The Myth of the Heel-First Landing – Part 2

In the first article in this series, I started us down the path to understanding why the proper landing for a horse at the walk has to be flat-footed by describing the anatomical differences between humans and horses, and pointing out the limitations of our ability to perceive fairly significant differences in how the horse lands without using slow-motion video techniques. In this installment, we’ll continue on by examining the orientation of the foot as it travels through the flight arc and prepares to make ground contact.

Diagram of major tendons & ligaments of the lower limb

– from Equine Locomotion, by Willem Back & Hilary Clayton. I added the color.

As you’ll note in the diagram above, the common digital extensor tendon (6) and the deep digital flexor tendon (2) both have insertions (attachments) on the coffin bone – the most distant (distal) bone in the horse’s limb that resides inside the hoof capsule. Like humans, the horse’s foot is stabilized by these tendons as well as ligaments, and, shortly after leaving the ground, its orientation to the rest of the limb immediately above it (long & short pasterns) remains essentially constant throughout most of the flight arc. And like us, horses don’t typically think about where to place the next foot when they walk. So although the tendons stabilize the position of the foot, the attached muscles do not actively pull or relax to reorient it as it prepares to land. In fact, James Rooney’s Biomechanics of Lameness in Horses, which I’ve referenced a number of times before, describes Dr. Rooney’s somewhat counter-intuitive discovery that transecting the extensor and deep flexor tendons in the vicinity of the short pastern really had no significant effect on the orientation of the hoof as it landed. He concluded, therefore, that (like humans) foot placement in “normal” walking is not an active effort. Keep in mind, by the way, that we’re talking only about A/P orientation in the horse; the M/L orientation, as we saw in Part 1, is dictated by the construction of the joint surfaces themselves.

Dr. Rooney’s comments inspired to me look at the center of coffin joint rotation and the center of gravity of a properly-trimmed bare hoof, which is fairly easy to do with my CAD system by importing an image of a sagittally-cut hoof, visually noting the center of joint rotation, and tracing the hoof’s outline so the software can then computer the center of gravity. Check out the images below of one of Brian Hampson’s feral hooves from Australia/New Zealand. And as an interesting  aside,  also note the bone remodeling evident at the tip of the coffin bone –

Center of coffin joint rotation on a feral hoof

The center of coffin joint rotation (the circle’s center) lies on a line indicating the top of the hoof capsule

Center of gravity of the foot

After outlining the hoof capsule, the software calculates the foot’s center of gravity, which lies directly below the center of joint rotation

The interesting thing about this observation is that it was consistent for all of the hooves I looked at in cross-section, regardless of their toe angle. What this means is that the hoof balanced for a proper (flat) landing is quite literally “hung in balance” on the end of the limb! Thus, Dr. Rooney’s experience with the severed tendons makes sense; because the mass of a properly-trimmed bare hoof is, by design, evenly distributed in the A/P direction, the tendons are only providing a stabilizing (damping) force rather than actively controlling pastern/hoof alignment. This is similar in function to the shock absorbers on an automobile; they don’t determine the position of the car body, but they help the body more quickly return to a neutral position after being displaced.

The point I’m trying to make is that the horse isn’t consciously “aiming” his foot so it hits the ground a particular way based on what’s happening with the bottom of his hoof. Instead, the orientation of the bones of his foot and leg are the consequence of masses, muscle tensions, and joint construction – all under the influence of gravity and other external forces (we’ll get there in a moment!). As designed, the hoof is bare, quite short, and (as we saw above) fairly even in its mass distribution; therefore, it’s able to quickly accelerate and decelerate to a position of proper alignment with respect to the limb above it, aided by the “neutral” tensions in the tendons/muscles, as it travels through the flight arc. So given a properly-trimmed bare hoof, the net result is that the internal and external structures of the hoof will be in proper alignment as the hoof prepares to contact the ground. Check out the following photo, taken from a video of a client, in which the hoof position has been captured as close to the ground as possible just before making contact –

