Hello and we're back again in this twelve-part series about the equine foot. New studies using new technologies are really shaking up the field of equine podiatry, illuminating many of the flaws of conventional wisdom in unexpected ways.
In Parts I-III we learned a bit about the foot's evolution, general biology, and physical structure, and in this Part IV we're going to discuss how it all works together as a mechanical whole. So let's get started!…
Foot Mechanics
It’s startling how much we really don’t know about the mechanics of the equine foot, especially when considering the long period of equine domestication, the pivotal role the horse has played in human civilization, and the importance of a sound foot for his functionality. Throughout history, various interpretations have persisted, some based on traditional thought, some based on practical experience, some based on biomechanical research and some based on field study. Essentially, we still can’t pin down exactly how it works and why, and only relatively recently has modern science turned a serious eye to the equine foot. This probably is why equine podiatry lacks an operating paradigm, a steady set of rules that work for all horses all the time. Skilled farriery can be considered more of an art than a science for that reason, and, indeed, sometimes it seems they work magic! Plus, when we consider therapeutic podiatry, the variables explode into countless differing hypotheses, most with sharply contrasting assertions, and all because we've taken so much about the equine foot for granted all these long years.
It’s useful to remember that multiple concepts exist about how the foot works, with more developing all the time as new data develops. Predictably, all these ideas have spawned just as many contradictory methods for dressing a foot. Because each method does seem to have a level of success with some horses some of the time, each proponent believes his way is the correct way, which causes heated debate. But we are coming closer to pinning down its structures and mechanics better and better as new technologies illuminate more every year. (For a foray into these issues, please refer to the Recommended Resources at the end of this series.)
Fortunately, as artists we’re in a better position to look in from outside the fishbowl. Being free agents, we can weight information with greater freedom and can live by this important fact—the proverbial fat mare hasn’t whinnied yet. Data is still incoming, but much of it seems to inadequately accounts for horsemanship. There’s still a dearth of comprehensive, long-term data, too, since modern science has only relatively recently started to rethink equine podiatry from the ground up. Hopefully, the near future will shed some definitive light on how the equine foot works and what conditions are genuinely ideal for a domestic foot so that a new standard operating paradigm can form.
But the one thing this new data is suggesting most consistently is that pretty much everything we thought about the equine foot was fundamentally wrong or incomplete, and that should give us pause, as horsepeople and as artists. As artists, we can either validate aspects about the equine that promote his well-being or inadvertently endorse things that cause him harm. Unless we know what we're doing then—unless we're currently informed—we risk the latter, and who wants that? Until a firmer grasp on foot function arises, however, our discussion can only present some general concepts that are currently regarded within the bubble of “correct."
So let’s start with three basic tenants strongly implied by current research:
- The equine foot is a sum of its parts, i.e. no feature within it is any less important than another, and, what's more, each part must be optimal to maintain optimal physiology. Nature created an astonishingly efficient mechanism that contains absolutely nothing superfluous; every element is there for an essential reason. This idea is in sharp contrast to traditional thought, which ignored or dismissed key features of the foot, such as the sole and bars, to focus almost entirely on a couple, such as the frog or hoof angles.
- Inseparably tied to the body, the equine foot adapts to the lifestyle of the animal and continuously changes in structure and growth to find equilibrium with the animal’s physiology and lifestyle. This means that each set of feet is unique and can be considered as distinctive as a fingerprint. This, again, is in contrast to traditional thought, which tended to ignore the overall management of the animal and focused instead on “fixing the foot" while, at the same time, perpetuating the idea that "one size fits all" horses.
- The basement membrane and lamellae are essential connective and functional elements in the foot, clearly illustrated when they fail. Nonetheless, it now appears that the mass of the animal doesn’t pass completely through the limb to the wall, suggesting that this burden is shared by multiple mechanisms within the foot working together in a specific relationship, especially the palmar foot. This is contrary to traditional thought that asserted the coffin bone is suspended and supported within the hoof capsule primarily by the lamellae, and that it transferred force to the hoof wall, which was the primary bearer of weight.
