Why Feet Are Weirder Than You Think

0:00Welcome to The Rest is Science. I'm Hannah Fry. And I'm Michael Stevens. Here's a question for you, Michael. What is going on with feet? With my feet? Yeah. Not just yours, but also yours. Okay. No one knows what's going on with my feet because I never show them off. Do you not? Okay. I guess I did once when I stepped on Legos, like for a bit to get views. But when I was a small child, like a child, I had hairy feet. Yes. And I was walking around barefoot one day and my dad's friend, whose name was Al, like I'm not afraid to name and shame. He looked at me and he goes to my dad. He goes, whoa, your son's feet have a lot of hair on them.

0:39Wait, how old were you at this point? Okay, in my mind, I'm like four, but I'm sure I was like a teenager. Are we talking about like sort of soft downy children hair or are we talking about like sick man hair? Like look at my beard and then imagine toes coming out of it. it's almost like that. Even at home, I've been married to my wife for 10 years now. I still just wear socks all the time, so she won't go whenever she sees me. Are they still hairy? Well, yeah, I haven't shaved them. Wow. But this episode is not about me. This episode is about our entire species. It is about our entire species.

1:16I hadn't really thought about it until you mentioned it, but they are a bit weird. They are very weird. They're very different to other animals' feet. And that is what we're going to get into it. But listeners, don't worry. We're going to come back to Michael's feet as well because I've got more questions. I've got more questions about that. I have no more answers. Or do I?

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4:22So I was in Cambridge the other day having lunch and I ended up chatting to a colleague of mine called Shreyas Mandre. He raised this question with me. I'm like, why are human feet so weird compared to every other creature that exists? Okay. If you think about it, the same solution has evolved over and over again in the form of hooves and trotters. What's a trotter? A trotter. Like a paw? It's like a, it's on a pig, you know? Oh, okay. It's got a split. It's got kind of like toes. But it's different than a hoof. Pigs don't have hooves, do they? It's sort of halfway between. It's slightly softer than a hoof.

4:58See, I'm not afraid to admit what I don't know. Do you know, I'm also now worried about the limits of my own knowledge of trotter versus hoof. Is anyone Jewish here? Because they would know. They've got that whole rule about you can't eat a hoofed animal unless it chews cud. Really? Yeah. Do you know what? I think trotter might be a kitchen word. Yeah, because if you look up pig's trotter on Wikipedia, it's a culinary dish. and the picture is pretty grim grim like it's good it illustrates the trotter that's what you need it does not look appetizing does not it looks quite undignified i would not want to be that pig put it that way okay hold on if you're gonna be a pig who's going to be slaughtered you could do worse than being the pig whose trotter gets pictured on wikipedia for eternity you're immortalized.

5:50I don't think I would care that much about legacy as a, as a. Oh, I would. I'm like, I'm in it. I'm in it for, for the legacy, for the fame. Yeah. Yeah. For the photos. The immortality of my trotter. My little trotter, my little trotters. Okay. Here's the thing, right? Hooves make really good mechanical sense. You know, you've got, you want to be a fast, heavy runner. You, you, you evolve hooves, right? It's not, damaging the ground. If you want to sprint and hunt, then you evolve paws. They're much softer. They're like squishy jelly bean pads. If you want to climb, then you evolve a kind of second pair of hands, which is what you get with monkeys.

6:30Human feet, what is going on with them? Right. It's kind of halfway between climbing and not climbing. Right. Running. It can't grasp. But monkeys have much more opposable toes. They do. In fact, my daughter has a book about human evolution, and it's got all these artist renderings of early hominids, and they all have like hand feet. So yeah, not only is that different, but also Homo sapiens are the only animals on this planet that wear shoes. Right. Now we put them on horses. Sure. When they're doing labor for us, we'll put them on dogs, but only when the dog is going to be walking on something we made, like asphalt or concrete.

7:10By the way, do you say asphalt or asphalt? Asphalt. Oh, you said felt. I don't know. It doesn't come up that often in my world, I'll be honest. Well, anyway. And so why? What's going on there? Our feet are weird. They are weird. They are weird. But I think the shape of them in particular. When people first started trying to make robots, okay, the first time that you sort of needed to like construct a foot to see how it could work. And they tried to make these like flat feet. It's just like kind of moving like this, right? Which is maybe that's how our feet have evolved. Maybe it was that we were previously climbing trees and then they just became slightly flatter, slightly less able to grip around the trunk of a tree and slightly more attuned to going on the ground.

