Most riders echo the sentiment expressed by this high-level professional dressage trainer, who said, “As a riding teacher, trainer and student of ‘the horse’ I’ve searched high and low to understand how to make the horses job easier for them and applicable for my students. Various training methods suggest ‘putting the horses’ head low’, while other say to bring it up. Some suggest to flex the neck left and right and yet others tell you to just ride the horse ‘forward and straight’. How is a teacher or student to know what is really the right answer?” “The Science of Riding with Feel: Horse Biomechanics and You,” written and illustrated by scientist Dawn Hill Adams, Ph.D. and horsewoman Jo Belasco, Esq. of Understanding the Horse, LLC, provides equestrians of all levels and disciplines with the knowledge to answer these and other pressing questions about the movement of the horse and rider.
“’The Science of Riding with Feel: Horse Biomechanics and You,’ came about because participants at our biomechanics seminars asked for it,” explains Adams. “People who love horses and want the best for them attended our clinics and seminars to learn how to help their horses move better. They told us the information on biomechanics that is out there either didn’t seem to address the kind of riding they did or was too complicated for them to apply to their riding.” Belasco continues, “A lot of people don’t work with an instructor on a regular basis, so they have to rely on resources such as magazine articles, books and DVDs. We wanted to give every rider, as well as trainers and instructors, a resource that contained helpful information in a way that they could use on their own with their horse. I think we are doing that with this book, and also with the accompanying Workbook and videos that we will be putting on the companion website.”
Adams and Belasco have been collaborating on practical applied horse biomechanics since 2005, working together to help people and their horses have a better experience. They’ve been collaborating on public education projects since 1999, first in the non-profit organization Tapestry Institute and more recently in Understanding the Horse, LLC. Dawn is a professional scientist in biomechanics with a doctorate from UC Berkeley, and she is also a scientific illustrator who is preparing all the book’s original illustrations. Jo is a professional clinician and trainer with a wide range of experiences — in different kinds of riding, with a variety of people, in a number of different types of learning venues including lessons, seminars, clinics, and Expos, using more than one cultural approach to horses and horsemanship.
This book is for the horsecommunity, all parts of it, which means horsepeople get to participate in making it the book they need and want. Adams and Belasco have launched a Kickstarter campaign to finance necessary material for the illustrations and photographs in the book and Workbook. The project became a Kickstarter Staff Pick within only a few hours of its launch. For as little as $1, supporters can vote on the breeds of horses and styles of riding and driving they want to see included in the book’s illustrations. Supporters can receive an autographed advance copy of the book and even combine the book with a series of horse biomechanics video tutorials at different support levels. At higher levels of support, horsepeople may suggest questions to be answered or additional topics to be addressed in the book during a Skype biomechanics seminar. A more limited number of supporters and their horses will receive a private session with Adams and Belasco, during which they will learn biomechanics’ exercises, as well as have photographs taken of them and their horse to be included as examples in the book and accompanying Workbook.
We are launching a Kickstarter campaign later this week to help us get our book, “The Science of Riding with Feel: Horse Biomechanics and You” published and into your hands. People at our seminars and clinics have asked for a book on horse biomechanics, and Dawn and I have been working hard on one. We have already written more than 60,000 words, but we need YOUR help to complete the project. I will have more information in a few days about how you can support this campaign for as little as $1!
So far, we’ve been talking about the hoof as if it’s a single structure, even though we know it’s really a composite of different parts. The terminal phalanx that I wrote about in my October 18 post, known as the horse’s coffin bone, is sheathed in a highly complex nest of tissues that are designed to safely handle stress. You can see in the stylized cross-section to the left that the “hoof” is made of hoof material itself, ligaments, tendons, bone, connective tissue, and skin. Many of these tissues are made of combinations of collagen and elastin, two types of fibers that both stretch and absorb energy in different ways. Cartilege is also a large component of these tissues, increasing with the age of the horse. (See this paper by Robert Bowker, VMD and PhD at Michigan State for more details.)
It is exceedingly difficult to measure the strength of such compound structures. In our last post we therefore generalized about the hoof as a whole to get a rough idea of what’s going on. And we’ll do that again in future posts. But in this post, we’re going to consider the issue of how various parts of the hoof transmit stress. Then we’ll return to considering the hoof as a whole.
In that first diagram above, the terminal phalanx or coffin bone is labeled p3 (for phalanx 3). You can see the hoof wall material all down the front of the hoof in front of that bone, and the frog is beneath it. A thick pad of special tissue called the digital cushion is also between the p3 and the ground. It’s important to remember that the hoof wall extends all the way around the foot, though, which you can’t see in the section shown above. It’s also important to remember that the horse has a sole across the bottom of its foot that isn’t very visible in the section above. Both are shown in this bottom-of-the-hoof view from the University of Missouri Extension website.
The history of putting protective coverings over the hooves of horses is nearly as contentious as contemporary discussions of whether horses should be shod, go barefoot, or wear boots. In 1934, Professor A. D. Fraser wrote in Classical Journal about the academic wrangling going on even then — 80 years ago! — about whether the Romans shod their horses or the practice began in much later Medieval times. But one thing that is clear is that two different types of hoof coverings have been used by Eurasian cultures when they did anything at all to a horse’s hoof. There were boot-type coverings that went across the entire bottom of the hoof, and shoes that were nailed only to the hoof itself, leaving the sole bare and elevated above the ground. Of course, we have both types of hoof coverings available to us today, for our horses, although it’s only in the last couple of decades that boot-type coverings have reappeared after being absent for quite a long time.
One of the most striking things about the underside of a horse’s bare hoof is the flatness or concavity of the sole. Some barefoot horses have flat soles that touch the ground all the way across, whereas others have slightly concave soles that are domed so that only the frog touches the ground. Of course, the imprint of the frog is visible in a barefoot horse’s print, regardless, because it is made of different material that leaves a distinctive impression in the dirt. You can see the impression of the frog and of the outer hoof wall on the modern hoofprint image (from a barefoot horse) in the top picture of the set to the right.
What’s interesting to me is that you see much the same shape in the image directly beneath that one. That image was carved into rock about 24,000 years ago in Le Cellier Cave in France, and is almost always identified by archeologists as the picture of a vulva. (You can see several other similar images carved into the same rock in other areas if you look closely.) But in that same cave, there is another carving of the same shape made over the top of a horse’s head and neck. A drawing of the shape is third in the column of images, and a photo of the original stone is beneath it at the bottom. An archeological description of the piece says “. . . it is a stone much less voluminous, carrying a bizarre design. The image appears to be the head of a horse, and on the right a vulva not well represented. These designs are united by a line which seems to indicate a real relation existed between the two.” One is tempted to speculate that the “real relation” might be that both images represent parts of the same animal. (You can read more about these shapes and other items found in the cave at a page of the Don’s Maps archeology website. The hole in the hoof/vulva is thought to have held oil, the stone being a small portable lamp.)
There’s no particular reason here to argue that these hoof-like shapes are actually hooves, that their having been carved into stone tells us something about how these ancient people felt about horses, or that the level of assurance with which contemporary archeologists identify the shapes as vulvas is also meaningful. But the carvings at least suggest the fact that people have been paying attention to the shape a horse’s hoof makes in dirt for quite a long time. And of course, horse hooves are important symbols to people even today — as you can tell by looking up “horse jewelry” online and counting the number of horse-hooves and horse-shoes that come up. (I should note in passing that there’s a good deal of professional and lay argument about shapes like the ones from Le Cellier even in ancient Celtic art that’s only about 2000 years old, with an additional level of discussion about a possible metaphor that may have linked women, fertility, and horses. But that’s fodder for a different post.)
The point is that the sole of a horse’s foot doesn’t always touch the ground, even when the horse is barefoot. And we all know that the sole can be bruised or “sored up” in some way by sharp or stony surfaces. In fact, one of the reasons shoes or boots are used is to protect this sole. However, it’s interesting to notice that there is a likely biomechanical reason for any concavity of a horse’s sole.
At left is a diagram (modified from one on a Federal Transportation Authority webpage) of two different types of reinforced concrete beams. The one on top is a “regular” beam that is cast in a flat shape. When force is applied to the beam (because it is helping to hold up a bridge, for example), the stress causes the beam to bend or bow in such a way that it cracks on the underside. The black arrows in that diagram show the force being applied, and you can see the way the cracks are represented. This really does happen in beams that experience loads greater than they can bear.