Now we can begin to understand why equine foot mechanics have been so elusive to identify, because with all these variables, at any given time, in any given situation, there’s a lot to consider!
Now one may wonder why a realistic equine artist needs to understand the structure and function of the equine foot. Aside from the ethical issues, artists should be familiar with how the foot is structured and how it changes during motion, as these are details of direct relevance to artwork. Indeed, artists should not only know the structure of the foot to create thoughtful sculptures, but also know when those structures change to remain true to life. For example, to sculpt a “loaded” foot as a “stance” foot is a biomechanical error on the artist’s part by not factoring in physics and foot mechanics into the creative equation. But it’s these details, born out of a deeper understanding of the animal’s biology, that can take artwork to new heights of authority and realism.
General Concepts
While all equine feet have the same structures and basic design, they exhibit individual variation even in tissue quality, dependent mostly on environmental factors. Nevertheless, the foot acts as a whole system, much like an expertly crafted complex clock, so no part or system can be dismissed. The lower leg lacks musculature, thus the soft tissues intended for motion consist mostly of tendons, ligaments and cartilages, or "cables," with muscle belly “servos” located high on the limb, above the knee and hock. Within the foot itself, the coffin bone is the basis of the front half of the foot and the lateral cartilages are the basis of the rear half, and they act together. The structures in the rear half in the foot are designed for traction, support, and energy management, whereas the structures in the front half are designed for breakover and protection.
The foot is highly adaptable, constantly reforming its structure, chemistry and function to harmonize the animal with the environment (Ramey, 2006). Under natural conditions, the foot comes into equilibrium between both growth, the environment and the horse’s physiology to become self-maintaining. On the contrary, domestic feet don’t have the luxury of consistent footing or conditions, or the ability to roam for miles every day, which may impede the foot’s adaptive abilities (Ramey, 2006). This means that we are ultimately responsible for developing a foot that can withstand the demands we place on it, though it may not always be so straightforward. For example, if the horse is exercised enough on the same footing, the foot will adapt itself to that kind of footing and motion, even if that horse spends considerable leisure time on different footing (Ramey). The foot also tends to adapt to the footing that presents the most “challenge”, or stimulation, which may present a problem between performance and stalling. In the end, it seems that adaptability is more a function of consistent force rather than time.
Regular exercise appears to play a pivotal role in foot mechanics insofar as the more regular the exercise, the more the foot changes into what nature intended; the more the foot is used, the sounder it becomes (Bowker, 2003). For example, dissections demonstrate that horses under the age of 5 years have internal foot structures that are rather similar, with thin, small lateral cartilages, underdeveloped venous plexuses and digital cushions made mostly of fat and collagen. As the animal is exercised adequately over time, particularly with heel-first landings, that consistent vigorous stimulation (especially on the sole, bars and frog) causes the lateral cartilages to thicken and enlarge, the digital cushions to harden into fibro-cartilage, and the vascular plexuses to increase in amount and extent (Bowker, 2003). Despite all the uncertainty about the equine foot, one thing is clear—nature never designed the horse for a sedentary lifestyle.
Foals
Foals have their own considerations since their hooves cannot be regarded like an adult thanks to their ever-changing size, shape, and characteristics. For example, the narrow chests and long wobbly legs of a foal often produce a base-wide stance (sometimes paired with lateral rotation of the limb), which is perfectly normal in a youngster up to 3-5 months old by providing more stability until his little muscles can develop. Nevertheless, this stance often wears his hooves down so that the medial side is slightly shorter than the lateral side.
The size and shape of the foal’s foot is also quite different from an adult foot. For instance, a newborn’s foot is shaped like an isosceles triangle, and it’s thought this shape helps with the birthing process. Also, the foal’s hoof capsule is about the same diameter as the pastern (when seen from the front), forming a neat, small hoof at the end of the long leg. A foal’s foot also lacks the conical shape of an adult, being more cylindrical or even narrower at the base than at the coronet, and only around 6 months old does it start to attain the conical shape and increased diameter of an adult foot.