7:52To walking, yeah. So when people first start building robots, they made these perfectly flat feet. And it was a total disaster because what that means is if your feet are completely flat, you can't roll forwards. Right. So instead, like the early robots like Asimo, which is, you know, a particularly famous one, they have to, if you look at them, they have to stand essentially with their knees permanently bent in order to be able to move forward. Right. It's really, really energy intensive to like kind of lift your feet up this way. And if you look at an ape walking, that's essentially what they're having to do.

8:24Oh, really? Because their feet are so flexible. Right. that when they are down on the ground, they have them very flat. And so their knees and their hips are continually having to be bent in order for them to propel themselves forward. It's like really energy intensive. They don't like doing it at all. Would much rather be in the trees. And so a firmer foot that's less flexible, like a human foot, is better for walking. Right. You sort of want the rigidity in order to be able to kind of press off, but you also don't want it to be flat. Right. Because the flat obviously takes a lot of energy. It needs to be somewhere in between.

9:00Yeah. But then here's a question, OK? You don't want it to be flat. You want it to be rigid. Why isn't it shaped like a rocking chair? That would help you propel forwards. Oh, yeah. I mean, that would make way more sense. I mean, it needs to be, I need to be able to move it. Like, I need my toes to be able to sometimes give more or less resistance to the rocking motion. Agreed. But, like, this kind of, this effect of, like, rolling forwards. And yet our feet are arched exactly the opposite way. They are, aren't they? Yeah. So this was a genuine puzzle. Why is it that our feet are arched that way around rather than the other way around?

9:30Can I make a guess? I would guess that it's like architectural for support, that that arch, just like an arch in architecture, holds weight better than like a saggy rocking chair type foot. I don't think that's the primary reason. So what other reason is there? It's probably because standing still would be really difficult if our feet were bent like a rocking chair. People have tried this with robots as well. So the next generation of robots had rocking chair type feet. But then standing still would require you to continually tense yourself. You'd have to like permanently be locked in place. And so it just doesn't work either.

10:11So instead with the arch the other way, it means that you can kind of create a triangle between the big toe sort of pinky side of the foot and your heel. And you're kind of sitting on a little tripod. Two little tripods. Yeah. Be in a relaxed state. But in the 1950s, people were like, this does still really not make a lot of sense. Like, why have we got this? Maybe the foot arch is there for structure. But, you know, it makes sense why we don't have these flat feet. But what is going on? Like, why are our feet in this particular shape? So in the 1950s, this English orthopaedic surgeon, he was like, guys, I've cracked this.

10:47I've worked it out. He was kind of looking at the human foot and he knew it couldn't be a rocking chair. But he also knew that if it was just this fluffy, fluffy bag of joints, you know, this insane number of bones. Like a monkey foot. Like a monkey foot. Then we wouldn't be able to push off and it would be really, really difficult. So he was like, right, there must be this kind of structure that's going on in here. This is the explanation that he managed to come up with. OK. Oh, you've got a cardboard model. I do. Looks like you built it yourself. I did. In the car on the way here. It's got like an arch.

11:18It's like a cardboard-shaped boomerang. Yeah, it's got three different parts to it. Okay, so those of you who are watching, you can see this is sort of like Learn With Hannah. Yeah. This is my model of a foot and how a foot works, or how they thought the foot worked in the 1950s. Okay, so let me say what I'm seeing. Go on. Three parts. One looks like a boomerang shape. Yep. And then attached to one end is a little ellipse that is hinged, like with a brad. so it can swirl around 360. Yeah, I mean, that's not what you want in your real foot. That's not what you want. And I can tell that because you've connected the far end of the boomerang to the far end of the ellipse with a rubber band.

11:57So it's almost like a bow and arrow type shape. Exactly. It's like a bow and arrow type shape. Exactly. So what you can't quite see here, I've essentially split the foot into three sections and then we have like a tendon running between them. So here is the heel. This is kind of the back of the foot. And then you have essentially the foot arch that goes up and over. And then this here is the toes. Okay. Right. So now. The ellipse is the toes. The ellipse that can move around is the toes. Yeah. The hinge is there. So here was his idea. He was like the foot being this shape, like a bow from a bow in our own.

