| Your One-Stop Farrier and Hoofcare Portal - Hoof Mechanics and Physics Fri, 24 Nov 2017 05:35:20 +0000 Joomla! - Open Source Content Management en-gb Addendum and Correction to The Basic Mechanics of the Foot and the Horseshoe

This addendum should be read in connection with the section: The Egg Bar and Support in the article The Basic Mechanics of the Foot and the Horseshoe.

Henry Heymering, always alert and thoughtful, called me to task for not giving the egg bar the respect and value that it deserves. As the result of Henry's doubts and a well-done demonstration, I have reevaluated the mechanics of the egg bar situation and wish to add to and correct my earlier statements.

Henry, as well as others, have indicated that the egg bar shoe is of value in the treatment/rehabilitation of legs with tendon damage, i.e, bow, suspensory strains, deep flexor check ligament strains. My earlier evaluation suggested to me that could not be the case, and I herewith wish to correct that incorrect evaluation.

Once again I must refer to the figure, Fig. A, which indicates the basic moment equilibrium equations and situation for the coffin and fetlock joints. DF, the tension in the deep flexor tendon operates around the moment arm b at the coffin joint while T represents the tension in the suspensory, deep flexor and superficial flexor operating around the moment arm c at the fetlock joint.


Pain arising from damaged tendons can be alleviated to some extent by decreasing the tension in those tendons. In this case either DF or T must decrease. The horse can do this by standing back, Fig. B. By doing so the line of action of F, the ground reaction force, is moved from 1 to 1', Fig.A. This movement clearly decreases the length of the moment arm, a and the moment arm, 1. That reduces the value of Fa and Fl, allowing a decrease in the tension of the deep flexor in the one case and all the tendons, T, in the case of the fetlock. This decrease in tension is, of course, mediated by shortening of the tendons.

The same effect can be achieved, that is, reduction tension in the tendons and suspensory by applying the egg bar shoe. When that shoe is applied, as indicated by the dotted line in Fig. A., the line of action of F will be moved closer to the coffin and fetlock joints just as when the horse itself moves the leg to the standing back position. And, again, this allows shortening of the tendons which decreases tension and, so, pain.

The four photographs, labeled Photos 1 through 4, are of the demonstration Henry Heymering provided which enforced my rethinking. In the Photo 1 the horse is standing back on the right fore with a regular shoe. Photo 2 is immediately after removal of the shoe. In Photo 3 an egg bar shoe has just been applied without any trimming of the foot. In Photo 4, 5 minutes after application of the bar shoe, the horse has moved the foot into normal relationship with the other foot. The cause for the standing back to begin with in this horse is moot. Henry feels it was suspensory pain; I did not examine this horse and do not know.

Photo 1
Photo 2
Photo 3
Photo 4
]]> (James Rooney, D.V.M.) Hoof Mechanics and Physics Thu, 30 Jul 2009 07:20:45 +0000
The Basic Mechanics of the Foot and the Horseshoe

The purpose of this paper is to discuss the basic mechanics of the horse’s foot and the horseshoe. A single, pilule major principle will be developed: The effect of the horseshoe on the movement or stance of the horse is completely determined at the moment of impact of the foot (and shoe) with the surface and at the moment of lift-off of the foot from the surface. When the foot is not moving relative to the surface, the horseshoe has no effect.

I shall not explicitly discuss various shoeing systems such as four point, Burgey’s, Duckett’s dot, etc. While these may be worthy of discussion, they do not directly relate to the present analysis. Some of these shoeing systems are said to be derived from study of the natural wearing of the unshod hoof. The nature of such wearing has been addressed by Rooney (1999).

In order to discuss the mechanics of the foot in definite, clear-cut terms we must define the forces which are acting on the foot. In mechanics there are two such forces to be defined: linear forces and moments. In what follows, a simplified and two-dimensional analysis is used. The real system is more complex and three dimensional, but for practical purposes this analysis suffices.

I realize from long experience that neither farriers nor veterinarians, on average, care to concern themselves with “mathematical stuff.” In order to have a clear understanding of how the foot works, however, that mathematical “stuff” is necessary. What follows is couched in mathematical terms but with immediate relationship to the physical situation. It is not difficult if the reader is willing to overcome fear and loathing and reread as necessary in order to understand.