The first days of a foal’s new life also present some particular considerations for the artist. For starters, a newborn’s distal foot is covered by a soft, fleshy cap of unpigmented horn, presumably to protect the mare’s uterus and birth canal, which separates from the cornified horn along a break-line when the foal stands (Pollitt, 2000). Once this cap has rubbed off, the distal surface of a newborn’s hoof, both frog and wall rim, are still covered with “feathery” fleshy extensions, which contain proprioceptors that are thought to help the foal learn to use his new legs and navigate the terrain in those first few days. The hoof capsule itself is also comparatively soft and so the frog bears most of the burden of weight-bearing, being quite large in a youngster, taking up the largest portion of the foot’s distal surface. Indeed, a foal’s wall will usually wear down to the plane of the sole in the first few days to place the large frog directly on the ground for it to bear the weight and begin to cornify. Moreover, the coffin bones in newborns are identical, and it’s only through wear and time that they begin to develop a “front” and “hind” coffin shape, often as early as 2 weeks old (though the hoof capsule can adopt these qualities by as early as 2 days old).
Additionally, a perioplic membrane encases a newborn’s entire hoof, but with wear, this membrane wears away and dries out, sometimes creating a kind of tourniquet around his hoof. This creates a slight hollow, or sulcus, below the coronet or mid-hoof which grows out at about 3 months, making it an important detail for sculpture.
It appears that ample exercise on firm ground plays a fundamental role in the size and shape of the coffin bones and in the overall long-term development and health of the foot, meaning that an artist should be aware of how management can impact the characteristics of a foal’s foot. Specifically, most domesticated management of foals entails their confinement on soft bedding, particularly for those born in the winter months, which can adversely affect their new feet. For example, confined foals don’t wear away the feathery extensions as quickly, nor is the wall allowed to wear down to the sole. Consequently, the frog isn’t placed directly onto the ground to bear weight and, failing to develop, often becomes atrophied, which can sometimes cause a foal’s foot to contract, compressing the soft and developing coffin bone inside (Ovnicek, 2002). In fact, current study is hypothesizing that foal management in the first few months, a critical “window of opportunity," may bear a significant influence on the long-term quality and function of his feet, perhaps for his entire life. So artists, beware!
Static vs Dynamic Function
While standing, it’s estimated that about 60% of the horse’s weight is supported by the forehand; however, this changes when the animal moves (Clayton, 2001). There’s a distinct difference between static and dynamic balance when considering foot function (Ramey, 2006). In the end, this may make the effects of impact (which includes the footing) more important than those of stance in the working horse, meaning that a foot adapted to motion can look quite different from a foot adapted to inactivity. What’s more, the foot at stance may look very different from that same foot during loading.
For starters, the moment the horse moves, much of the energy and weight of motion is shifted to the hindquarter, the center of propulsion and momentum (Ramey and Clayton, 2003). In other words, the horse doesn’t drag himself along by his forelegs, but uses them as pole-vaults for his ever-launching hindlegs (and these dynamics either become disrupted or enhanced depending on horsemanship). Even the hindquarter musculature of the animal bears witness to this biomechanical dynamic, being arranged and designed far more massive and powerful than the forequarter for the express purpose of forward motion.
Furthermore, the hoof changes shape in rather complex ways when loaded. For instance, studies show the foot probably was intended for an arched design, from back to front, like the arch of our foot, partly created by the external arch of the sole’s vault paired with the mechanisms inside the foot that work to depress and expand during impact. But it's also a function of the inherent arched shape of the internal aspect of the foot, as described in the Internal Arch Apparatus Theory™ (discussed shortly). Even so, this arch creates a depressible bowl that allows the foot to flatten and distort, and then snap back into shape, much like a plunger, which may be how the foot is supposed to function. Indeed, the concavity of the solar surface flattens and stretches when loaded about 1/4 inch, and the hoof capsule expands, becoming markedly different from its stance characteristics (Ramey, 2006). Even a frog that may appear to have only minimal contact with the ground at stance actually may be adapted so that it’s appropriately loaded at impact (Ramey). Again, dynamic function trumps static stance. And expansion not only happens at the posterior foot, but also around the entire perimeter of the hoof capsule in varying degrees.