However, if the beam is cast in a bow or curve that faces against the direction in which it will be loaded, there’s a different outcome. This type of beam is called “prestressed” to indicate the way it can “resist” the stresses caused by bearing a load. Now when the load is applied (again, see the arrows), the stress flattens out the beam instead of bending it. So cracks do not form. Many domed or bent structures are actually designed to resist force. Skulls, for instance, are domed outward to resist the stress of fairly large force against the head from the outside (as from falling and hitting the ground). So you can see that the degree to which a horse’s foot is slightly concave on the underside allows it to carry stress better without failing. This doesn’t mean that a flat sole that touches the ground all the time is “bad”, though. In fact, a hoof that’s slightly concave when you pick it up off the ground to look at it might flatten out and touch the ground when it’s bearing the horse’s weight. We are not establishing a value system here, but merely considering the possible adaptiveness of certain aspects of hoof structure.
One of the more interesting things to consider is how a typical metal horseshoe affects the way that stress is transmitted through the horse’s leg. Gail Snyder, a hoof care professional who also has a Mechanical Engineering degree, wrote an excellent overview of this difference in a 2012 issue of Natural Horse (that you can download here). She assumed that a typical barefoot horse has a flat sole and therefore distributes force across the entire surface area of the hoof — just as we assumed in our calculations of Part 1. She assumed a smaller sized foot for her calculations than I did, so her figures came out slightly different, but the important issue is how adding a horse shoe changes the stress in a horse’s foot. Once a horse shoe is in place, as shown in the figure to the left (from her paper), the surface of the foot that’s in actual contact with the ground is much smaller, restricted to the bottom of the shoe itself. The stress in the weight-bearing part of the hoof therefore goes up — by over 230% — simply because the shoe is so much smaller than the whole foot. (Remember, Stress = Force divided by area. The shoe is smaller, so the area is smaller, which makes the Stress larger for the same Force.)
There’s an intriguing corrollary to this, too. To understand it, you have to realize that force also comes upinto a horse’s foot and leg from the ground. If you hit the wall next to you with your knuckles, reading this, you will feel what I’m talking about: you hit the wall, but the wall also “hits” you back and smacks your knuckles. (If the wall did not “hit” you back with the same force that you used against it, your knuckles would go through the wall. The wall would not resist your force with one of its own.) If you hit the wall harder, the wall “hits” back harder. A person who’s really angry may hit a wall so hard as to suffer bruises and possible broken bones as a result of the fact that the wall will hit them back with the same huge force. You might remember this returned force from studies in high school: Newton’s Third Law of Motion states that for every action there is an equal and opposite reaction. When it comes to forces that react to an animal standing on the ground, such a reactive force is called the ground reaction force.
So not only does a horse experience force, and therefore stress, from gravity pulling down on its body mass so that its feet strike the ground, a horse also experiences force and therefore stress from the impact of hoof on ground in which the ground exerts force against the horse. You can see this, and very clearly, in an astonishing series of slow-motion video images made by Sky Sports UK of Hickstead horse jumping, at YouTube, here. I tried, however, to capture some small piece of what’s visible in a series of three images that are reproduced to the right. When the horse’s hoof comes down and hits the ground, you can see a wave of force returning up the horse’s pastern, from the ground impact. I have pointed to one of those waves with a red arrow in the second image. The apparent distortion of the top of the pastern area in the last image is due to similar waves continuing to propogate through the soft tissues. If you watch the video, you will see that these waves move upward and are caused by force coming from the ground, due to impact.
(Yes, the fetlock drops very low in the last image. We will not discuss that here, as it’s an adaptation related to elastic storage of energy. All you need to know right now is that this image does not show a dangerous degree of flexion.)
The interesting thing about what you see in these images is that the ground reaction force is being transmitted upward through the skin layers of the horse’s foot and leg, as well as through the stronger bones in the “core” area. It’s possible that there’s proportionately more “shallow tissue transmission” when the ground reaction force is directed ONLY through the shoe, since the shoe is going to transmit force through the hoof wall it’s nailed to, and this force is then very likely to be transmitted on upward through the skin and deep dermal tissues that are in line with the hoof wall. If so, then the wraps or boots around the cannon of such a horse may well keep those forces from adversely impacting tendons that aren’t designed specifically to transmit those forces but are quite coincidentally sitting in their path. (If anyone who designs such boots and knows the biophysics of their design elements wants to educate me about this, I’d love to know what data exist.)
On the other hand (and there is always an “other” hand), remember that the terminal phalanx sits just inside the front portion of the hoof wall. So forces being transmitted through the hoof wall will also, at least to some extent, enter the chain of bones nearby. The question is how ground reaction forces are transmitted through the legs and feet of shod and unshod — and also booted — horses. And as far as I know, we don’t have good data on that. But it’s certainly something to bear in mind when you’re weighing the decision each of us has to make about shoeing, booting, barefooting, or whatever. While you are taking into account the kinds of surfaces you ride on, your horse’s personal history and past injuries, and the sorts of activities in which your horse engages, remember that the different kinds of things we put on our horse’s feet change the way that force is transmitted — to the ground and also back up into the horse. There aren’t any easy answers. But it is probably safe to say that it’s more important to take extra steps to protect the hoof wall and tendons if your horse wears standard shoes than if it goes barefoot.
Next up: What happens when the horse starts moving
How does a horse who is outstanding in his field carry his weight? Well, if he’s out standing in his field, as opposed to trotting or running, he carries it on all four of his legs (bum-dum-ching). Here’s an example, to the left. Whatever this horse weighs, gravity is acting on his mass to pull it towards the earth with a calculable force. That force travels through each of his four feet to the ground. So a very simple way to begin thinking about force and stress at a horse’s hooves is to think about the force caused by gravity pulling on the whole horse’s body mass, and then simply dividing this force by four to estimate how much is supported by one of the hooves.
You can try this out for yourself on a bathroom scale. Stand on it with both feet and see what it says you weigh. Then balance yourself so you’re standing on it with just one foot, the other off the ground somehow. (Don’t balance yourself by touching a hand to the sink or a wall right now, as it will mess up your experience. You’ll do that in a moment for another reason.) Although you will see your weight wobble a bit on the scale as you move around to get to the one-footed position, in the end you will weigh the same amount. Only now you can feel that your weight is all being held up by that one leg and foot rather than two. Your body weight is your body weight, and it is going to push down the same amount on the scale whether it’s held up by one leg and foot or two of them. The difference is how much the leg/foot you’re standing on, that’s holding you, has to hold.
Now, put a hand on the bathroom sink while you’re standing on the scale. You will see your weight go down. That’s because you are supporting part of your weight through your arm and the sink, so less of your body weight is going through your legs to the floor. If a horse “leans” towards its front end, more weight goes through its front legs and less through its hind legs, just like you can lean more of your weight on the sink and watch the scales show you less and less weight going through your legs.
What you probably did not realize is that if you live in the United States and measure your weight in pounds, your bathroom scale does not measure your body’s mass, but the force with which your body’s mass is pressing down on the scale. A bathroom scale is a very simple type of machine that measures how much force pushes against it. A spring inside the scale deflects (gets squished) when you step on it and thereby apply force to it. A rack-and-pinion mechanism inside the scale turns the spring’s deflection into the motion of a needle on a dial, so that it points to a certain number of “pounds” depending on how far the spring has been deflected. So weight, in the US, in pounds is a measure of FORCE. (Also, see the section “spring scales” on this page about weighing scales.)
To explain how this happens, consider the equation for force, F = ma. F stands for the “force” an object exerts; “m” is the mass of the object; and “a” is that object’s acceleration. So the equation can be written in English, with specific reference to you on a scale, this way: the force your body exerts against the ground (or the scale) is equal to how much mass your body has, times how quickly your body is accelerating.
At this point, you are liable to say, “Wait a minute. My body is not accelerating if I’m simply standing on the bathroom scale.” But it is. The Earth’s gravitational attraction is pulling down on your body all the time. It pulls so hard that if your foot slips on some water as you get off the scale, you’ll crash to the tile floor hard enough to regret it. And it pulls so hard that when you hit middle age, various parts of your body start sagging down towards the earth’s surface because of gravity’s pull on them.
Gravity pulls on things enough to cause them to accelerate if they fall, and anything in freefall accelerates at exactly the same amount: 32 feet per second per second. That means that with each second that passes, the falling object goes faster than it did before. The diagram at the left (adapted from one on a UC Berkeley website) shows the position of a falling ball as gravity makes it go faster and faster, or accelerate, in free-fall. You can see where the ball is at the end of 1 second, 2 seconds, 3 seconds, and so on. Notice how much farther it goes in the 4th second. The speed of the ball, or its velocity, is recorded at the left as 32 feet per second at the end of the first second of its fall, and then 160 feet per second at the end of 5 seconds. You can see, here, what it means to say that gravity accelerates objects by pulling them towards the earth. The ball goes faster and then even faster, second by second, as it falls.
