A couple of weeks ago there were a bunch of articles about a new study discussing why humans walk heel-toe. The articles related that to dogs and cats, who walk on their toes.
Unsurprisingly, I have some bones to pick with the articles. (Hell, I have bones to pick with just about everything, right?)
Here’s the article that seems to have been shared the most: Why we walk on our heels instead of our toes: Longer virtual limbs.
But there were also others, like Human Heel to Toe Stride Finally Explained Through Treadmill Study; Why humans walk on their heels: Researchers find our stride has evolved to make footsteps far more efficient; Humans hacked walking by stepping on the heel not the toes, like other animals do; and Why we walk on our heels instead of our toes. They all seem to be based upon this press release from the University of Arizona, where the research was done: Why We Walk on Our Heels Instead of Our Toes.
The article bugs me. (Yes, I know, I lot of things bug me.) There are a couple of comments comparing human locomotion to that of dogs and cats that just strike me as odd. Yes, I realize that it is always difficult to explain a study and this is an attempt to do so by making a comparison. However, the comparison fails.
I do note, however, that the research paper itself doesn’t make this mistake. So, in this blog post, I’ll first address the popularizations, and then down at the end I’ll discuss what the paper actually says (which is interesting in its own right).
Here are the statements that bothered me. The first is from the article itself:
His most recent study on walking, published in the Journal of Experimental Biology, specifically explores why humans walk with a heel-to-toe stride, while many other animals — such as dogs and cats — get around on the balls of their feet.
The second is a quote directly from the lead researcher of the paper, James Webber. Webber himself is a barefoot runner. Here’s the quote:
Humans are very efficient walkers, and a key component of being an efficient walker in all kind of mammals is having long legs. Cats and dogs are up on the balls of their feet, with their heel elevated up in the air, so they’ve adapted to have a longer leg, but humans have done something different. We’ve dropped our heels down on the ground, which physically makes our legs shorter than they could be if were up on our toes, and this was a conundrum to us (scientists).
Quite frankly, that shouldn’t be a conundrum. The real reason we don’t walk on our toes like dogs and cats is that it was evolutionarily inaccessible to us.
Basically, our remote ancestors had certain characteristics, and lived in what is called an evolutionary landscape or a fitness landscape. As an environment changes (or an organism changes to fill a new niche) the changes have to be incremental. It has to build on what is already there, and the difference between dog and cat paws, and their entire locomotion anatomy, would require massive simultaneous changes. Evolution just doesn’t happen that way.
Let me start explaining the human foot situation by asking a simple question. First, look at this picture of a rhea (as a prototypical bird) and ask yourself the question: Why do birds knees bend backwards?
Think about that for a minute before you read on.
That’s not the knee; that’s the ankle (and heel). And it’s bending the right way for an ankle.
Let’s go all the way back to one of the first tetrapods, tiktaalik.
Mammals, birds, reptiles, and amphibians are all tetrapods. The limbs of tiktaalik are basically fins, and all those different finny bones developed, in one way or another, into the appendages (hands, feet, hooves, paws, etc.) of today’s tetrapods (including us).
What has happened through evolution is that the bones (and of course all of the surrounding tissues) have modified. Some get longer, or thicker, or they merge with nearby bones, or shrink down to nothing (or almost nothing). For the ones that disappear one can find remnants during development or in the DNA.
But when you do comparative anatomy you can pretty much identify which bones correspond among the different tetrapods. This works with extant species and with fossils.
Here’s a picture of a T. Rex where you can see those bones and how they correspond so closely to those of the rhea. You can also see the “ankle”.
Let’s look at how it is that we (and our various primate ancestors) are plantigrade, that is, walking on a sole that stretches from an ankle to the base of the toes. Here’s a picture of Aegyptopithecus, an old world monkey from about 34 million years ago.
In the reconstruction, one can see that the phalanges (fingers, toes) are adapted to curl around branches, while the carpals and tarsals (what in humans make up the palm and sole) up near the “heel” are still not used for grasping.
By the time of Proconsul (a primate, closer to 20 million years ago), the reconstruction shows the carpals and tarsals involved in grasping tree branches.
There is even the beginnings of a pronounced heel bone.
This morphology works great for climbing around in and jumping about in trees. But there are a lot of primates that still spend a lot of time in trees, but also spend a lot of time on the ground. They end up using their whole soles to walk around.
We can see that in this baboon.
We can see that in this chimpanzee.
We can see that in this gorilla.
Those are the kinds of feet that would have been evolutionarily modified as humans minimized climbing trees and starting walking long distances.
Now let’s look at dogs.
As with birds, their “ankle” is way up in the air. Dogs, like a bunch of mammals, are what is called digitigrade, that is, walking on their digits (fingers and toes).
Here’s another picture (of my son’s dog, Tali) doing a better job of showing the “ankle” bend.
As you look at the pictures of the (non-human) primates and the dogs, you ought to easily be able to see the large morphological changes that would have had to have happened for humans to have started walking on their toes. It’s just not easily accessible on the evolutionary landscape and we instead evolved along a different path.
There’s another problem: the mammals we’re looking at are all quadrupeds. They can walk on their toes because being four-legged is inherently stable. But if you do what humans did, and went bipedal, walking on just the toes is inherently unstable. (Not to mention that even more so trying to just stand on toes for any period of time is even more unstable. Note that running on our toes, or the balls of our feet, is not inherently unstable because we are moving fast, just as a horse galloping with just one foot touching the ground at a time is not unstable.) Birds dealt with that long ago by expanding the sizes of their toes. You can see that in this skeleton of a prototypical bird.
That kind of change is also not easily accessible on the evolutionary landscape.