12:28Yeah. This makes perfect sense. Because when you put your foot down, you've got lots of nice kind of elasticity. So your foot doesn't get damaged. the kind of the string from the bow hits the ground first and absorbs lots of the energy as you hit the ground. But if you move forward by lifting your heel up, then what you're doing is you're tightening this bow, you're tightening the tension between your heel and your toe. And that's the thing that's giving your foot this structure. That's the thing that's giving your foot this strength, this stiffness, in order for you to be able to push off and move forwards, right it's something that just made absolute perfect sense yeah as a result of this explanation this explanation by the way stood for like 70 years everyone was like super happy with it and it led to something called jack's test which you can do it on yourself there was a bit of me michael that was going to suggest that you get your feet up for this but maybe not maybe maybe later maybe later but if you stand barefoot it's for the rest is science after dark yeah that's uh the subscription yes yes you know i think lily allen once said she's got only fans but she only shows pictures of her feet okay and she said that she makes more money from that than she does from her actual music i believe her that's incredible isn't it yeah well she probably doesn't have like ticket master fees and a recording label to pay very true she just snaps them with her phone ka-ching ka-ching hello i'm gonna come on to that a bit bit more later the foot fetish stuff it wouldn't be a proper episode about feet if i didn't mention no we have to we have to we have to okay so so there's this thing called jack's test if you stand barefoot with your weight on the floor and you have somebody else pull your big toe backwards so you're kind of standing like this and then you get someone else to effectively tighten so they like this elastic they pull your big toe up yeah then you will see your arch tighten Basically, you see your arch kind of getting tighter, more stiff, essentially.

14:30And everyone was like, well, this is the proof that his explanation is correct. It's very clear that moving the big toe moves this tendon, the plantar fascia that runs between the front and back of your foot. And everyone was like, well, there you go. This is it. We worked it out. This is absolutely fine. And then what would happen is this kind of like really fed into the medical understanding of what it meant for feet to be working correctly. So you might have heard this idea of people saying you're very flat footed and that being considered a real problem. Right. And it comes from this exact same explanation that if your foot is too flat, if your arch is too narrow, then you can't get the tension properly and it affects the way that you end up walking.

15:16There was some confirmation bias going on in this, too, because people would go to doctors and they would say, we've got loads of pain in our feet. And the doctors would look at their feet and say, well, it's because your feet are flat. I can see that there's a flatness to your foot and that must end up being the problem. The slight issue, right, with this explanation, well, there's two. The first is, if this really is the way that feet work, well, then how come people who do have completely flat feet can walk? Yeah, they can. They can. It is possible. You don't actually need the art. Well, I remember being told as a kid that I had flat feet by my doctor.

15:50Why? But I didn't know what the symptoms were supposed to be. Right. I could walk fine. I could run fine. It's never been a problem. What did they say about it? I don't remember anything outside of he made me walk around barefoot in the doctor's office. Maybe he just wanted a free sweeping of his floor. They're not that hairy, guys. Maybe he thought your feet were so flat because he couldn't see through all the hair. He couldn't see through all the hair. It was like this forest. No, but I remember him telling my mom, yeah, he's kind of got flat feet, but then nothing ever came of it. Right. And it doesn't matter at all because while some people who have pain in their feet have flatter feet than others, there are also loads of people who've got incredibly flat feet and run marathons and do all the stuff and they're absolutely fine.

16:37They're completely fine. That's what I was thinking. Yeah. And there are plenty of people with really nice arches who have... Absolutely. Completely. the other big clue that this this explanation was completely wrong is that for this explanation to work the way that they constructed it is that you you sort of have to fuse the heel and the arch you kind of this particular joint is like quite rigid in the explanation but then it turns out that there have been some surgeries where people who've got really bad arthritis have had that joint physically fused and it makes it really really difficult to walk right so how can it be And this is the joint between the heel and the rest of the foot?