Properly-trimmed hoof about to make contact

Note, however, that I’ve been using the phrase “properly-trimmed bare hoof.” Within some particular range of variation from “proper,” the internal structures of the foot will still be in the correct orientation with respect to the leg and the ground, but the external structures will not be. You have only to look at the preceding photo to understand that if some part of the hoof is left long with respect to the other parts, the longer part will end up striking the ground first. So a long toe will yield a toe-first landing, and long heels will produce a heel-first landing. As you’ll learn in later articles when I discuss the origins of navicular disease, this is an extremely problematic situation because, by design, the coffin joint is not intended to be rotating at the instant of contact! But if a horse is contacting the ground any way other than flat, the coffin joint will, necessarily, be rotating when the hoof “slaps” the ground as the leg comes under load.

But the situation gets worse. Once the hoof exceeds this range of variation from “proper” (and I don’t pretend to know precisely what those limits are), the internal hoof/limb orientation, and therefore the internal and external hoof/ground orientation, will be further mitigated by three factors, particularly at the beginning and end of the flight arc. These are: 1) hoof length, 2) hoof weight (mass), and 3) hoof weight (mass) distribution.

Remember Newton’s First Law of Motion?

Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.

Because the coffin joint is fairly flexible, departures from this “intended design” of a short, properly-balanced bare hoof can have a significant effect on the motion of that joint during the flight arc, with profound consequences on the form of the landing. Probably nearly every horse owner has handled a horseshoe before, but relatively few have held a cadaver horse foot; I can assure you that most steel shoes are a significant percentage of the weight of a foot! And as we add weight to the end of the limb in the form of excess untrimmed growth and/or (more significantly) a shoe, we start changing the dynamics of the flight arc. In essence, the lower limb of the horse begins to act more and more like a double pendulum – the long & short pasterns forming one pendulum suspended from the fetlock joint, and the foot with whatever extra weight has been left or added suspended from the coffin joint. The resulting motion, and the mathematics describing it, can be quite complicated, but we can make some general statements about the effects of length, mass, and mass distribution:

  • As the length of the hoof increases, the time it takes to accelerate from the ground or decelerate for a landing increases. This is because the timing (period) of a pendulum is a function of its length; the longer the pendulum, the longer the period.
  • As the mass of the hoof increases, the time it takes to accelerate from the ground or decelerate for a landing increases. This is a consequence of both the center of mass shifting away from the coffin joint, effectively lengthening the pendulum and therefore the period; and the increase in the rotational force (moment of inertia) on the coffin joint, to which the period is also proportional.
  • As the mass distribution of the hoof moves away from being symmetrical, there will be increased unilateral A/P torsion forces on the joints, affecting both the effective length of the pendulum and the center of mass.

What does all that mean in practical terms? It means a foot designed to contact the ground in a particular orientation with respect to the limb above it may not be able to do so because whatever has been done (or not done) in the trimming/shoeing process has altered its dynamics. This is why a shod horse with obviously-long heels can have a reasonably-flat, or even toe-first, landing. The shoe, whose weight distribution has shifted the foot’s center of gravity toward the toe, is modifying the flight arc such that it’s keeping the toe from rotating properly right before contact. But when you remove the shoe, the horse immediately starts contacting the ground heel-first! Similarly, horses with any type of bar shoe, where the weight of the shoe is much more A/P symmetrical or even back-heavy (depending on the specific design – think about wedges!) tend to really come down heel-first, because the weight of the shoe is keeping the heel down plus the horse’s heels are already too long. And the added weight also increases stresses on joints and soft tissues, because that extra weight must be accelerated and decelerated with every step.

So we’ve now added yet another variable to the “landing picture.” Not only do horses not walk like humans, and not only can we not reliably observe landings at normal speed, but the landings we do observe can be a far cry from “proper” because of how the aforementioned factors can modify pastern/hoof alignment. But the problem is this: as Dr. Deb Bennett says, “What people come to see every day, they naturally come to consider ‘normal’…But what a culture or a nation comes to expect is usually common instead of normal.”

