Proc. R. Soc. A doi: 10.1098/rspa.2008.0367 (2008)

Credit: P. GREENHALGH (UKPIX.COM)/ALAMY

It's the way people balance themselves rather than the timing of their steps that makes certain bridges wobble, John Macdonald of the University of Bristol, UK, has found.

The Millennium Bridge (pictured), a footbridge straddling the Thames in London, closed just days after it opened in June 2000 because it was wobbling sideways. It looked as if pedestrians were synchronizing their steps in time with the wobble, and that this was exacerbating the movement.

But Macdonald's model says that wasn't what was happening at all. By looking at the biomechanics of how humans balance, Macdonald has revealed the effects of the adjustments people make to keep their balance, by sticking one foot further out to the left or right. These add energy to the bridge's natural wobble. Once a critical number of people start trying to balance in this way, that extra energy becomes significant, and increases the bridge's sway.