The scientists expected that the knots would come undone slowly. But their slow-motion footage — focused on the shoelaces of a runner on a treadmill — showed that the knots rapidly failed within 1 or 2 strides. To figure out why, O’Reilly and his colleagues used an accelerometer on the tongue of a shoe to measure the forces acting on a knot. They found that when walking, the combined impact and acceleration on a shoelace totals a whopping 7G

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