A person walking normally on a treadmill (left) and with their ankle rolled inward when a block in the sole of the shoe is extended (right). Photo credit: Nicholas Kreter.
Even on the smoothest path you can sometimes miss your step and when hiking on uneven trails you're constantly at risk of twisting an ankle. But could you reduce your chance of injury if you knew when your foot was about to be displaced? This was the question playing on the minds of Nicholas Kreter and Peter Fino (University of Utah, USA) and Keith Gordon (Northwestern University, USA); ‘Daily environments feature uneven ground that can shift where the pressure is under our feet’, says Kreter. To find out how people adjust when thrown off step and whether they can adapt if warned in advance, Kreter and Carter Lybbert (University of Utah) designed a pair of athletic shoes equipped with instantly deployable extendable blocks embedded in the sole of the left shoe, which could tilt the foot and roll the ankle either inwards or outwards as the wearer strolled along.
Recruiting 13 young adults from the local community, Kreter equipped them with the custom-built shoes and set them walking at a comfortable speed on a treadmill while recording their every move with a multicamera motion capture video system like the ones used in Hollywood. Then, after 2 min of sauntering, Kreter twisted the walker's foot 6 deg by triggering the block, rolling their ankle outward, sometimes warning the walker 2, 1 or 0.5 strides before with a loud beep so they could prepare for the disruption, or springing the stumble on them without warning. ‘When they first experienced the disturbance, some participants chuckled at the surprise of what it felt like. One person even tried to look at the bottom of their foot to see what they had stepped on’, Kreter laughs. Then, he wrapped up the experiment by tweaking the walker's ankle inward (extending the block under the outside of the walker's foot) after a warning beep and then outward again, for comparison, before analysing how the volunteers shifted their bodies as they responded to the instability beneath their feet.
Calculating how far apart the volunteers straddled their legs for stability during each stride and where their centre of mass was thrown as they stumbled, the team was surprised that the walkers seemed to make no attempt to protect themselves from taking a gentle tumble by setting their feet wider apart when alerted during the preceding stride. However, when warned two strides in advance, all of the walkers widened their strides for stability, but this strategy did not necessarily help them to recover faster. When the walkers knew how their left shoe was going to try to send them off course (rolling either inward or outward) they recovered within two steps, in comparison to walkers that were less certain how their shoe was going to destabilise them, which took four steps to recover.
‘The quicker recovery when people knew what was coming suggests that individuals could prime a response and anticipate how the block will roll their ankle, enabling them to recover more effectively’, says Kreter. And how could this discovery help people with impaired mobility? ‘This study tells us that it's important for individuals to understand how their walking environment affects them’, he says, adding, ‘In the future, practice with our shoe may help people better anticipate the effects of obstacles in their daily environments, enabling them to recover their balance more effectively and prevent falls’.