No matter what you drop, the speed of descent is going to be same. That’s what Galileo demonstrated in his famous experiment at the Leaning Tower of Pisa (which may have been just a thought experiment), where he dropped two objects of different masses from the balcony to see when they would hit the ground. “Common sense” held that the heavier object would fall faster. But in fact the two objects hit the ground at the same time. This is because the gravitational constant is simply and only 32 feet per second per second — mass isn’t in there at all. So the mass of the object gravity is pulling on has nothing to do with the speed at which it falls.
However, we all know intuitively that although a bowling ball and a tennis ball dropped from the Leaning Tower of Pisa might hit the ground at the same time, they will not hit the ground with the same force. We might be willing, if suitably encouraged, to stand where the falling tennis ball could hit us on the head. We would run like mad to get out of range of the shards of ball and pavement that would fly through the air from the much bigger force of impact with which the bowling ball would hit the ground. The force with which the tennis ball and the bowling ball hit the ground is described by the equation F = ma. Mass, m, is a part of Force. So the bigger the object that the earth’s gravity is pulling on, the more force that object applies to the ground. And this brings us back not only to the force at a horse’s foot, but our own bathroom scale.
American bathroom scales already factor in the acceleration due to gravity when we get on the scale. They do not measure our body’s mass. They measure the FORCE that results from gravity acting upon our body mass. That is what a pound measures on a bathroom scale in the U.S.
But, as you learned if you stood on the scale on just one foot long enough to do the exercise I described above, force isn’t the only thing that matters. Standing on the bathroom scale on two feet isn’t difficult. Standing for a long time on the bathroom scale on just one foot is. If you tried this part of the exercise, you might have noticed that you had trouble keeping your balance, that your joints hurt, and that your foot felt more “smashed” by having all your weight (force, remember!) on that one leg. The difference you felt was in the amount of stress that was in your foot.
In physics, stress is not a measure of how desperate you feel when you’ve missed a project deadline, have an overdue bill, and your car breaks down all on the same day. Stress is a measure of how much force is acting on a given area of an object. Stress, S, is also defined in an equation: Stress = Force divided by unit area. In other words, stress is how much force is being transmitted through any given part of an object. It is stress, not force, that causes structures to fail (if they fail). The area you measure — and this is important — is at a right angle to the force. So in your foot, the picture looks something like this:
What matters, in measuring the stress in your foot, is how much force is pushing down on it and how large the cross-sectional area is. In the picture on the left, we are looking at how much stress there is in the lower part of the shin area, just above the ankle. That’s where I’ve drawn a green ellipse that represents the cross-sectional area of the leg right at that point, as if you sliced through it horizontally. That plane is the one that the body’s weight, or force (the red arrow labeled “F”), is acting through. If the diameter of the person’s leg here is 3-1/2 inches (not too uncommon in a woman), then the area of the leg’s cross-section there is the area of a circle, pi times the radius squared. (Do I hear groans as thoughts of geometry flood back? Hang in there! We’re about to get back to horses, and this will all be worthwhile!) The radius of this part of the leg is half the diameter, or 1.75 inches, and pi times this radius squared is 9.6 square inches. So if our woman weighs 130 pounds, the stress this part of her leg is experiencing is 130 pounds divided by 9.6 square inches (because Stress = Force/area). That’s roughly 130/10, which is 13 pounds per square inch. That is the amount of stress going through this part of the woman’s leg in our example.
IF she is standing on one foot. If she is, all the force of her body (created by gravity acting on her mass) is being supported by one foot. But if she’s standing on TWO feet, then they are sharing the force. In that case, the stress in this part of the leg — the other one also being on the ground — is half her weight, or 65 pounds, divided by about 10 square inches, for just 6.5 pounds per square inch of stress. As you can see, stress is twice as high when she stands on one foot as when she stands on both — 13 pounds per square inch compared to only 6.5 pounds per square inch.
Let me pause here and say that if you’d like a reasonably good idea about how much stress this is, measure off a one-inch by one-inch square on a piece of paper. Look at how big it is. Now think about something that weighs 13 pounds — a medium-sized watermelon, for example. And imagine balancing the entire weight of that watermelon on that one-inch sized piece of paper. That’s how much stress is on the leg of that 130 pound woman if she stands on one foot. If she puts the other foot on the ground, the stress is half of that (about large cantaloupe-sized).
But remember we are heading for a horse’s HOOF here, not its leg. So let’s go on down to the part of a human leg and foot that is against the ground: the sole of the foot. How much stress is in that part of the body? The force or body weight is the same: it’s still 130 pounds if the woman is standing on just one foot, and 65 pounds (half of 130) if she’s standing on two. But the surface area of the sole of a foot is a lot bigger than the cross-sectional area of a shin. Look at the bottom of your own foot, and compare it to an imaginary slice through your shin bone, and you’ll see what I mean.
A common estimate for the surface area of the bottom of a woman’s foot is about 25 square inches. So our woman who weighs 130 pounds (who is, remember, exerting a FORCE of 130 pounds on her foot) experiences only 130/25 = 5.2 pounds per square inch of stress on the sole of her foot if she’s standing on one leg. And it’s half that — 2.6 pounds per square inch of stress — if she puts the other foot on the ground to hold up half the load.
Notice that the stress in the woman’s foot is much lower than it is in her shin, even though her weight (or force) is the same, regardless: she weighs 130 pounds. But since her shin has a smaller cross-sectional area than does the sole of her foot, the stress is higher there than in her foot: 6.5 pounds per square inch in her shin compared to 2.6 pounds per square inch in her foot (if she is standing on both of her feet). So now you understand one reason why it’s more common to hear of someone breaking their shin bone than breaking a bone in their foot. The basic stresses in a shin are higher than they are in the foot because the shin is smaller in diameter.
And now we can talk about force and stress in a horse‘s foot . . . er, hoof.
Let’s consider a horse that weighs 1100 pounds. If this horse is standing at a tie rail and has 4 feet on the ground, each of those four feet carries about 1/4 of his total body weight, or 1100/4 = 275 pounds.
I have to pause here to point out that this means if you want to lift up his hoof to pick it out, and if he doesn’t do anything to change the situation, you are going to have to pull up against those 275 pounds to get that hoof off the ground. That’s what he’s putting through that leg and foot if he’s just standing there eyeing you. I point this out to remind you that horses are actually fairly cooperative. Most of them “lean away” when we go to pick up their hoof, and redirect a lot of their weight to the other three legs so we can lift the one we’re after. But a horse that’s determined not to pick up a foot can not only brace itself but lean more heavily on the foot the person wants to lift up. I’m sure you’ve seen a horse do this. And when you see that, you’re not looking at a force-and-stress problem, but a relationship one.
So, back to force and stress. We have a horse hoof with 275 pounds of force going through it. The amount of stress on the hoof is going to be this amount of force (275 pounds) divided by the area of the hoof. This means we need to estimate the area of the bottom of a horse’s hoof, that’s against the ground.
The Davis Boot company makes standard sizes of boots for horse hooves to soak in, and they provide the boot dimensions on their website. They say that “size 2” is for average horses but that the boots are intended to go over a bare (unshod) hoof and to fit loosely enough for soaking solutions to be added. So we’ll select the boot that’s one size smaller, the size 1, to represent the dimensions of our 1100 pound horse’s foot. That boot’s dimensions are 5-1/4 inches wide by 5-5/8 inches long. Notice that the boot is not perfectly round, but longer than it is wide. This reflects the shape of horse hooves, as you can see on the right.
Since the shape of a hoof is not a circle, we are going to average the two measurements (5-1/4 inches by 5-5/8 inches averages to 5.35 inches) to approximate a circle of roughly the same size. The radius (half the diameter) is therefore about 2.7 inches. We can now calculate the surface area using the standard “area of a circle equals pi times the radius squared” equation. Multiplying this out gives us a figure of about 23 square inches for the bottom surface of this hoof that is against the ground.
