your laces. A weak knot will typically have one loop pointing toward the toes and one toward the ankle. A stronger knot typically has the loops oriented on opposite sides of each other, Daily-Diamond said. For the shoelace-challenged, there are, of course, other options, such as loafers or sneakers with Velcro fasteners. “We spent countless weekends walking up and down the hallways and staring at shoelaces, watching them coming apart,” Gregg said. Initial experiments involved so-called barefoot running shoes, dress shoes, running sneakers and hiking boots. Gregg said she spent hours sitting on a table, swinging her legs to see if the movement had any effect on the knot. It didn’t, and neither did merely stomping her feet. The force of her feet hitting the floor when she was walking or hitting the treadmill when she was running, combined with her gait, eventually caused the shoelaces to come undone. In an article published on Wednesday in the Proceedings of the Royal Society A, the researchers wrote that the unraveling was “a sudden and catastrophic phenomenon.” The researchers used a high-speed camera to record Gregg running, and an accelerometer was attached to the knots to measure the forces on her feet. Many variables, such as shoe and lace types as well as the tightness of the knot, were not assessed. “If you do a whole bunch of other variables, it just explodes,” O’Reilly said. Coming up with the hypothesis and testing it took two years. O’Reilly added that the work was a “labour of love” that took place on nights and weekends after the researchers were done with their full-time jobs. How does this research matter beyond the inconvenience of having to retie your laces? It has practical applications to things such as improving surgical sutures, he said. O’Reilly said the project began to germinate when he was teaching his daughter, Anna, now 14, how to tie her shoelaces when she was about five. He said that for just about all of his 52 years, his laces have come undone. “I didn’t want her to inherit my problems,” said O’Reilly, who wears Converse sneakers. So, after his extensive research, how have his laces been? He said with a laugh, “It’s catastrophic. Every day.”