Ever walked on an escalator and found that it was "frozen"? You stumble and feel unsteady as you attempt to walk down or up its "dead" steps. That unsettling feeling tells us something unexpected and important about our normal walking–and the complex neurology in our brain that makes us human and unique.
Walking is much much more than the apparently obvious activity of repeatedly putting one foot before the other. For a start the way we walk is different from that of our close cousins the Chimps. They do not normally walk upright like us with a stiff leg gait and vertical upper body but with bent-hips and bent-knees (see left photo below). If we walk like this, it is when do Groucho Marx impressions or when we are on soft surfaces–it also makes good sense on flexible tree branches since it minimizes steps creating a destabilizing wobble underfoot. But we normally do not walk like this (Groucho looks funny) but chimps do it all the time even on firm ground.
This body bendy walking has a great energy cost. Bent limbs drain energy though the need to maintain muscles in isometric tension: dogs hold their limbs bent when standing. The mere fact of that increases their energy consumption 70% more than when laying down. Humans in contrast use only 7% more energy standing than when supine. Similar efficiencies exist when walking with a stiff upright rather than a chimp-like bent body. Not only is holding the body erect more efficient but the bent-hip bent-knee walking does not reuse the energy of the swing foot. Because the leg is stiff, it raises up the hips a few centimeters (see the stick person below on the right) which then can power swing forward of the other leg with a fall. In chimp or Groucho walking the hips stay level (see the stick walker on the left below).
Part of the explanation why humans but not other animals walk like this is that it takes a lot of skill. Macaque monkeys (see right photo in white shorts above) in Japan are taught for entertainment purposes to walk in a stiff manner–but it takes years. The first thing is to patiently teach them to balance themselves statically in an upright fully erect stand. After mastering that they can start to stiffly walk like us. Even so they never fully align their bodies in the column anatomical alignment with which we carry our bodies–though that is perhaps to do with the limited adaptability of their hip and knee joint anatomy.
Brains are important: the way we walk requires a robust sense of balance–and that needs both big brains and lots of practice particularly if walking is to be combined (as normal with humans) with other activities that complicate balance like carrying mobile objects (think of protesting babies).
Walking because it is so easy tricks us to think there is not more to it than, well, walking. But we are in fact engaged in a dance of our whole bodies that is needed to keep ourselves balanced as an erect vertical column–and as importantly–keep at the same time our head and eyes and our vision steady. Our bodies to control its center of gravity make all manner of adjustments in advance of each step. That not only keeps our bodies vertically upright and in balance but also aids the efficiency of walking by using slight but controlled center of gravity instability to aid push the body forward.
That adjusting is fined tuned in regard to challenges that our brains expect to encounter when we walk. The broken escalator reveals how subtle that fine tuning can be. We quickly adjust how we walk on working escalators to take account of the momentum difference caused by its forward movement. And that experience–research shows–is automatically used to adjust our balance. That anticipation even acts before we put our foot on the escalator–if we "know" in advance that its forward momentum is there to counteract.
So those anticipatory adjustments get activated when we get on escalator and the result is that we feel unsteady when it does not "move" as our bodies anticipate. Our balance is upset since it is adjusting our bodies to a moving escalator that is not in fact moving.
The broken escalator effect is a small window to a world that is normally hidden to us. We balance as narrow based vertical columns so easily either when standing, walking or running that we assume that we can do nothing but be as we find ourselves–upright and erect. Yet this state is biologically unique to us. No other animal stands their whole body "carriage" vertically erect–those upright penguins and meerkats balance with their tails tripod-like. All other bipeds such as birds hold their thoraxes level or near so not upright into precarious columns. And we do it with anatomical alignment. Chimps stand semi-vertically–they lack the gravity line down their skeleton that we have from the base of our skulls down to our ankle heel. That skeletal vertical alignment makes it easy for us to stand for long periods without strain and walk in a stiff manner–gravity and compression holds us not muscles engaged in active isometric work keeping limbs and body bend.
That we hold our bodies in this way is thanks to the extraordinary smart balance that comes from our large brains and their very prolonged immaturity. Children may seem to walk and stand but they lack the robust balance agility that comes with being an adult. Young humans are still refining their balance as walkers even into adolescence. Children notably lack the adult robustness to pushes and shoves. So it is perhaps not surprising the way we walk with our bodies vertically erect is unique–no other animal has the big brains nor the prolonged period of immaturity to master the superbalance which makes it safe.
And so the broken escalator shows like a rock fall exposing a fossil a hidden side to our origins. It reveals the incredible smartness of our walking and upright vertical balance. Of course our superbalance does not end with walking. Human bipedality allows much more than simple walking and running. Look at Nureyev or Federer–we have a capability toremain on our two feet in complex quick movement that defies science to understand. No engineer knows how to make a robot that can double rond de jambe en l'air or sprint into an accurately played high backhand volley. Even simple humanoid robots attempting anything other than a slow walk crash themselves on the ground (the robotists carefully make sure we see only their successful walking).
That unsteadiness on the broken escalator tells us we are rather more special than we realize.
The broken escalator phenomenon. Aftereffect of walking onto a moving platform. Reynolds RF, Bronstein AM. Exp Brain Res. 2003 Aug;151(3):301-8
In this site see my: Human origins, and bipedality, dexterity and speech/song
For just the section in the above on bipedality here
Also see Skoyles. JR Human balance, the evolution of bipedalism and dysequilibrium syndrome.