GAIT DIFFERENCES AND WHAT THEY MEAN

The way each of us walks is unique and recognizable, whether it’s John Wayne’s slightly off-balance swagger, Dorothy’s skip toward Oz, Mae West’s exaggerated hip swivel, or the lope of Shaggy from Scooby-Doo. This observation is more than anecdotal.

One of the first experiments to test whether people could identify one another by the way they walk involved recording individuals walking and then converting their bodies into a series of small lights, similar to the motion-capture technology used in Hollywood and video games today. That way, study participants could not pick up on cues such as hair colour or body shape. Even when people were turned into a string of lights, their friends were pretty good at spotting them.

Repeated studies have confirmed that we are skilled at recognizing friends and family members solely by the way they walk. As it turns out, regions of our brains are fine-tuned to accomplish this.

A 2017 study had 19 participants lie in an MRI and watch videos of familiar people coming toward them. A region of the brain just behind their ears (the bilateral posterior superior temporal sulcus) activated when they recognized people from their walks. When the walkers were close enough for their faces to be identified, a different area of the participants’ brains lit up.

But the way a person walks signals more than their identity. We are skilled at detecting moods, intentions, and even personality traits from the way someone walks. Slumped shoulders and a plodding gait are recognized as signs of sadness. A bounce in one’s step communicates happiness. A loud stomp can signify anger. Research shows that these influences are not just a matter of intuition.

However, people aren’t always 100% accurate in interpreting these cues, and some of us are better at it than others. We perceive others as adventurous, warm, trustworthy, neurotic, extroverted, or approachable by the way they walk, but the walkers often don’t think of themselves that way at all. It appears that the inferences we draw this way are sometimes wrong.

But, as it turns out, some of those who are particularly good at it are psychopaths. When videos of undergraduates walking were shown to 47 maximum-security prisoners, they were asked how vulnerable the walkers were on a scale of one to ten. The prisoners, especially those characterized as psychopaths, revealed in follow-up questioning that they used gait cues to identify those who were frail or otherwise vulnerable to being preyed upon. Given the same task, undergraduate students were blind to these cues.

The implications were chilling. Ted Bundy, who confessed to raping and killing thirty women and girls in the 1970s, once boasted that he could “tell a victim by the way she walked down the street, the tilt of her head, how she carried herself.”
It makes evolutionary sense that all animals – including humans – would be fine-tuned to identify different species and different individuals within those species, and even to recognize their moods by the way they move.

Given the evidence that different hominin species walked differently from one another in the past, it would have been beneficial, and perhaps even a matter of life and death, to know whether a group of hominins foraging in the distance belonged to your species or another. Subtle gait cues might have helped with these identifications.

As it happens, our gaits aren’t the only things about walking that betray our identities. Footprints are like fingerprints. Omar Costilla-Reyes developed an algorithm that identifies individuals by the footprints they leave behind. He identified 24 ways in which footprints differ from one person to another, and it is accurate 99.3% of the time. In the last two decades, the research community has, by and large, moved to facial recognition, deemed a superior approach. The wrong camera angle, variations in the walking surface, or carrying a load are enough to alter gait and impact accuracy. Individual gait signatures would be challenging to extract from a crowd. Facial recognition is cheaper and more effective.

Gait analysis has possibilities for health professionals. One of the first symptoms to appear in dementia is a change in gait.

In 2012, an app was developed that allows a smartphone to recognize the gait of its owner. Tiny gyroscopes and accelerometers inside smartphones can detect subtle differences in how someone walks. Since everyone has a unique gait, the phone will remain locked if it doesn’t recognize the user’s speed and movement.

Walking has always been about more than moving from one place to another. It is, and always has been, a social phenomenon. Today, we celebrate the solitary, cerebral Thoreau, Wordsworth, and Darwin, but rarely in our evolutionary history has walking alone been a.  good idea. Until very recently, a lone, contemplative walk would have surely ended in the piercing grip of a leopard’s jaw.

Often, we walk collectively, like a school of fish, and it seems likely that our ancestors did too. It has long been known that people walking together subconsciously coordinate their gaits. One study had 14 middle school girls walk down a school hallway. When walking in pairs, the girls synchronized their gaits. Wearing blinders that blocked their views of one another did not change the results. Synchronization was easiest when the girls held hands. People walking on adjacent treadmills at the gym synchronize their gaits. All people, regardless of their background, synchronize their strides, even strangers. Sometimes, however, synchronized walking comes at a cost.

At just 18 years of age, Stephen King wrote his first book, The Long Walk. In it, one hundred teenage boys and young men line up at the Maine-Canada border and walk south at four miles per hour. If they go under this speed threshold, they are issued a warning. Three warnings are given, and they are executed by soldiers riding alongside them. Crowds lined the streets, cheering them on. The long walk ends when there is only one competitor left standing.

What makes The Long Walk so gripping is the speed threshold: 4 miles per hour. A cross-cultural study on over 2,000 people from thirty-one different countries found that human walking alone on flat city streets averages almost exactly 3 miles per hour. The Irish and the Dutch tend to be slightly faster (3.6 miles per hour), and Brazilians and Romanians stroll at a more casual pace (2.5 miles per hour).

When walking speeds of 338 people of different ages were studied, an average of 2.8 miles per hour showed gradual slowing with age. At this speed, humans can walk and walk and walk without exhaustion.

But the cost to move our bodies increases with speed. Those boys in King’s novel were exhausted, mentally, emotionally and yes, physically. Having to sustain four miles per hour to stay alive is what makes The Long Walk horrifying.

There are many reasons, some cultural and some anatomical, for why people naturally walk at different speeds, but one of them involves fundamental principles of energetics. You can try walking at your normal pace. Now accelerate and walk faster. It takes energy to do this. But if you slow to a snail’s pace, it also takes energy to resist your body’s preferred speed. Everyone has an optimal walking speed, so what happens when people with different optimal speeds walk together?

Picture two walkers, a fast one and a slow one. Does the slow one speed up and absorb all of the energetic costs, or does the fast one slow down and take on that burden? What happens in a large group of people who all have different optimal walking speeds? The answer is that people tend to meet in the middle, subconsciously settling into an optimal speed that minimizes energy costs for the entire group.

However, a twist occurs when two walkers are romantically involved. The male in a heterosexual relationship absorbs the entire cost. This may be chivalrous, but not entirely fair from a physiological point of view. Women’s wide hips also give a wider range of optimal walking speeds than men have. When women slow down or speed up, they don’t expend as much energy as men do.

Nevertheless, walking has always been something we do together. For 97% of our species’ history, and for 99% of the time that bipedal hominins have walked the Earth, we have been nomadic hunters and gatherers. We roamed the landscape, walking from one food source to the next. We established temporary camps, and when the resources were nearly exhausted, we packed up our few belongings and moved on together.

Some human populations, including the Hadza of Tanzania and the Tsimane of Bolivia, still live this way, but today most people live in permanent settlements and eat farm products. We drive cars and fly in airplanes. And many of our cities, where more than half of all humans live, are designed in a way that makes walking from place to place difficult or even dangerous. Walking – the thing that made us human – isn’t nearly as common as it used to be. There was a time when everybody walked; they did it because they had no choice. The moment they had a choice, they chose not to do it. As a result, our health has suffered.