And now we can calculate the stress there. We have a hoof with 275 pounds of force going through it, and its area is 23 square inches. So the hoof is experiencing 275/23 = 11.9 pounds per square inch of stress. . . if all four feet are equally on the ground and the horse is not moving. That’s quite a bit more than the 2.6 pounds per square inch of stress that we calculated in a 130 pound woman’s foot if she’s standing with both feet on the ground. Notice why: the surface area of a hoof (23 square inches) is about the same as the surface area of the sole of a woman’s foot (25 square inches). Horses are carrying much more weight, and experiencing much greater forces, through feet that have about the same amount of surface area in contact with the ground that our feet do. Granted, horses have four feet instead of two to carry the load, but they also weigh a great deal more — even proportionally.
However, there is obviously far more to the story of stress in horse hooves than this. Horses don’t simply stand still at tie rails, and they often have only one or two feet on the ground at a given moment. Now that we have gone through this set of calculations, though, we can take things to the next step (pardon the pun!).
I had a friend several years ago who was an artist, who became absolutely captivated by tornadoes after we saw one once, and lived to tell the tale.
I am supposed to be writing about horse hoof stresses in this post. I’ve sat here quite a while today, typing variations of the lead sentence: “There are several factors that influence the stress on a horse’s foot.” And it just won’t go anywhere, because it’s “the artist and the tornado story” that wants to come out today. You’ll see why by the end of the post. Trust me.
The artist and I stood literally shoulder to shoulder for what felt like a week, mesmerized, watching the clouds spin down beneath the central updraft, coalesce into a funnel, fall apart, and pour upward back into the core like an inverted waterfall, over and over again. It wasn’t a very big tornado. That was probably a good thing. (Don’t try this at home, adult supervision only, your mileage may vary, etc.) I have to admit, it was an astonishing thing to see. As far as I know, the artist never quite got over it.
Every time I talk to people about horse biomechanics, something comes up in our conversation that makes me think about form and function . . . and the enormous problem people have with remembering that the “function” part matters. A lot. The artist brought this aspect of peoples’ thinking home to me real fast that day, maybe the first time I’d ever realized how separated form and function can be if people aren’t used to thinking about it. Because after the funnel was carried away on the moving squall line, and the storm had rained itself out over the tops of our pointed little heads (we were lucky not to have been pulverized as well as mesmerized), the artist turned to me with rivulets streaming out of her soaking hair and running down her forehead, and asked, “Why is a tornado shaped like a triangle?”
She was thinking about the shape shown in the NOAA photo to the left. It’s a common enough shape for a tornado, but it’s certainly not “standard”. Here are more NOAA images to give you an idea of the range of variation.
The thing that really threw me about the artist’s question was that the tornado we’d been watching as it tried to form and kept falling apart never looked like any of these pictures, though. It looked a lot more like the still image below from video of a rotating wall cloud near Terre Haute, Indiana in 2007. The red arrow points to a dark circular structure with a fairly narrow, paler rim of clouds that can be seen, in the video, to be rotating slowly with the entire cloud immediately surrounding it. (It is, in fact, an example of the infamous “rotating wall cloud” of severe weather alerts.) This is where a tornado funnel can form. It was a structure like this that we saw forming “sides” several times before falling apart and “pouring” over its own lip and up into the clouds again, a number of times in succession.
In other words, the image we actually saw, the artist and I, was one of circular rotation. Furthermore, it was absolutely clear as crystal to both of us that within the “funnel-to-be” that kept forming and falling apart, the overall direction of movement was up. And that is how tornadoes work, really. They are a vortex, having a spiralling motion that goes from the ground up into the clouds, spinning as it goes. It is the spinning winds of the vortex that create the distinctive “tornado vortex” signature on radar that identifies a tornado’s likely existence even at night when it can’t be seen by weather spotters. Because the tornado is spinning in a circle that’s parallel to the ground, Doppler radar shows winds on one side of the funnel as going away from the station, and winds on the other side of the funnel as coming towards the station. Since the radar creates different-color images for winds blowing away and winds blowing towards, there are two different direction-colors of wind smack next to each other on the radar. That’s the vortex signature, and it’s created by the rotating circle of wind.
Interestingly, about five years before the artist and I encountered the nascent tornado I’ve described, I had spent several long months putting information about things like “tornado vortex signatures” onto a series of educational pages for the non-profit I worked for, using tornadoes to explain the power of intergrating different ways of knowing, learning about, and responding to the natural world. The artist knew that I had spent hours going over reference materials, talking to experts at NOAA and the NSSL, and writing everything up because she had proof-read and edited every one of those pages for me at the time.
So although I understood why she figured I would have at least a semi-reasonable answer to her question, “Why is a tornado shaped like a triangle,” at the same time I was stunned. Because not only had she read, in detail, about how tornadoes function, she had just seen it with her own eyes: rotation and updraft, both. Yet somehow she had fastened onto a stereotypical shape she’d seen somewhere, a form — one she certainly hadn’t gotten from the clouds that were still trailing over our heads that afternoon — as the “meaningful” shape of a tornado. And the evidence that she saw the triangle form as meaningful is that she asked WHY the tornado was that shape.
The question “why” only has meaning if a person wants to know the connection between a form and its cause. And the “cause” or “reason” for form is almost always function.
If you look at the line of pictures above, again, you will see that understanding how tornadoes work, functionally — that they are a rotating vortex — makes sense of the wide variety of forms shown there. Sometimes the clouds are way up high above the ground and the rotation’s diameter is narrow, so the tornado’s form is that of a long rope. Sometimes the rotation’s diameter is so large that a tornado is wider than it is tall, and the form is then like a wedge that blots out much of the sky at the horizon. When the clouds above blow along faster than the base of the thing moves, the tornado leans sideways, the part on the ground trailing the part at the cloud. When condensation doesn’t happen around the vortex, you see debris flying around in a circle but not much else — sort of an “invisible” tornado. And, of course, if the clouds and the ground are at a particular distance apart, and the diameter of the rotational part in the cloud is a certain proportional size, and the debris field around the base on the ground isn’t very big, and if you happen to be standing in just the right place with a certain point of view — then the tornado looks like a triangle.
A cloud that happened to hang down, maybe like a rain virga, and was shaped like a triangle, that did not rotate and did not have updraft in the center of it, would not be a tornado. It would not cause damage. It wouldn’t be dangerous. It might be an interesting phenomenon, but it would not be a tornado. The form of a tornado is a product of its function. Details of the specific situation, such as how high the clouds are and how wide the rotation diameter is, create variation in the form. But a tornado is a function-based phenomenon. Even the very oddest-looking tornadoes in the line of photos above is still a tornado because it functions as one.
The bottom line of form — the thing that produces form — is function.
So what does this have to do with horse biomechanics? Remember, I said at the outset of this post that sometimes it’s hard for me to realize how far form and function are divorced from each other in people’s awareness. I have the artist and the tornado to thank for bringing this to my attention the first time. But now that I’m working to help people understand horse biomechanics, I discover that the separation of form from function in people’s minds is extremely common. And so is the focus on form, all by itself.
If meteorologists thought that tornadoes really were triangles, if they did not understand that how tornadoes function is the most important thing to understand, that form is merely an outward manifestation — almost a side-effect — of this critically significant function, we would not have radar warnings to tell us to take shelter. The ONLY way people could know a tornado was in the area would be if a spotter happened to see it and see it from exactly the right “triangle” vantage point. It is understanding tornado function that allows us to predict them, to identify them on radar, and to design special structural elements that help keep buildings from flying apart in rotating tornadic winds.
Horses have anatomical systems of bone, muscle, connective tissue, and nerves that function, together, in a range of very specific ways. Certain patterns of horse anatomical function produce forms that we recognize, just as certain functional patterns of wind rotation produce the recognizable form of a tornado. The parts of a horse’s body are connected to each other in complex ways. When certain nerves fire, they cause certain muscles to contract, which causes specific movement of certain parts of the skeleton. The end result of this chain of causes may be that the horse’s head rises from a neutral position. If so, this is a side-effect of the entire functional chain. The raising of the head is a result, not a cause.
Yet, what I have learned is that many horse people see that a horse someone has identified as being collected has an elevated head — and conclude that this form of “elevated head” is the cause of collection, that it is the actual functional mechanism of collection. They therefore pull on the reins and on a horse’s mouth to raise its head and “pull it into a frame”, thinking this will functionally produce collection. But functionally, it can’t. Bodies don’t work that way.
Human beings tend to put their arms up and out when they leap into the air. But you can’t “make” a person jump by having them raise their arms. It doesn’t “go that direction”, functionally. If humans were trained to jump by aliens (E.T.s) who did not understand the functional mechanisms of jumping, by aliens who focused instead solely on the one aspect of form “arms raised”, then these aliens would try to train humans to jump by lifting their arms over their heads with ropes and pulleys. Presumably the aliens would be frustrated and irate when no one jumped as a result.
