Force, Stress, and the Hoof, Part 2

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.

Roman “hipposandal” boot-like hoof covering made of metal, about the first century CE.  From Wikipedia.

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 up into 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