Please don’t fall into the trap of thinking that just because you observe a lot of heel-first landings, they’re supposed to land that way. Those landings are merely common, they’re not biologically normal! That’s simply not the way the equine limb is designed, and we’ll continue to provide further evidence to support this claim in the near future.

More to come…

Comments

  1. Sandy Judy says:

    I get what you’re saying and you’ve got a lot of good info here but…you’re leaving out an important factor that affects landing: pain; the presence or absence of it. The muscles DO affect the landing and the horse DOES consciously affect landing if he is in pain. And consciously, if he’s trained to a certain way of moving, say a Spanish walk. I think heel-first landings are often pointed to as healthy. The shod horses will often land heel-first. However, when the horse is in pain, it is diagnostic to helping locate the pain as caudal or distal based on the horse’s landing.

  2. Steve says:

    Thanks for pointing that out! Yes, I should’ve qualified my statements to include only sound horses, because, as you mention, a sore horse will absolutely consciously place his feet (just as a human will), and it is an excellent indicator of where pain is occurring.

    Steve

  3. Kim Truitt says:

    Steve,

    Another rip-roaringly good article! Regarding how the authentic natural trim positively allows the hoof to land as designed, I will think about the hoof landing as you’ve explained it here. After having read this, I had a lot of fun thinking of simple, sensible ways to explain this to clients!

    Thank you.

    • Steve says:

      Thank you, Kim. That’s the beauty of what we do; we allow the hoof to land as it’s designed to do, rather than attempt to create or cause a landing that conflicts with the laws of nature and common sense. To me, this is all simple and logical. When someone has to struggle to “explain” why what they’re promoting is “correct,” we should immediately be suspicious, and be prepared to run the other way!

      Steve

  4. Saskia says:

    Thank you for your article. I’m studying technical cybernetics and was recently rather interested in the mechanics of barefoot runners versus barefoot horses.
    I was wondering about why horses would land heel first, when humans didn’t. A good barefoot runner lands toe first because of the damping effekt and force distribution, so I started thinking why should horses land heel first in higher speed? It didn’t make sense to me. Just the thought of landing on my heel after a high jump, pains me.

    • Steve says:

      Yes, and it pains the horse as well!

      Steve

      • Caroline Trayes says:

        The equivalent would be the horse landing on his hocks and elbows. You cannot compare a horse heel landing with a human one.

        • Steve says:

          Hi Caroline –

          I certainly agree that human and horse landings cannot be compared! Sadly, many people do, and therefore believe that horses should land heel-first.

          Steve

  5. I share this on fb and this was a response from a barefoot trimmer i respect who studied with Andrew bowe – known in Australia as ‘the barefoot blacksmith’- I am interested in ur response to her (Tanya French’s)comment:

    I really can’t agree with this. If the horse has well developed frogs and digital cushion then it usually comes hand in hand with good proprioception and there fore the horse knows exactly where its hoof is and can adjust accordingly for landing. The heel first landing helps to build up that digital cushion and frog, they go hand in hand. The article makes no sense..

    • Steve says:

      Interesting, but I believe to be in error. What builds digital cushion and frog is not a heel-first impact; that’s simply not logical. What builds digital cushion and frog is the loading on the foot as the foot expands and the frog comes into contact with the ground. In human medicine, it’s now been acknowledged for quite some time that the strength of tissue is proportional to the demands placed on it as it’s growing. No physician tells you to heal your sprained ankle by sitting on the sofa and tapping your heel on the ground because no loading is taking place. But when a horse’s weight actually loads the foot, the tissue is being subjected to reasonable forces; something that’s not happening when the heels strike the ground a few milliseconds before being loaded, and the toe slaps the ground. Plus, as I’ll get to in future articles, a heel-first landing eventually leads to navicular bone and DFT damage, as Dr. Rooney pointed out.

      Steve

  6. Katherine Mills says:

    What’s your take on hoof boots regarding their effect on landings? Thank you!