It is a trainer’s responsibility to learn what the anatomical functions are, that produce particular forms such as collection. This is absolutely what the great trainers of the last few centuries have done. They may quibble about the details (scientists do too!), but they understand very well that form is not a cause but a result. If a rider trains her or his own horse, then they have just picked up the trainer’s level of responsibility in this matter, whether they know it or not. And because the responsibility exists, so does the power to harm a horse that is forced to move in a way that produces only a meaningless form, on the assumption that the rest of a functional sequence will somehow follow even though the real cause-and-effect line flows the opposite direction.
Given that bones and muscles are largely hidden by the horse’s skin, how is a rider or trainer to understand the functional mechanisms that produce the forms they desire in their horses? There are plenty of books and articles on horse functional anatomy and biomechanics available, and this blog and my seminars are two small contributions to that field of knowledge. But any time you read one of those that tells you to focus on FORM as the most important part of anything, as the part that produces function . . . stop and ask yourself our riddle: “When is a horse like a tornado?” Because right at that moment you will have encountered the answer: “A horse and a tornado are alike when the person thinking about them focuses on FORM rather than FUNCTION.”
By the way, it turned out the artist had not really registered either the circular rotation or upward movement we had witnessed. Instead, while watching the spectacle, spellbound, she had reflected on the photographs of tornadoes she had seen. This is what prompted her question. When I pointed out the functional aspects of tornado shapes, she explained that it didn’t matter at all to her because, as an artist, she only cared about representing images of tornadoes on paper or canvas — not creating functional ones. I strongly suspect that most horsepeople, to the contrary, do want a functional horse rather than 2-dimensional representations of one. So although horses and tornadoes may be alike when people think about them the same form-based way, the people who do the thinking — horsepeople and artists — may be rather different.
The slogan Jo (my business partner) and I use for our horse work is “Balance, Center, Connect.” This is the logo we designed for the first horse program we ever ran, which was at that time part of the work in which our non-profit, Tapestry Institute, was engaged. We coined the phrase to refer not only to our horses’ own best functional states, and to the states that allow us to have our best riding experiences, but also, purely and simply, to ourselves as human beings.
We all know it’s difficult for a person who’s not physically balanced to ride well. They continually throw their horse off-balance with their own off-kilter weight (which produces the “eccentric load” I talk about in my biomechanics seminars), accidentally asymmetrical hands and feet, or body stiffness that translates itself right into the horse. The same is true of being physically centered. And of course we need just the right amount of physical connection with the horse we’re riding to communicate responsibly as well as effectively.
But emotional balance matters, too. Who among us has not had a miserable time riding, fussing with our horse for being “difficult,” only to realize with a sudden pang of guilt that we went out to ride that day already angry or upset about something else? An emotionally unbalanced state communicates itself to our horses just as freely as it does to the humans around us, who can at least use spoken words to ask us to “lighten up” or “take a walk to calm down” before dealing with, and upsetting, them further.
When it comes to understanding how horses move and support themselves and their riders, it’s also important to balance differenttypes of knowledge. But doing so is extremely difficult because of our culture’s deeply-embedded assumption that logic, reason, and mathematics are the most valid ways of understanding reality. “Factual” information is seen as trumping any other type, “science” is seen as producing the most reliable facts, and information that is mathematical is seen as being the “best” type of science. Here’s an example of the result of this type of worldview.
Hilary Clayton, Ph.D., at Michigan State University’s College of Veterinary Medicine, has carried out studies on the biomechanics of bits. Clayton is a solid researcher who holds excellent credentials and has published a number of books. But her work can be a bit daunting for many people to read. Her biomechanics and physiology books, in particular, are ones I would personally select for my students to use if I was teaching a graduate course in biomechanics, but be reluctant to recommend as an easy-to-use resource for a rider who doesn’t want to get a graduate degree. Yet Clayton gets her work “out there” in clinics and talks — and is frequently misunderstood by people who are looking for “facts”, particularly mathematical ones they can hang their hats on with pride of knowledge.
Clayton’s work on horse bits was summarized and reported in Horse Science News in an article titled “Myler Bits Act Differently on Horses.” More than half of the three-paragraph summary was devoted to mathematical data, such as: “The jointed snaffle and Boucher bits both averaged 3.3 centimetres (1.3 inches) away from the teeth, whereas the Myler snaffle lay just 2 cm from the premolars. The other two Myler bits were located even closer, at 1.3 cm from the premolar teeth.” A reader studying this report is left with the comforting notion that “something important” has been learned about bits, particularly about the place in the mouth that myler bits sit compared to other bits, and that single-jointed snaffle construction and rein tension are somehow relevant factors. At the same time, I think it would be difficult for most riders to apply any of the information presented. I also think, from my years of doing public education with Americans who both respect and dislike science (thanks in large part to nasty grade- and high-school experiences) that most of the people who couldn’t make heads or tails out of what to do with this information would assume the problem was their own — that they didn’t know enough, or weren’t smart enough, to see at once how to apply it.
The trickier part of this is that I also know, from long experience, that there are people who do bitting clinics as part of their general horse business who, when they come across this numbers-filled article, will figure out a way to say that the figures support whatever view of bits they propound in their clinics. If you think people don’t do that, just watch the labels on (and ads for) products in the grocery store the next time there’s big news about a scientific study linking a particular nutrient or compound to some aspect of staying healthy. And it makes sense if you think about it — as long as the “facts” have been appropriately represented in the press to begin with. And that, as the PLoS Medicine article I just linked to shows, is the rub, right there.
On the other hand, take a look at this article summarizing Clayton’s bit research at Horse Journal on the Equisearch website. (That’s not a rhetorical statement. I really recommend you read the material to which I’ve linked.) This article, titled “Dr. Hilary Clayton Offers Many Prescriptions for Bits,” has an entirely different theme: “Dr. Hilary Clayton has been studying the way bits act on horses’ mouths for more than 20 years. But even after all those years of systematic research, she still says that ‘finding the right bit is more a matter of trial and error than a scientific process.'” The italics are my addition, to highlight the part of this statement I find particularly meaningful. The article then goes on to list some generalized findings that will come as a surprise to many horsepeople — one of which is that many horses prefer smaller-diameter to larger-diameter bits because their mouths are smaller than we think they are — and then quotes Clayton categorically stating that the rider has to figure out which bit their own horse is most comfortable using: “‘So the size and the shape of the bit is individual to every horse, meaning you have to keep trying until you find a bit they’re comfortable with”. This is repeated in the article’s last line, which concludes: “. . . Clayton still believes that finding the right bit for your horse is a matter of simply trying different bits.” Please notice that this research scientist is emphasizing the real value of experiential knowledge here, rather than “fact”-based knowledge, in two ways: (1) the rider is to use a personal trial-and-error method to try different bits and see how each one works, and (2) the rider is to assess each bit tested by using their own individual perception of howcomfortable their horse is with that bit in its mouth.
You don’t have to look far to find publications in which people who claim “horse expert” authority denigrate the very idea that horse “comfort” matters or that humans can assess a horse’s comfort. Further, such “experts” claim to use science, of all things, to give authority to their denigratation of experiential modes of understanding and to support their own fact-based system of information as not only best but also the only reliable source of knowledge. This is what leaves horsepeople feeling stuck, unable to get themselves and their horses out of situations that don’t feel right.
Yet here is a highly-credentialed research scientist who’s emphasizing experiential learning, the significance of a horse’s comfort, and the reliable ability of a horse owner to tell if their horse is comfortable or not. Given that the non-scientist “bit expert” who denigrates the horse owner’s experiential learning is claiming the authority of science (supposedly), then should not the actual scientist have the higher authority in this case? If so, then the paradoxical situation is that it is the scientist who turns over the tables on what type of knowledge has real value, placing the final authority squarely in the hands of a rider and the responses of her horse. And if that rider says, “But I don’t know about bits,” then look again at the article I linked to and you will see that it addresses this issue. Clayton points out the salient factors that usually impact bit comfort, that we need to pay attention to, including how high or low in the mouth the bit rides, whether it’s single jointed or has a flat area over the tongue, whether it touches the roof of the mouth, and the importance of paying as much attention as possible to the size of the mouth cavity since it’s not directly comparable to the horse’s body size. In other words, there are pieces of “factual information” she’s learned in 20 years of bit research, and she’s passing those on in order to give riders information about what to look for as we run our own tests. She is teaching us what to look at, how to see.
So that then we can make our own decisions.