In addition, the foot doesn’t just change on a horizontal plane, but the internal structures experience impact on the vertical plane through the heels, frog and bars. Plus, the hoof undergoes diagonal shear and distortion as each side of the heel vertically receives impact relative to the other side. In fact, if we manipulate a barefoot healthy foot, we can gently squeeze the heel bulbs together and vertically shift each heel independently with relative ease, allowing the foot to adapt to irregular ground surfaces and to accommodate the angles encountered in situations such as turns in order to minimize shearing forces on the limb. Interestingly, it’s been theorized that this wall distortion may play an important role in the development of the internal foot, by providing stimulation in all directions (Bowker, Strasser).
Some field studies of feral and wild feet have noted that while almost all the sole and frog contact the ground for weight-bearing in unloaded stance, very little of the outer wall does (Bowker, Ramey). Some have even observed that ground contact occur mostly at four points (Jackson, Duckett, Strasser, Ovnicek), for "Four-Point" structure. The two front points are located on either side of the toe and referred to as “front pillars.” The two back points are associated with the two points on the heel that impact the ground first and are referred to as “back pillars." Interestingly, this four-point support and wear pattern can often be manifested on the sole as well, as slightly raised buttons of calloused tissue just to the inside of the four points on the wall, particularly at the toe. In addition, the toe tends to be worn back into a blunt shape between the front pillars, creating a somewhat square-shaped hoof. However, not all feral or wild hooves exhibit this four-point wear (such as those living in soft, grassy habitats), so environmental conditions may be a deciding factor in whether the foot assumes this structure or not.
Nevertheless, what’s interesting is that when the arch of such a foot is loaded, it’s believed the entire perimeter of the foot is pressed onto the surface as the hoof capsule expands and distorts, which is thought to be the mechanism that produces the wear pattern (because it’s believed the quarters are the weakest areas of the wall). However, this interpretation is debatable since it can be easily argued that if the four-points were active participants in hoof dynamics, they should be worn down level with the rest of the rim (Bowker, Ramey). Suffice to say, it’ll be interesting to see what research reveals over the years on this subject, especially as to how it applies to domestic horses.
Feral and wild hooves also seem to show that the outer, pigmented layer of the wall, the one containing the highest tubule content, may not actually be the dominant weight-bearing structure (Jackson, Ramey). Counter-intuitively, such hooves wear down to walk on the more flexible waterline, with breakover finishing there, as well, as seen on a naturally worn “mustang roll" (rounded, beveled toe and hoof edge).
Generally speaking, it seems than the unloaded foot bears weight on the posterior of the foot (frog and bars), the toe callous around the perimeter of the white line, and on the waterline. In contrast, however, a loaded foot bears weight on the totality of the distal surface of the foot and all the internal mechanisms within it, physically changing its appearance, and creating an important detail for sculpture. Indeed, it would be an error to sculpt all four feet as stance feet if our sculpture depicts motion!
All the same, some studies are suggesting that an insufficient amount of foot stimulation has resulted in an endemic underdevelopment of the foot mechanisms intended for circulation and energy management, which may be the core reason why foot problems in the domestic horse are the norm rather than the exception (Bowker, 2003). The culprit seems to be conventional management practices of domestic horses, such as inadequate exercise with long periods of confinement, improper footfall (through trimming or shoeing), unhealthy foot structure (breeding for type rather than function), inconsistent footing and improper diet (Bowker, Ramey). As a result, it's thought that most domestic horses have insufficient foot circulation, lateral cartilages, digital cushions, sole quality and lamellae function, ultimately resulting in a mature horse essentially having the foot of a yearling, a structure woefully ill-equipped to deal with the weight and forces of a 1000+ pound adult.
