Our brains always plan one step ahead of our bodies when we walk
Keep your head up. Today, navigating the urban jungle can be challenging, with uneven sidewalks and errant kerbs presenting obstacles to easy walking. So why do we rarely trip up even though we hardly ever give walking our full attention? It seems that all we need is a brief glimpse of what’s coming next on the road in front of us, just one step ahead of time, to keep up upright.
Humans have a unique kind of locomotion – we’re bipedal, meaning we move around on two legs rather than four. Scientists are still struggling to unravel the mystery behind our shift to two legs – for instance, some suggest it freed up our hands to carry food.
Others point out that our human gait is much more energetically efficient. Our walking style exploits external forces like gravity and inertia to use as little muscular energy as possible so that we actually fall forward onto the lifted foot with each step.
Jonathan Samir Matthis at the University of Texas at Austin wanted to know how we aim and control this forward motion – particularly since the way ahead is rarely level and obstacle-free. “We have to be much more careful about where we place our feet than we would if we had four legs on the ground,” he says. “Because if we do it wrong, there’s serious consequences like breaking your leg.”
To explore further, his team asked 44 people, aged 18 to 22, to walk across a flat surface while a motion capture system kept track of their movement. The team told the volunteers to step on illuminated “targets” on the ground – but software made sure that these targets were visible only during particular stages of each volunteer’s walking cycle.
Hit the target
It turned out that the participants’ accuracy in stepping on these targets was greatest when they saw them between 1 and 1.5 steps in advance.
Matthis says we walk too rapidly for our brains to micromanage the individual movements of each step. So rather than constantly have our muscles guide where we place each footfall – an energetically costly task – we plan ahead using what we can see of the route in front of us, tailoring the direction we’re going to fall forward one step ahead of time.
“You’re making a plan, and at the same time you’re making that plan, you’re making a movement based on the stuff that you saw a second and half in the past,” says Matthis.
The researchers also tweaked how long the target remained visible before disappearing from view. It turns out that even if the target disappeared after being seen 1 to 1.5 in steps advance, the walkers still had a high chance of stepping on the spot where the target had been. Walkers’ accuracy also improved when they could see two upcoming targets at once.
Our hominin ancestors lived in environments that were anything but flat, working to find food and avoid predators while saving energy. “Vision undoubtedly played a crucial role in these activities,” says Dan Marigold at Simon Fraser University in Burnaby, British Columbia, Canada. “So optimising when to use visual feedback regarding the terrain to ensure efficient locomotion makes sense.”
Matthis says that understanding how far in advance we need to look up when walking could be important for helping people with Parkinson’s Disease or age-related conditions that make walking tricky.
Marigold thinks it could also help better understand the challenges walking poses to those who can’t see very well. “It might be useful for orientation and mobility specialists who teach people with eye diseases how to efficiently use their remaining vision to negotiate the complex world we live in,” he says.
Journal reference: Proceedings of the National Academy of Science, DOI: 10.1073/pnas.1611699114
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