That’s what science is about. It’s about learning what factors exist, since usually we aren’t aware of even a tenth of them in normal practice. It’s about learning which factors that we never even thought of are really important to pay attention to. And then it’s about learning how to integrate information about these factors into a much larger scheme of understanding the subject so that we can make personal decisions that are wise and informed. This is real science — which, interestingly, is based on trial-and-error (which is called “experimentation”) and personal assessment of results (which are called “sense-data”) — the two specific methods Clayton recommends horsepeople use to select the right bit for their own personal horse. Oddly enough, the fact-based, supposedly “science”-based system we find so often in the horse world, that tells us horse comfort is an illusion, that riders can’t possibly decide what’s best for their horse (but must hand over decisions to Experts), and that all horses in a certain discipline “must” use a particular bit because “we know” that bits work in such-and-such a way is actually not scientific at all. And I am saying that as a scientist with a doctorate from a major university, who has participated in policy discussions about the nature of science and of communicating science to the public in formal and informal education at the highest levels possible in the US.
All this being the case, my goal for this blog and for my seminars, both, is to share what I know about horse biomechanics and horse movement to help people learn how to see their horses better. I want to share information that helps riders see and appreciate the really astonishing power and subtlety of response that exists in a horse’s systems of adaptation to stress, and to learn what to look at when they assess how their horse is moving. I want to help people better understand the basic biomechanical adaptations of their own human bodies, to feel how subtly but profoundly those operate, and then use that information to understand the operation of those same systems in their horses’ bodies. And I want them to be able to use this information, even twenty or thirty years from now, with everything else they know about horses in general and their own horse in particular, to feel confident enough about their own authority to tell a trainer, when necessary: “No. I realize the method of moving you use on horses may work for many, but it’s not good for my horse. Thank you for your efforts to date, but I am discontinuing his training with you.”
The problem with my original blog post on force, stress, and the hoof was that it focused on numbers. It focused on calculations. It focused on apparent absolutes that really do not exist except in the most artificial situations imaginable (where, for example, a horse is literally not moving a muscle). And in doing all these things, it took us all to exactly the place I don’t want to go.
The first time I took it down, it was because I thought about all this. What I did was put it back up with a paragraph added near the beginning: “I want to say at this point that calculating, or even measuring, force in a living body is extremely difficult — sometimes impossible — for practical reasons having to do with the incredibly responsive systems of animal support, balance, and locomotion, and the fact that contraction in even a single muscle is generally along a wave that makes its force vectors change rapidly and dramatically. So the goal of this blog and the next is to look at things in just enough detail for you to get a working idea of the levels of load on horse hooves, and the sorts of things that impact those loads for better or worse.”
But the post still bothered me. Because I knew very well that my disclaimer — which was the real point of all the horse biomechanics work I do — was going to be monumentally outweighed by the correct but almost meaningless calculations I had gone to such great lengths to explain, about force and stress in the hoof. And finally I realized I have to go about explaining this a different way — one that focuses on the issues of subtlety and responsiveness in the horse’s living systems of adaptation. One that leaves the numbers out unless there’s an essential reason for them, and then makes very certain to keep them in perspective. One in which all the different ways of knowing about and understanding how horses move are in balanceand connected.
I close with the same Calvin and Hobbes cartoon with which I opened my ill-fated post of hoof stress calculations, this time focusing your attention on all the different ways of knowing that Hobbes combines to be an effective tiger. The laws of physics and the principles of biology are important factors, but so are art and ethics. All I can say is: right on.
So now I’m in balance again, not focused all to one side of things with purely intellectual and mathematically-based knowledge. And I’m centered once more, squarely in the framework of the goals I have for this blog, my seminars, and my life work in general. The result, I hope, is that I’m better connected to you, the reader.
And NOW we can go on to consider force and stress in the hoof . . . this time, in a way that has a little more meaning, and meaning you can use.
See you next week.
This post was a featured blog entry on BarnMice, Jan. 2, 2013.
The post that originally occupied this space is gone. If you’re one of the small group of people who’ve started following this blog since its beginning earlier this month, you may have seen the post on force, stress, and the hoof appear, disappear, and then reappear within a span of three days as I wrestled with my goals for posting it. It wasn’t a small matter that I put it up to begin with . . . and it wasn’t a small matter that I finally decided to take it down. Both things matter enough — to the goals I have for writing this blog to begin with, and to the goals many of us have for our horses — that I’m going to express my struggle and the reasons for my final decision in a new post later today or tomorrow.
And after that, then I’ll get back to hooves. But we’ll be starting on the right foot this time.
The amount of information available about horse hooves and their trimming, shoeing, barefooting, wrapping, booting, soaking, and “orthotic-ing” is mind-boggling. So the first thing I want you to know in this post is that I’m not going to summarize all that or wade in to the “natural hoof care” debate or encourage you to purchase a product of some sort. My goal in this post is the same as it is for the blog as a whole: to offer you a different point of view on a subject — one that doesn’t seem to be readily available otherwise — that is based in biomechanics and good, solid biology. It will be your job to take what you learn here and figure out how you want to integrate it with what you already do.
This post will discuss specific aspects of the biology of hoof structure, and then the next post will consider functional aspects of those structures in terms of basic biomechanics. In both posts, the place we’re heading is a consideration of the nature and size of the element called the “coffin bone” by horsepeople and the “terminal phalange” by anatomists, and how that element functions in the stress environment of a large animal’s hoof.
We’ll start by looking at what bones are in a horse’s legs and feet. These bones have names as well as “definitions” based on where they’re located in the body and what other bones they connect to on the ends. For instance, animals have a lower jaw bone called the mandible. In mammals, the mandible is entirely mde of a bone called the dentary, which bears teeth on its upper surface (if the animal has teeth in its lower jaw at all, which some do not). There is a right dentary bone and a left dentary bone, one on each side of the animal’s lower jaw, and they join each other in the middle at the front (to form what we would consider the animal’s “chin”). The two halves are “sutured” there (that is the anatomical term) by a joint that is fixed or immovable and that looks like a wavy line of back-and-forthing such as might be done by a needle and thread. As the animal ages, this line disappears and there seems to be only one bone instead of two. On the back end of each mammal’s dentary bone there is a process that sticks upward, that makes a fairly high crest and that has a lower piece sticking back with a roller joint on it. When the lower jaw is put in proper life position against the skull, this roller joint articulates or fits against a socket in the skull, and the jaw rolls to open or close the mouth at this joint. The high crest projects up through a part of the skull that sticks out from the rest of the bone as a big arch. In life, it can be seen that strong jaw muscles attach to both the crest of the dentary bone and that arch on the skull. These muscles help to pull the jaws together so the teeth can chew food. Here is what the dentary looks like in a horse, when it is not attached to the skull.
The picture below (left) shows you how the dentary bones look in position, articulated with the skull. You can see the crest goes through an arch on the skull and that the roller joint articulates with or fits up against a part of the skull that is shaped to receive it. The dentary in this picture is the white (uncolored) bone. The lower teeth are hidden by the upper teeth, which “lap” around them to the outside as this is drawn.
The skull to the right of the horse, above, is that of a human. It shows the human dentary bone and the way it fits to its skull, for comparison. I have colored the dentary a light bluish-purple. The point is to notice that the basic shape, position, and relationship to the bones around it are the same for the human dentary bone as in the horse. That’s why we consider both bones to be dentary bones, is that they meet the same “criteria” of definition for that bone. The dentary of the horse is therefore said to be homologous to the dentary of humans. It should be added that homologous structures may be seen to develop from the same embryonic tissues during development of different animals. So the similarity of structure that we call homologous reflects a real commonality of genetic origin and development.
Comparative anatomy gives us powerful tools for understanding animal structures. That’s probably more true for the hoof than for just about any other structure, because we all know how vitally important a horse’s hoof is. “No hoof, no horse” is an adage that’s been around about as long as people have been riding and working horses. And it turns out that the hoof is an astonishingly “little” bone.
Here is a horse skeleton with the bones of its front leg colored in a specific way. The circle I’ve drawn is around the bone horsepeople call the cannon bone, to serve as a place-marker with which you’re familiar. I’m not going to worry here about what other names horsepeople have coined for different parts of a horse’s limb anatomy, because that will actually confuse matters. I’m going to use terms that tell us about the homologies of these structures instead. Next to the horse skeleton is the skeleton of a human arm. I have colored the bones there so they correlate to their homologues in the horse skeleton. As we run through these bones, you can use your own arm and shoulder to help you learn the structures experientially.