Loading and Breakover
Without a doubt, loading and breakover (i.e. alternating movement) is the basis of the equine foot. Exactly how it all works is still a mystery; conventional foot management has proven to be incomplete. Modern technology has allowed science to peer into the foot with greater clarity, and field studies of feral and wild feet have also provided additional insights, but we have a lot more to learn. So until we develop a more thorough understanding, below is what generally is thought to occur with loading and breakover:
- When landing, the heel and frog receive impact first, absorbing high frequency oscillations and energy through compression. As the heels expand, they help to spread apart the lateral cartilages and the hoof capsule, initiating the expansion cycle of the foot.
- As the foot continues to land and increases ground contact, the buttress of the heel and the bars begin to receive impact and act as vertical supports for the internal foot as well as absorbing energies and continuing to transfer them to the flexible lateral cartilages, which continue to spread apart. The digital cushion begins to be initiated as a shock absorber, as well as perhaps a shield for the vulnerable coffin joint and navicular bone area. The wall broadens its expansion by about 1/4 inch while also offering vertical support as the pastern starts to descend in loading.
- The sole begins to depress fully on the ground, or “bottom out," to support the coffin bone. As the pastern descends, it further helps to push apart the lateral cartilages and hoof capsule as well as to tighten the coronet’s noose around the foot. As the foot mechanisms continue to receive, absorb, and redistribute energy away from the coffin bone, the sling and structure of the digital cushion and lateral cartilages help to support the descending bony column, too. While the wall continues to share the energy of impact, even microscopically as the tubules compress, it’s believed most of the initial impact energies and frequencies are dissipated by this time, by as much as 60%-80% in some studies, being transformed through these distortions.
- The flexible suspension of the coffin bone by the lamellae allows it to descend into the hoof capsule when fully loaded, while the digital cushion, lateral cartilages and sole (particularly the toe callus) offer further support for the loaded coffin bone. At this time, it’s thought that the hoof capsule has expanded by about 1/4 inch to 1/2 inch and the vascular systems within the foot are fully pooled (hemodynamic theory), and perhaps facilitated by the coronary band (Suspension Theory of Hoof Dynamics™), and are fully initiated as a cushion gel pad and series of heat diffusers that buffer and transform the remaining impact energies and frequencies. With all these systems, it’s thought that relatively little energy is directed onto vulnerable foot structures, the bony column, or the rest of the body.
- Breakover begins and the pastern starts to ascend, by which time all the impact energy has been dissipated, absorbed or transformed into propulsion energies. As the foot lifts off, it contracts back into its unloaded shape, squeezing the internal structures and releasing the heel, sole, coronet and lateral cartilages. This raises internal fluid pressure to pump blood out of the foot (which is further assisted by the flexion of the foot and leg itself) and readying it for the next footfall.
Also involved may be the Internal Arch Apparatus Theory™ (La Pierre, 2006), an idea that considers why the coffin bone still holds its orientation to the coffin joint and the 2nd phalanx even if the hoof capsule is removed. Its postulates a the spring mechanism created by the internal arch of a properly balanced foot which stores and converts energy into forward momentum (this idea supersedes absorption and dissipation of impact energy in preference for a transformation of that energy). Specifically, at impact, the heel and bars make initial contact with the ground, with the first high frequency energies mediated by the inner wall so that the dermal layers are protected. At impact, both heels are fully in ground contact and the wall begins to store incoming energy through distortion and compression, which initiates pastern motion. As the leg loads, so does the internal arch, and the wall continues to distort to store more energy. Meanwhile, the dermal layers begin to meet the resistance of the wall, and pressure within the foot builds. Correspondingly, the pastern descends into the hoof capsule, blood flow diminishes and then stops entirely at full loading, sealing off the internal foot in which the pressurized blood, fluids and loaded arch have stored vast amounts of energy, yet are still in perfect equilibrium to the forces that generated them (see the Suspension Theory of Hoof Dynamics™, discussed later). While some of this energy is transformed into heat and dissipated through the foot’s vascular systems, much of it remains to be released through the energy of depressurized blood at the first moment of breakover, when the pastern begins to rise out of the hoof capsule. Simultaneously, the impact energy that was stored in the internal arch is transformed into momentum, with any excess dissipated through the rapid surge of blood out of the depressurized hoof capsule. So in a way, this concept regards the hoof capsule as a kind of pressure pot that helps the internal foot transform impact energy directly into forward momentum.