So, when Webber says that it’s a conundrum why we’ve dropped our heels onto the ground, that’s just wrong. Our ancestors didn’t drop our heels onto the ground in the first place (it was proto-simians that dropped their “heels” in the trees, anyways). And there’s really no conundrum about why we didn’t evolve dog-like structures for walking. It’s evolutionarily inaccessible from the morphology of our (non-human) ape ancestors, and physically unstable at that.
Also note that in our evolution, our legs did lengthen, by quite a bit.
But there are still interesting questions: why is human walking so efficient? And with walking on our toes evolutionarily inaccessible is there something else that happened that took its place? Did evolution “find” some different path up the evolutionary landscape?
Webber’s research says “yes”. And demonstrates the answer.
So now, let me talk about the research paper, The role of plantigrady and heel-strike in the mechanics and energetics of human walking with implications for the evolution of the human foot. The paper itself is quite interesting and useful despite my misgivings about the popularization.
The first thing I’d like to note is that the paper itself doesn’t talk about dogs and cats at all. It does, however, talk quite a bit about primates. (HS=Heel Strike.)
Non-human apes may offer more context into how and why habitual HS (heel strike) walking evolved in humans because we often assume that traits shared among apes may have been present in our prebipedal ancestors. Extant non-human apes use an array of plantigrade walking gaits, some of which lack consistent heel strikes. For example, researchers have shown that our closest living relatives, bonobos (Pan paniscus) and common chimpanzees (Pan troglodytes), use a wide range of landing postures, from a traditional human-like heel strike, to landings where the heel does not touch down until the second half of stance phase. Most often, these apes use gaits where the heel and mid-foot contact the ground simultaneously, which also differ from human HS walking where initial ground contact is made by the heel alone. Thus, the key human evolutionary shift from the non-hominin ape foot landings appears to be the consistent use of heel-only landing postures.
The goal of this study is to better understand the advantages and disadvantages of this shift to consistent HS gaits.
[References removed here and in all quotes.]
What the study did was measure (using force plates and video recordings with markers placed on bare feet) various aspects of the difference between heel strike (HS—heel to toe) and non-heel strike (NHS—toe to heel) walking. This included the cost of locomotion, when subjects transitioned from walking (either HS or NHS) to running, and impact transients
The study was designed to further test the results of a 2007 study by Pontzer, Predicting the energy cost of terrestrial locomotion: a test of the LiMb model in humans and quadrupeds. Pontzer put forth the idea that heel-to-toe walking effectively increases the length of our leg, illustrated by this diagram.
The rolling of the Center of Pressure of our footstep from the heel to the ball of the foot effectively increases the length of our legs, which gives us the greater efficiency of longer legs without actually having them.
When Webber looks at the two strides, he did find a marked difference in the effective leg lengths (ELL) between HS and NHS.
I found the following statement in the paper the most interesting (L’ is the HS effective leg length):
Additionally, human HS plantigrade gaits increased L’ significantly more than adding the utilized length of the foot to the hindlimb, as in a digitigrade gait (the limb length gained by the translating pivot point was 24.19±7.16% higher than the sum of foot length and hip height, P<0.0001).
What that says is, even if we somehow could walk around on our toes like birds or dogs (ignoring stability and the evolutionary landscape), the effective leg length doing so would still be shorter, and less efficient for walking, than the peculiar roll from heel-to-toe technique that we actually do use.
Webber also addresses the kind of impact transient that Daniel Lieberman found comparing barefoot and shod running. (I wrote about Lieberman’s study in New Study on Why Barefoot Running is Better.) Webber did find impact transients in heel strike walking that aren’t there in non-heel strike walking. Unfortunately, he doesn’t compare the sizes of walking transients with the running transients found by Lieberman, so it is difficult for us to know whether they are significant. On the other hand, I personally think they pretty obviously less for walking. On top of that, Webber does say that the transients increase with speed.
Anyways, here’s what he says (IT=Impact Transient):
While there are important benefits to using an HS gait, we also found that NHS postures effectively reduce ITs during walking, similar to results of studies examining ITs in NHS running gaits. These results suggest that the evolution of consistent HS walking in our hominin ancestors came with tradeoffs and that foot anatomy specific to our lineage (e.g. a robust calcaneal tuberosity) may reflect the tolerance of consistently high ITs.
I think the key is the word “tradeoffs”. We have a much larger heel than other primates; that’s there for protection from the transients when we walk. When we start to run, the transients get much greater and we transition to a front-to-back landing that can take advantage of our Achilles tendon and cushion every other part of our body. It’s all about tradeoffs (which describes evolution in a nutshell).
Webber finishes by discussing evolution from earlier human-like species:
Foot anatomy in early hominins suggests that the use of a consistent HS may have been beneficial across much of human evolutionary history. While the feet of early bipeds lack derived features seen in the genus Homo that allow for effective endurance running, they may have been well adapted to HS walking. For example, the relatively short toes, and therefore shorter feet, seen in modern humans are advantageous to endurance running because they reduce plantar–flexor moments at the metatarso–phalangeal joints and probably evolved with the origins of the Homo genus. Australopithecus, a hominin genus that preceded Homo, had relatively long feet, which Rolian et al. hypothesize would have detracted from running performance. Our results suggest that relatively long feet in australopithecines may have led to improved walking performance through increased translation of the COP, and therefore increased ELL. Thus, foot proportions in fossil hominins may reflect competing selection pressures for walking and endurance running.
He also goes on to discuss other species like Australopithecus afarensis, Australopithecus sediba, Homo floresiensis (the hobbits), and Homo neanderthalensis, and how the relative lengths of their feet and legs and toes can give us information on how good they would be as walkers and runners.
It was too bad that the popularization of the paper was nowhere near as interesting as the paper itself. I hope I’ve given you a better feel for it here.