At the top of the of the arm, the first good-sized bone that moves the arm in a mammal, is the scapula or shoulder blade bone. It’s colored pale turquoise here. In a horse, the scapula lies beneath (and gives the sloping ridge-shape to) the withers. In a human, the scapula is “down behind” our arm in our back because of the way our chest is flattened out side to side. There is a bone called the clavicle or collar bone in humans that is shown in white on the diagram above, that is not present in the horse. Then the next main bone in the sequence is the humerus bone. We looked at the humerus bone already in our discussion of the trotting gait in horses, and it’s colored in red on the diagram. It’s also in red on the human skeleton. In you, the humerus is inside your upper arm. (You’ll have to disregard the horse’s back leg here, on which I previously colored the femur red as well for a previous blog. The horse’s back leg is homologous to a human’s leg, too, but we are going to focus on the foreleg and arm as our example for this blog.)
Beneath (or “outward from”) the humerus there are two bones, side by side: the ulna and the radius. Here, I have colored the ulna a light green and the radius yellow. In humans (and many other mammals), the two bones are totally separated their entire length. They twist back and forth across one another when we rotate our forearms to turn our palms up or palms down. (If you sit with your right arm bent at the elbow, with your right hand just above your lap, you can move your arm in a way that allows you to feel these two bones. Put the fingers of your left hand across your right arm, about halfway between the inside of your elbow and your wrist. Press down firmly. Then, without moving your upper arm, simply twist your wrist to rotate the palm of your right hand so it faces the floor, then faces the ceiling again. (Your right thumb will face left when your hand is palm-down, and right when your palm is face-up.) The fingers of your left hand should feel the radius cross over the top of the ulna when you do this. The ulna is the longer of the two bones, and the back end of it “sticks out” to form our elbow.
Horses have an ulna and radius, too, but theirs are fused together not too far below the elbow area. (The next time you saddle your horse, take a good look at the part of the horse’s arm just in front of the cinch or girth. You will see a very obvious elbow there.) It’s hard to see what they look like in the skeleton above, so here’s a closer picture of just those two bones in a horse. I colored the ulna light green and the radius yellow. You can see the dark line between them near the top of the radius, and then you can see that the ulna fuses to, and essentially becomes one with, the radius farther down the shaft. This is because the separate ulna and radius allow the hand to be twisted around the long axis of the leg — as is so in us — and that would be a really bad way for a horse’s leg to move. Can you imagine what would happen if a horse was running at 30 miles an hour and suddenly twisted its front hoof on its leg? So there is an adaptation of this structure that keeps the twisting motion from taking place. The radius and ulna are “locked” down.
Beneath the radius and ulna are the wrist bones, or carpals. In the big skeleton, those are not even drawn in, they are of such little consequence to many horsepeople learning anatomy. But of course, those bones are extremely important and you sometimes hear about one or more of them being injured in a performance horse. Since we’re heading for the hoof, though, we’re going to skip the carpals for now and head on downward. So the carpals are simply white on the human skeleton as well.
Beneath the carpals are the finger bones. The first of these is a long metacarpal bone that is actually positioned inside the fleshy part of the hand (where the palm is). Then, at the end of each metacarpal, there is a series of three little bones, each called a phalange. There are three phalanges on each finger except the thumb, which has only two phalange bones. If you take a look at your hand, you will see all this. On the human skeleton, I have colored the metacarpal a dark blue, the first phalange dark red, the second dark green, and the last orange or dark gold. However, I have only colored the bones of the middle finger — because that is the finger homologous to the hoof bones of a horse.
Look at the colors of the bones in the skeleton above, showing the horse hoof. You will see that the cannon bone is actually a metacarpal. And the bones that make up the pastern and the hoof are phalanges. Here is a diagram that shows you the end of this sequence a little more clearly. It’s a photograph of a horse hoof that was injected with plasticine so it could be sectioned cleanly and show you the structures. On the left is an image without any modification, and on the right is the same image with the bones colored so they correspond to the key I’ve just run through. (The diagram, from Wikimedia commons, is labelled in German.)
What should blow you completely out of any sense of complacency about your horse’s abilities, here, is realizing how very tiny the bone is that’s inside your horse’s hoof. I mean, LOOK at it! Here’s a nice shot of this coffin bone from a horse care website, that happens to have the edge of a person’s thumb and thumbnail in it, for scale. Take a look at just how small this bone really is. And if you consider the fact that the “hoof bone” or “coffin bone” is, in fact, a terminal phalange — the last tiny bone of a single finger — the fact that it bears so much weight, running at high speeds is nothing short of astonishing.
We are impressed, and rightfully so, by ballet dancers who leap and pivot en pointe, with their feet positioned so that their weight is borne only on the tips of the toes. Depending on the gender of the dancer, they may weight perhaps 80 to 150 pounds a person. What you can see here is that horses are also en pointe, minus the ballet slippers, but carrying closer to 1000 or 1200 pounds when they run, jump, and spin on those toe tips. It takes nothing away from human ballerinas, but I think it certainly adds to our appreciation of horses.
Or at least, I hope it will after we consider the consequences this has for the stresses (force per unit area) in the bones of horses’ feet. Coming up, next time . . .
This post was a featured blog entry on BarnMice, Dec. 1, 2012.
You can take the title of this blog as meaning either or both of two things: horses that are moving (in which case the word “moving” is an adjective), or the things a person does to move a horse (in which case the word “moving” is a gerund). The two things are related, which will turn out to be the theme of this blog.
This entry is the joint product of two people: myself as the anatomist and biomechanics scientist, and my business partner Jo Belasco as professional trainer. Jo and I have worked on what might be called “applied horse biomechanics” together for over ten years now, and although I wrote the words you see here, they only came into existence after lengthy conversations with her and discussion about this very post.
The impetus for writing this entry was that a horse friend wrote me in response to my post about the walk, saying that although he’s heard many trainers insist that a horse cannot move the leg it’s standing on, and that cues to move a specific foot need to be provided when that foot is off the ground, he doesn’t see how this can be so. “If it were true,” he wrote, “there would be no way to start a horse from the stop. Maybe the ‘rein or seat’ qualifiers are important here, but I had a horse that was so forward he would move if you simply relaxed the reins. . . All with no reference to influence with the foot on or off the ground.”
My friend’s comments made me think about the glib way we all short-cut biology to smoosh terms around in ways that give them meaning they were never meant to have, and the impact such short-cuts have on how we think about and deal with horses. So here’s some additional thought about the walk and the training practice of learning to cue a horse’s foot when it’s off the ground.
When people say you can’t move a horse’s foot unless it’s not on the ground at the time, they are really making a short-cut statement. The real statement should be: “If you cue a horse to move its foot when that foot is bearing weight at the time, the horse won’t be able to respond immediately and/or in the way you’ve asked. And because horses move quickly, even at a walk, if you cue them any later than the moment you feel their foot come up, you will be too late. They will have put their foot back on the ground before they have a chance to respond. Furthermore, you want to cue them early enough that their foot is not only still in the air, but able to be redirected to come down somewhere else than where it would have gone otherwise.” Yes, that’s a lot to say, but of course it’s why we speak in a short-cut.
Let’s consider, first, where this idea comes from, which is ostensibly the nature of the step cycle. A step cycle is the complete range of motion made by a limb from the time it leaves the ground to begin a step, through the time it advances and comes down again to the ground and bears weight, until it leaves the ground again to take a new step. The sequence of still images below shows one complete step cycle of the left hind limb of the same horse that was pictured in the walking blog I posted on October 12. The screen grabs are from the same slow-motion video I used there. The blue arrow I’ve added to the first frame shows the foot coming up off the ground to begin the step cycle. It’s a little hard to see that the heel is up in that image because of the way I reduced it to fit on the page. But the heel is up off the ground where the blue arrow is.
You will notice several things in this sequence, if you look closely, all of which give you a little better understanding of the walk. First, the horse is moving its body forward (the whole point of walking) by pushing the foot back against the ground during part of the step cycle. So the position of the foot we’re watching changes with respect to where the horse is. When the foot is coming up off the ground in the first frame, it looks to be about 4 feet to our right of the vertical wooden post, which is at that time behind the middle of the horse’s neck. Mid-way through the step cycle, in frames 6 – 9, the horse’s left hind foot is fully on the ground and supporting weight. It has been brought forward about 2 feet, so now it is only about 2 feet to the right of that post. At this point, the post is behind the horse’s withers (in 6) and then the end of the back (9). By the time we get to the last frame, where this back left foot comes up off the ground again, it’s still about 2 feet to the right of the post — but now the horse has advanced so far forward that the post is behind its rump.
