Researchers at the University of California, Berkeley, have unraveled one of life’s enduring mysteries: Why do shoelaces come undone?

Forces at work

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 Ph.D. 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.

Knots matter

The researchers identified “strong” knots, those commonly associated with square knots, and “weak” ones tied in the style of a “granny” knot.