Now as for breakover, this is an equally important part of foot dynamics. Breakover should be early, easy, fast and fluid, with good limb extension resulting in long strides and heel-first landings (discussed later). Traditionally, there are many interpretations of “breakover," the dominant concept being that the point of breakover for the front foot is the middle of the front rim of the wall and for the hind foot, just to the inside of its point. However, new information is suggesting this has been an inaccurate assessment all along. Using a far more dynamic interpretation of breakover, it’s becoming clear that the true point of breakover is the area on the foot still on the ground when the heel first lifts off, placing it behind the white line and around the toe’s entire solar surface, especially at the toe callus underneath the coffin bone (Ramey, 2005). And because feet don’t always leave the ground flat and forward, but usually at angles due to balance adjustments, uneven terrain, turning and maneuvering, the point of breakover isn’t a specific spot, but an area, meaning that breakover is literally anywhere along the front perimeter of the solar surface.
Breakover is also linked to heel-first landing (discussed later), as biomechanical studies are confirming the long lever of an overly long toe forces the horse to lift his legs off the ground much faster and more vertically, which ultimately results in short strides and toe-first landing (Bowker 2003, Ramey, 2005, 2006, Clayton, 2001). In contrast, when breakover is brought back onto the sole, the toe ceases to act as a long lever, allowing the horse to keep his foot on the ground longer to produce longer, lower strides with breakover becoming easy and quick, and automatically producing heel-first landing to boot. All of this is in sharp contrast to the traditionally held philosophy of long toe-low heel dynamics and trimming, which typically is still seen on racehorses, hunters, and western pleasure horses.
All the same, the mechanical effects of natural breakover can clearly be seen on feral or wild hooves with their very short hoof capsules (with toes about 3 inches long), the “mustang roll” (the bevel along the outer rim of the wall down to the waterline), the thick sole callous along the perimeter of the white line and the long, coordinated strides (Ramey, Jackson). And some feral and wild feet even have a “backed up toe” with the four-point wear pattern, shortening breakover even more (Ovnicek, 1997).
Even so, many domestic feet tend to experience a very different type of breakover due to their pathological construction. Here the wall is forced to become the primary weight bearer, or "peripherally loaded" (either with trim or shoe), with the breakover point ahead of the bony column (Bowker, Ramey 2006, 2005). This not only appears to change the foot internally, but its mechanics as well. Indeed, most domestic feet have long toes, which changes the point of breakover too far forwards. Sometimes hooves are made even longer by trimming for the long toe-low heel structure (Clayton, 2001). The LT-LH trim, being unnatural, tends to produce underrun heels, overly long hoof capsules, and dishes or flares, while creating an unnaturally long lever at the front of the foot that delays breakover and pulls on the toe.
This has two effects: one is contracted heels because of the repeated pulling on the toe that momentarily narrows the hoof capsule each time the toe is pulled at breakover. The other is an altered flight pattern by forcing the limb to assume choppy strides that tend to hit toe-first, rather than heel-first. Ultimately, this imposes unnatural forces to the foot, limb and body, resulting in a host of problems, such as injured ligaments and tendons, contracted heels, trauma to the navicular apparatus, tearing and bruising around the toe, the tearing of the wall’s corium at the toe, and exhausting, uncoordinated motion, stumbling and interference. And when a long toe is combined with underrun low heels (discussed later), the problems compound even more. We'll talk more about the LT-LH trim later.
Blood Pump
Because a healthy foot alternately expands on impact and contracts upon release, fluids in the vascular and lymph systems are believed to be alternately sucked in and squeezed out, to perhaps aid the heart and lymph systems against the forces of gravity in a long, vertically oriented limb (indeed, the horse’s heart is only about .5%-.6% of his total mass, which is the same ratio as a mouse or average man) (Strasser, Bowker, La Pierre).