17:18Yeah, let me get the exact... I didn't know there was even a joint there. I mean, there's lots of joints. I think there's 30 joints in the foot in total. Oh, goodness. I mean, this is sort of pretending that there's two. Yeah, right. But yeah, there's... It's called the navicular medial cuneiform joint. Okay. Okay. And if you fuse it, in the existing idea of how feet work, not a problem at all. but in reality makes somebody's ability to walk significantly worse so it's like well this this is like there's something wrong going on something missing something is wrong going on here very recently in 2020 this paper that my colleague at Cambridge was was involved in a group of scientists mechanical engineers and biologists essentially were like something's going on here something something not right so they decided to look at the foot from from a completely different angle because this entire time when we've been thinking about the mechanics of the foot we'd been looking at the arch that runs from the toes to the heel yeah but this group were like well hang on a second there is another arch going on in your foot your foot doesn't just bend front to back it also bends left to right right like if you think about the way that you're like a pringle like a pringle exactly you're looking at your feet now yeah i know but i've got shoes on so i'm like no it's pretty darn flat but yeah i know what you're talking about your converse so underneath yeah exactly and actually they were like well hang on a second maybe it's the maybe the stiffness because you need your your foot needs to have this stiffness in order for you to be able to push off and propel yourself forward so they were like maybe the stiffness is coming from the arch that runs in the other direction instead rather than front to back maybe that one's not as important maybe it's maybe it's side to side from the big toe to the pinky toe um so what they did got a whole of some cadaver feet uh-huh gets gruesome keep going they mounted these feet into this like quite heavy duty materials testing machine where they would like press weights down on the foot to see how much force it would take to bend the midfoot.

19:29And these were cadaver feet that were not compensating for the extra weight. They were just looking at the structural properties of the foot. Structural properties of the foot and nothing else. And then once they'd done that, they then got out some scissors and they started snipping the joins between the toes. Right. Not just between the toes, but in that direction, right? Trying to break the arch that runs from left to right. And watch when it fails. And it turns out that this like, you know, really decades and decades and decades old understanding of the foot just crumpled as the snips took place.

20:03Wow. Because almost all of the rigid structure of the foot comes from the left to right arch, not the front to back up. Wow. Like completely shattered this biomechanical dogma that existed. Yeah, when they ran the numbers, the front to back arch only accounts for about 25 % of the foot's rigidity. Okay. Now, the reason why this works, the reason why you get this stiffness, something called Gaussian curvature, the reason why you get stiffness from there being a bend. It's a bit like, you know when you have a slice of pizza? Yep. So a slice of pizza, if you try and get it in your mouth, I'm doing this with a piece of paper.

20:43It would be much more fun if it was actual pizza. I'm imagining. You're imagining. Okay. If you have a slice of pizza and you try and get it into your face. Right. You're holding it from like the crust end. So the tip is going to flop down. It flops down really easily. Yeah. But what people automatically do is they've realized that if you fold it along the length of the pizza, then when you try and get it in your mouth, it remains straight in the other direction. Now, I've seen the famous like TEDx animation about this pizza, why pizza folding makes things rigid. but you're saying that this is also what makes our feet rigid and great for walking.

21:20Yes. So it all comes down to this idea of Gaussian curvature. So Gaussian curvature is something that measures the curviness of a surface. And it's made up of two numbers. So one number is how curvy it is in one direction. The other number is how curvy it is in a perpendicular direction. A perpendicular direction. Exactly. So if you imagine that you're on the surface of a globe, right? say you're at the equator one number would be north to south how curved it is in that direction and the other would be east to west now the key thing it's positive if it curves away from you okay okay now the key thing about gaussian curvature is that you cannot change the gaussian curvature of a surface unless you cut it or warp it right so if you start off with something that is completely flat you know like a piece of paper or a piece of pizza if you start off with something that is completely flat, it has zero Gaussian curvature.

22:13Yeah. Which means that even when you bend it, it has to end up with zero Gaussian curvature. Right, so when you bend a sheet of paper, you produce, say, positive curvature in one direction. All on one axis. Yeah. Now, the only way those two numbers multiplied together can still make zero is if the other direction is zero. Right? You've forced it to have a positive number in one direction. And a positive number times zero is still zero. So you have forced it to have zero curvature in the other direction. The curvature of a flat sheet of paper is zero. Zero times zero. Zero times zero. Yeah. Oh, so it's the product that matters.

22:51Yes. So if you create a curvature of, say, one, I don't know what the units are, but positive one, that means you're like turning your page into a taco. The curvature in the perpendicular direction needs to be zero because one times zero is still zero. Exactly. Exactly. So this is why the folding pizza thing works. You force it to have a positive curvature in one direction, which means it, by the law of like curvature, has to not have curvature in the other direction. The only way it could have a different curvature in the perpendicular direction now is if it warped or broke. Exactly. Which it's strong enough not to.

23:26Exactly. It's the same reason why corrugated cardboard works. Because you take the paper, I mean, it's actually quite thin, corrugated cardboard, right? But you take it and you curve it. You deliberately give it a curvature in one direction, which means that it then can't bend in the other direction. Right. So it's the same with your foot. You deliberately curve it in one direction between your little toe and your big toe. And it means that then it cannot curve in the other direction, which means that it has this stiffness as you propel yourself forward. I want to take a quick tangent and talk about globes.