    • Steve says:

      That’s a great question, for which I don’t yet have an answer! I had suspected that a particular boot was causing a toe-first landing because it’s quite toe-heavy, but when I shot slow-motion video of the same horse with and without boots, the landings appeared to be identical. Apparently the total mass and mass distribution don’t significantly impact the landing, although the added length may be a factor on some horse/boot combinations. More research!

      Steve

  7. Rafa says:

    Very good article and good exposure. I always thought I was landing heel, which affect strength over time, the distal interphalangeal joint and soft tissue hypertrophy.

  8. Ruth says:

    Correct landings is a subject that is absorbing me at present. I have been rehabbing a chronically foundered little pony mare with pedal bone rotation on all four. She had heels longer than her toes (3″!), layers of dead frog, severely contracted heels and no colateral grooves. She had plenty of thrush and still grows out evidence of numerous infections and absesses -when she grows at all; her hooves have been disconcertingly static. She had received a ‘trim’ a few weeks before I took her on in which her toes had been drastically axed which reportedly left her unable to walk. Her soles were convex (still are slightly in the front) but have now hardened enough to walk comfortably, I have carefully removed layers and layers of (dead?) laid over bars riddled with thrush and gingerly filed her heels every few days. What puzzles me is that even a 16th inch off the heel will make her almost stand on her toes. She is not the first foundered pony I have rehabbed but she is the first to consistently prefer to stand on the tip of her rotated pedal bones rather than her heels. The thrush is obviously creating it’s own pain biomechanics which improve dramatically when I am winning the thrush battle but I am at a loss as to how to make her consistently comfortable. The sole has never indicated it could tolerate any paring/shaping yet but has steadily compressed and hardened on it’s own. I am suspecting some serious age related pathology in the back of the hoof that makes her guard her heels so. The thrush is being treated almost constantly and reappears as soon as we have wet conditions. She is hand walked, or ridden when able, daily and lives on a strictly hay diet. I have a devil of a time coaxing mineral into her so offer a tiny handful of pellets sprinkled in mineral every other day. Her coat and overall condition are lovely now despite her age (20+yrs) and I have been working on her for just over a year. I have achieved a level landing on her backs and a near level landing on her fronts but the frog is nowhere near touching the ground and she cannot grow a healthy bar at all. This must be adversely affecting the health/bloodflow of the hoof. I have some pics from throughout the year and would appreciated any help you or someone you know may be able to offer. On barefoot trimming forums I am afraid the discussion is enthusiastic (dare I say fierce) but polarized and not particularly useful…

    • Steve says:

      These cases are always interesting. I, too, have never had a laminitic horse that preferred to stand on his/her toes beyond the first few days following a trim. Clearly there are some other issues at work here; perhaps DFT/navicular bone damage, abscesses from compacted sole, and/or significant bone remodeling to the point of bone/joint alignment being considerably different from the orientation of the hoof capsule. Radiographs would be interesting. It certainly could be a combination of factors as well. The only horse I’ve ever seen who preferred to stand on the toe of one foot had a bone spur on the back of his navicular bone. Interestingly, after good hoof care for a couple of years, he began to once again bear weight on the heels of that foot!

      Steve

  9. Harold says:

    Thinking about the calculation of the hoofs center of mass. Was the center of area actually used with the assumption that the hoofs mass was equally distributed within that area? Or was there some consideration to that?

    Great article! Thanks!

    Harold

    • Steve says:

      Thanks, Harold! You’re absolutely correct that the center of mass calculation was done as if the foot were of uniform density rather than made up of tissues of varying density. So it’s definitely a first-order approximation of the situation. But I suspect that if actual structures and densities were used, the result would be very close to the same.

      Steve

  10. Penny says:

    Has anyone here read Dr Glenn Ramsey work from Auckland University. NZ.Research paper on The influence of loading conditions on equine hoof capsule deflections and stored energy assessed by finite element analysis. Available on line at http://www.sciencedirect.com

Leave a Reply

Your email address will not be published. Required fields are marked *