There’s still debate as to how the vascular system actually works as a blood pump (and energy manager through pooling blood). One hypothesis asserts that the laminar corium acts as a “sponge” through stretching from the pull of the expanding hoof capsule, sucking in blood at impact and then squeezing it out when the hoof snaps back into shape (Strasser). This idea makes sense when considering the structures at the back of the foot, the frog and bars in particular. However, another idea suggests the opposite, that as the hoof capsule expands under impact, the thick corium under the coffin bone compresses and pushes blood to the posterior of the foot, to the microvessels around the navicular bone. But rather than stretching the laminar corium, the hoof expansion at weight-bearing actually compresses it, with the solar corium right below the coffin bone’s vault appearing to be thickened and the solar corium beneath the distal rim of the coffin bone being compressed (Ramey, Bowker 2007). In this way, the primary function of the lamellae is not as a “blood sponge," but to simply grow hoof wall and sole (Bowker).
It’s interesting to consider both of these ideas in relation to the structure of the coffin bone, with the depressed rim acting as a dam and the coffin’s open back acting as a possible “shunt” for blood directly to the microvessels at the back of the foot. Likewise, if the wall is unable to expand, it’s conceivable that the descent of the coffin bone would stretch and pull the lamellae downwards, causing persistent trauma to the lamellae, which seems widespread in the domestic shod population. However, what if coffin bone descent is part of foot mechanics (Stashak,1995, La Pierre, 2001)? Add into this mix the hemodynamic theory and the Suspension Theory of Hoof Dynamics™ (discussed later) and things really get interesting. Perhaps all these concept are correct and working in unison.
Regardless, as the foot loads, the hoof capsule, lateral cartilages, coronary band and the vasculature expand, causing arterial blood to be sucked inward throughout the foot (diastolic phase) (Pietra et al., 2004, Hoffman, et al., 2001). As the foot lifts, these features snap back into their resting form, squeezing out the venous blood from the foot and into the coronary plexus (systolic phase) (Pietra et al., 2004, Hoffman, et al., 2001) and other venous systems in the foot, which have strategic back valves to prevent the outgoing blood from returning to the foot (Pollitt, 1992). As the horse takes another step, this blood is pushed further up the leg, and so it goes back up the limb. This effect appears only to be possible with repeated alternating steps (i.e. movement), and maximized with proper foot structure. A possible confirmation of this concept are the relationships of the fluid management systems themselves, the myriad and convenient routes for blood and fluid intake and drainage, the absence of back-flow valves in most of the foot’s veins except for a strategic few, and the rich double-layer of venous plexuses around the lateral cartilages and coronary cushion that all work together to uptake, pool, equalize and drain fluids quickly inside the foot in synch with motion.
Footing also may play an important role, since research has found that blood and fluid flow to and inside the foot (perfusion) is improved on a depressible surface, such as pasture, dirt, rubber, shavings, foam, sand, gravel, etc. In contrast, perfusion is significantly impaired on non-deformable surfaces, such as cement, brick, metal, asphalt, tarmac or wood, by as much as 2/3 blood volume, according to some statistics (Welz, 2007, Bowker). Current thought asserts also that the entirety of the foot’s mechanisms are most stimulated when its total surface area is stimulated by deformable surfaces, whereas non-deformable surfaces bypass most of the foot’s mechanisms by only stimulating a comparatively small surface area, predominantly through the frog and peripheral loading of the wall (Ramey, Bowker). This may explain why barefoot horses (both wild, feral, and domestic) that spend a considerable amount of their movement time on hard, flat terrain such as packed clay or asphalt, develop soles and frogs that grow to “reach down” to the flat surface (Ramey, 2006). This is another important detail for sculpture if we have a narrative in mind.
Sum Up
There's a lot to think about, isn't there? Knowing how a foot loads and releases, deforms and snaps back into shape is an important detail for our clay. It means we have to pay attention to the imagined physics and lifestyle of our composition when we make our creative choices in regards to the foot.
In Part V, we're going to continue with mechanics, then follow it up in Part VI with some of the ongoing debates about the equine foot. Meanwhile, take it one step at a time…
"You are always the student in a one-person art school. You are also the teacher of that class." ~ Irwin Greenberg