DAVID GREENE, HOST:
Now another issue that comes up during workouts, and this is in all cultures. It is keeping those shoelaces tied. NPR's Rebecca Hersher says there's this new theory about why they come undone and how you can tie your shoes better.
REBECCA HERSHER, BYLINE: Personally, my shoelaces come untied all the time, like, every day. So I identify with this guy.
OLIVER O'REILLY: My shoelaces are chronically coming untied, and this has been happening for probably well over 40 years. I'm Oliver O'Reilly. I'm a professor of mechanical engineering at UC Berkeley.
HERSHER: About a decade ago, O'Reilly started Googling around looking for an explanation for why his shoelaces were coming loose.
O'REILLY: I was trying to teach my daughter Anna how to tie her shoelaces.
HERSHER: There were videos of how to tie good knots, but no one seemed to have the answer to why the knots were coming undone in the first place. So he and two graduate students investigated.
O'REILLY: My friends essentially went - shoelaces, really? Yeah.
CHRISTINE GREGG: I mean, we did all sorts of stuff.
HERSHER: Christine Gregg is one of those grad students. For one experiment, she ran on a treadmill while another researcher pointed a slow-motion camera at her shoes. In another...
GREGG: We swung our legs back and forth while sitting on tables. We were wandering around the halls of the mechanical engineering building just staring at our shoes, watching them come untied. So - makes you very popular at parties.
O'REILLY: We spent a lot of time at parties by ourselves.
HERSHER: Staring at your shoes?
O'REILLY: And staring at other people's shoes, going, wow, that's a very interesting knot.
HERSHER: So after all that, the answer to why shoelaces come untied? Imagine you're running. The impact as your foot hits the ground shakes the knot loose with each step. And the swinging of your legs swings the free ends of the laces.
GREGG: And so you have this interaction between the impact of your foot and the ground that's loosening the knot and the whipping motion that's trying to pull at it and pull it apart.
HERSHER: One interesting thing they found is that catastrophic, fall-on-your-face-type shoelace failure kind of happens all at once.
GREGG: Your lace can be fine for a really long time. But as soon as you get a little bit of loosening, it's all over in about one or two strides.
HERSHER: But you can protect yourself. There are two versions of the common shoelace bow tie knot, and one of them is a lot stronger than the other. It's all about which direction you go in when you get to that final loop in the bow tie. Make sure you go clockwise - it's stronger. You can see photos at npr.org. Check it out. Why knot (ph)? Get it? Rebecca Hersher, NPR News.
(SOUNDBITE OF OSAMU SAKAGUCHI AND RIE OTSUKA'S "SHOELACE")
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