With the help of a slow-motion video camera and a few experiments, engineers had received footage of a disintegrating shoelace knot. They didn't need to go far for that: Study co-author Christine Gregg, who is also a runner, laced up a pair of sneakers and ran on a treadmill, while her colleagues filmed the process. Using sensors, the research team measured the dynamic load the lacing were being exposed to.
When running, the foot strikes the ground at seven times the force of gravity, scientists found out. In response to that force, the knot stretches and then relaxes, which is why it loosens. The stomping of the foot applies an inertial force on the loop ends of the laces, until they finally fall apart.
According to engineers, the exact description of this process will help understanding the features of other nodal structures, such as DNA.