Many trainers say a rider could ask this horse to move that back left foot in about frames 1 through 3, when it’s come up off the ground and has not yet been set back down. So they teach riders “feel” of the feet so they can tell when that particular foot is in the position shown in frames 1 through 3 — and that’s when they cue the horse to set that foot down someplace other than they were about to put it. These trainers believe that if the horse is asked to redirect that foot between frames 6 and 11, the horse won’t be able to respond even if it wants to because it is standing on that foot and therefore unable to move it (unless it breaks its gait by hitching or stumbling). In the case where you are asking a loping or cantering horse to change its lead, there is even the possibility that it will respond on one end (that can respond at that moment) but not the other (which can’t), with the result that it cross-leads if you ask for a change of lead while the back foot that needs to move is on the ground.
Because of my biological background, I was actually suspicious of this idea for quite a while. The reason is that neurological researchers spend a lot of time considering what they call Reaction Time (or “RT”, because of course it has to have a catchier and geekier name than just “Reaction Time” if you study it in your lab). To get an idea of the level of discussion about how long it takes a body part to respond to a stimulus, and the kinds of factors that impact this response, here are abstracts of a couple of scientific papers on the subject from the National Academies of Science and the American Psychological Association. What you will see, besides the designation “RT” and other jargon that might make you reach for a pain reliever, is that moving a foot in response to a cue is not a simple matter of stimulus-response. There is a lot that affects how quickly a motor response is expressed when any animal hears, sees, or feels something — which is to say that how quickly a horse moves a foot in response to the touch of a heel or rein is not a simple matter of picking the “one right part” of the step cycle to offer the cue (stimulus).
Researchers in RT want to know how quickly a human being can respond to switch off power to an engine if an alarm sounds, for instance, so most of the work on Reaction Time is on human beings. If you wade through those abstracts, you will see that their work suggests that a person’s reaction time can vary according to things like practice, the kind of stimulus presented, how many response options exist (in other words, do you have to decide whether to flip the lever or push the button, or is there only one choice), the nature of the on-going task or activity, and how much attention is being paid to specific parts of the environment. Just to show you how important these factors are, visualize for a moment the memory of an experience I’m sure you’ve had — a sudden loud noise or sharp motion startles a horse, which proceeds to leap straight up into the air with all four feet at once. It might have had all its feet on the ground a split second before, but the nature of these other factors changed its “normal” Reaction Time dramatically.
Most of us hope we don’t generate that kind of reaction in our horses. The point is that when we talk about “when our horse can move a foot in response to our cue,” we are actually talking about Reaction Time whether we realize it or not. And, given that we are, it’s a much more complex phenomenon than we imagine. A lot of things, including training, can impact how quickly our horse responds to a stimulus-cue. Furthermore, even though the Reaction Time for a foot placed squarely on the ground may be longer than if it was in the air when we cue it, that Reaction Time still exists. A foot that is on the ground when we cue it can and will (if the horse chooses) react to our cue when it come up off the ground a few seconds later.
In the worst case scenario, the horse might react to the cue with a different foot and get a bit tangled up, which is how it’s said that cross-leading can happen by accident, or the delay of responding with the correct foot might lead to a bit of ungainly movement. But most of us who ask at the “wrong” or “sub-optimal” time don’t usually wind up with our horses tripping all over themselves. Usually what we get is movement in the direction we want, just a bit later than we asked for it — which is actually usually ok with us. Few of us are engaged in activities that require our horse to move a particular foot at a particular time in order to, say, keep from falling into a hole in the ground. So a little-bit-late response is just fine.
Or at least, this is what I thought until I saw a Tom Dorrance video about learning to move a horse’s feet individually while riding it. In this particular video, Tom Dorrance worked with some riders and their horses and a pattern of tires laid on the ground, to help the humans figure out how to tell where the horse’s feet were at all times and respond to their horse and the tires. The point of the exercise was to develop feel. The demonstration of feel was that the rider could move a horse’s foot exactly and precisely at the time desired, to the place desired. So there was discussion in the video about feeling when the foot came up off the ground and giving the cue then, as is standard in this type of training. The result was astounding and unexpected. Remember, I was pretty jaundiced about the idea that a cue has to be tied so closely to the step cycle because I had a healthy respect for the complexity of Reaction Time.But every rider who engaged in this exercise, and learned how to feel where the feet were and to cue them at the “appropriate” times developed an incredibly light and responsive riding relationship to the horse s/he was on.
Jo uses some of Tom Dorrance’s methods when she gives riding instruction, and she’s the one who developed the closed-eyes on a bareback horse walking exercise I described in my previous post. Sure enough, she found — and I saw it was so — that when people learned to feel their horse’s feet coming up off the ground and paid attention to it enough to cue them “at the right time,” they got light and responsive movement from the horse. I also saw other trainers working on teaching the same “learn to feel when the foot comes up and cue it then” method in clinics in a variety of riding disciplines, and again I saw that when riders began to develop feel of their horse’s foot positions, a light and responsive relationship blossomed into existence. The uniformity of this response shocked and amazed me, especially given the huge variation in different trainers’ means of explaining and teaching the concept. But as long as the rider finally learned to feel the horse’s feet, the response materialized. So clearly something important was going on. But what?
Remember that we know Reaction Time is not solely or even primarily dependent on the timing of the stimulus or cue, but instead a complex phenomenon. So we have an “effect” here, which is a light and responsive horse, the “cause” of which cannot be, biologically, simply applying the stimulus at precisely the right time. To find the actual cause, we must consider what else all these experiences have in common. And what they have in common is this: the human learns to feelwhere the horse’s feet are all the time, and learns it so well that a cue can be offered at one particular point in the step cycle of one particular leg. Notice which words are in bold and italics. It’s about what the human learns.
A human who learns to feel where his or her horse’s feet are is thinking about, and even physically connected with, the horse. That human’s mind is no longer inside their own ego and their own intention — at least for a while. Instead, they are focusing on the horse. They are feeling the horse, quite literally. And Jo and I both suspect, very deeply, that what happens is that this change in focus and concentration allows the horse and rider to communicate simply and effectively in an entirely different way than the one we usually think of as “stimulus-response.” Which means, of course, that Reaction Time is irrelevant. What matters is simply the development of feel. And an exercise in which you learn where your horse’s feet are — with such precision that you can cue one at one particular place in the step cycle — will teach you that. But the exercise is not about cuing the horse. It’s about teaching the human to feel.
If you read Tom Dorrance, Bill Dorrance, or Ray Hunt you will see that this is pretty darned close to the way they talk about “feel”. And given that they coined the concept, at least in contemporary equine society, that’s enough for me to take it as a “good fit” to the only explanation I can see for what happens in these riding exercise — given that the neurobiological “reason” usually given (Reaction Time) is actually not valid. Did Tom Dorrance know this when he used a line of tires to teach a rider how to tell where his horse’s feet were, and how to move each foot at just the right time? Was he actually and simply trying to teach “feel” in a reliable way? Yes, I think he was.
In my walking blog, when I suggested ways a person could develop feel of their horse’s walking foot positions, my thought was that I’d give lip service to the “reason” this is usually said to be a good thing to do: Reaction Time, or “you can’t move a foot that’s not off the ground.” It was, after all, good enough for Tom Dorrance in that clinic I saw in a video. But my real thought was that anyone who did that exercise would develop proprioceptive awareness of the horse they were sitting on. And I do believe, after all I’ve seen and after the training work Jo has done to explore this, that if you do this exercise you will develop your sense of “feel” and, as a result, start to develop a relationship of light responsiveness between you and your horse.
If it works better for you to skip all the talk about “feel” and use the short-cut of thinking that you can’t move a foot that’s on the ground, that’s fine. You will still learn where your horse’s feet are and when to cue a foot to move. And that will get you light and responsive movement from your horse. But you’ll need to pretend you don’t know about the complexities of Reaction Time if you do that — which is ok by me. The complexities of the natural world get abridged in common understanding all the time, largely for the reason of generating actions that have simple and predictable outcomes that seem to be based in science. But, as is usually the case in all such situations, understanding the science a little better actually takes you to a place you might not expect to wind up.
So if your horse already knows how to “go” and all you have to do is relax the reins, you’re in good shape in lots of ways. But an exercise that teaches you to feel where your horse’s feet are, well enough that you can cue one foot at one particular place in the step cycle, will still give you something new that’s worth developing: feel. Horses already know how to walk, after all. It’s we humans who have to learn how to go along for the ride without messing them up.
This post was a featured blog entry on BarnMice, Nov. 22, 2012.