23:59Yeah. Because the Earth is going to have positive curvature to the same degree in both east-west and north-south directions on those axes. So its curvature, if we call it one in each direction, is one. One times one. What can I do to a globe such that the product of those two curvatures is still one? So actually, with the globe, this is the reason why maps don't work. Yeah, because if I try to flatten it, I'm changing its Gaussian curvature from one to zero. And so it doesn't work. It'll get ripped. Right. Without you ripping it, cutting it or warping it. Are there other shapes I can bend it into that still have a Gaussian curvature of one?

24:43Yes. Not a cylinder because that's got a Gaussian curvature of zero because it's bent in one direction but not in the other. Yep. Not a Pringle because that has got positive in one direction and negative in the other. it's still got to be positive positive because that's the only way that you get a positive number overall so you're you're looking at like oblate spheroids right you know sort of like deformed balloons that's essentially all you got okay all right but you know let's save this for the globe episode let's see what kind of crazy deformed balloons we can make and crazy deformed maps and crazy deformed maps yeah yeah but back to the foot though please this is like okay so so maybe this is what was going on all this time and it has since been confirmed they've done high-speed x-rays of what is going on with the foot so where you can take an x-ray as somebody is walking or moving their foot you sort of move away from the cadaver feet at this point watch a runner essentially move around while uh having x-rays with a slow motion x-ray i would love to watch no actually in fact i think i have some how did they get volunteers for that because Because x-ray videos are amazing.

25:55Like I've seen x-ray video of people swallowing and chewing. But of course, it's a carcinogen. All these x-rays being shot at you. So it's not a good idea. They used to, of course, have actual x-ray boxes at shoe stores. Yeah. You could put a shoe on your foot and then stick your foot in the box and look through this window and see the bones of your foot and how they were being squished or not. by the shoe. Yeah. Not a good idea. That was a bad idea. But they were helpful if you didn't want to live a long time. And have comfy feet. If you wanted comfy feet, if you wanted the next few years to be really comfortable, they were great.

26:36I think you can think of everything on a spectrum, right? Between how much you care about comfy feet versus how much you care about wanting to live, right? And in the limit, if my feet were excruciating all the time, I'd be fine with a shorter life, you know? So this is just... Yeah, there you go. Exactly. It all balances out. It all balances out. It all balances out. Yeah, when you do these x-rays, it's like the foot is this spring-loaded trampoline, essentially. Just the second before your toes leave the ground, the arch just visibly recoils. It kind of springs back into its curved shape. And that elastic bounce releases all of that storage energy, throwing the body forward.

27:12Moving you forward. Moving you forward, yeah. Wow. Isn't that cool? Yeah. I really like that a lot. They ended up going further because they were sort of, OK, well, if it is about this way around rather than that way around, when you go back in the fossil record, it was before this paper came out about the direction that we know that it matters in. There was a bit of a question about this because some of our ancestors, Lucy, for example, the one that was found 3.2 million years ago, right, has like these very flat feet. Flat in the direction of toe to heel. Flat in the direction of toe to heel, but not flat in the direction of big toe to little toe.

27:51Okay, so is that evidence that she was a walker? It's evidence, yeah. A bipedal walker? Yes, that it was like this. She was essentially transitional. She was somewhere between climbing trees and walking upright. So then what's the point of the curvature from toe to heel? So that still helps with the elastic and propelling foot. It's a quarter of the support. It's just not providing the stiffness. Right. It's helping you propel forward. It's part of the kind of trampoline-like spring loading. It's just not the thing that's giving you structure in order to be able to lever yourself forward. But it looks as though this evolution of the big toe to little toe arch came about one and a half million years before our genus.

28:37Really? Before Homo? Before Homo, yeah. Yeah. This is a key Eve Luce Ruseppe as to how we learn to stand. Oh, interesting. It's good, isn't it? Yeah. Isn't that good? Here's the thing I really like about this is that this paper was published in 2020. Yeah. You know, like no time ago, six years ago. Yeah. So has this been studied further? When are we going to see it folded into what podiatrists recommend and the way shoes are designed? Great question, because I think it's going to probably take quite a long time. Because you're right that actually a lot of the time, the way that shoes are designed is about thinking of the arch, thinking of the front to back arch and trying to support it and so on.