How many years have you been tying your own shoes? Chances are, you've been doing it wrong all this time. That's the result of extensive research conducted by a team at UC Berkeley led by Oliver O'Reilly, professor of mechanical engineering.
O'Reilly first became intrigued with shoelaces and how they come undone while teaching his daughter to tie her shoes. It's the sort of question he studies, so he began a series of experiments in which subjects ran or walked on a treadmill, stomped up and down, or simply swung their legs back and forth in the air, to see whether impact helps untie shoelaces. In all, the team spent about 100 hours over two years testing shoelaces and watching how and why they untie. They found that both impact and forward motion are essential to the process. When they stomped their feet in one place, or swung them back and forth, the shoelaces stayed tied. But with a combination of impact and forward motion, shoelaces loosen slightly with each step. You might not notice it for quite a while. But then, the shoelaces reach the point where they're loose enough to start slipping. And then they untie fast, often within a couple of strides. O'Reilly and his team published their results in Proceedings of the Royal Society A, in a paper titled "The roles of impact and inertia in the failure of a shoelace knot." (You can't make this stuff up.) Seriously, though, they believe this research could have real implications for things like sutures and working with DNA.
They also discovered that there's a right way and a wrong way to tie your shoes--at least if you want them to stay tied. Most people tie their shoes with a weak knot that will unravel relatively easily, instead of a strong knot that has much more staying power. With a weak knot, the tied shoelaces tend to lie vertically along your shoe, whereas with a strong knot, they tend to lie horizontally across it. There are several diagrams in the shoelace paper that illustrate the differences between the weak and strong knot, which is also detailed in a popular three-minute TED Talk by Terry Moore.
Most people learned to tie the weak knot. If you're one of them, you can change to the strong knot by wrapping the shoelace around to create a bow in the opposite direction from what you're used to. (I seem to have been taught the strong knot, something of a waste in my case because my father instilled the habit of double-knotting my shoes in me so strongly that I can't help but do it, so they almost never come untied anyhow.)
O'Reilly's experiments proved what Moore learned by experience: Tie your shoes with the strong knot and they're much less likely to come undone. And, as Moore points out, your shoes will also look better, because the bow on them will lie straight across your foot instead of turning itself sideways.
In other words, if you've been using a weak knot all these years, it's probably worth the effort to switch to the strong one.