The experienced farrier may say that all this is unnecessary, that experienced “rack of eye” is what is needed to properly shoe a horse. That is often the case. One hears and reads frequently, however, that if one thing doesn’t work, try something else, perhaps the exact opposite! That alone says that rack of eye is often guesswork without theoretical foundation. If the art of farriery is to become more science and less art, that theoretical basis must be understood and used. And if that is to happen, at least the simple mechanics and mathematics given here need to be thoroughly understood.

Linear Forces

Figure 1: The linear forces acting
on the hoof of the standing horse.

The linear forces acting on the foot of the standing horse are given in the following equations, and figure 1:

  • F-W=0
  • R-R=0
  • H-H=O

In the standing horse there must be equilibrium between the downward force of body weight and the upward force of the resistance of the surface upon which the horse is standing.

The force, H, is a frictional force between the hoof and the surface which prevents the foot from sliding forward under the influence of the force, -H. That frictional force is:

That is, the amount of friction is a function of the vertical force, F, and the coefficient of friction, µ. The latter is an empirical measure of the roughness of the surface. Empirical means that µ is determined by experiment and cannot be calculated.

F is an upwardly directed force, the resistance of the surface, and -W the downwardly directed force of the body weight on the leg (downward forces are minus by convention). For equilibrium: F-W=0. Also, for equilibrium, H-H=0. The result of adding the vectors F and H and -W and -H are, respectively: R and -R. These latter are called resultant forces or simply resultants and are the actual physical forces going up and coming down through the digit.

The vertical force, F, is spread over the bearing surface of the hoof wall on a firm surface. It may also be spread over the frog and sole if the surface is soft and/or yielding. Stress, S, in mechanics is force per unit area; that is the amount of force experienced by some unit of the bearing surface such as pounds per square inch, kilograms per square centimeter, etc. Please note carefully that this is an exactly defined mechanical meaning of the word stress.

As noted the force F is exerted all over the bearing surfaces, those surfaces of the hoof in contact with the ground. In mechanics one considers that “spread-out” force to be concentrated at a single point called the center of pressure. That is done in order to simplify the calculations. It does not mean that “all” the force is concentrated at that point; it means that one can account for the mechanics of the foot if one considers that the dispersed forces are all concentrated at that one point.

Figure 2: The center of pressure
balancing the foot.

If a triangular support is placed precisely at the center of pressure, Figure 2, the horse could stand naturally and in balance. (I grant that is more easily said than done!)

No matter what type of shoe one puts on the horse’s foot, the stress S - force per unit area - on the bearing edge of the hoof wall remains constant so long as the bearing edge is the only part of the hoof in contact with the surface. Put another way, there is no way in which one can reduce the stress S acting on the bearing edge of the hoof wall by changes or modifications of shoe type unless one includes not only the bearing edge of the hoof wall but the frog and/or sole or parts thereof. One cannot change the value, the amount, of F by any method of shoeing.

To take an extreme case, nail a large snowshoe onto the hoof. The larger the snowshoe the less force there will be per unit area S experienced by the snowshoe, but the force per unit area experienced by the bearing edge of the hoof wall will not change. One can visualize this as the force “flowing” through the contact between the snowshoe and the bearing edge of the hoof wall.

Another example might be helpful. When you stand on a scale to weigh yourself, the area of the scale makes no difference. You weigh the same on a bathroom scale or on a platform scale for trucks.

To repeat: there is no type of horseshoe of whatever configuration that can reduce the linear force experienced by the bearing surface of the hoof. It is, however, possible to reduce the stress, the force per unit area, by the use of, for example, a bar shoe if the bar is in contact with the frog. If a wide-webbed, concave bottom shoe is in contact with sole and/or frog the stress will be reduced although, again, the total linear force remains constant. There are problems, of course, with the application of force to the frog and sole.


Figure 3: The moments acting on
the hoof of the standing horse.

Moments are turning forces such as one uses to unscrew a bottle cap or tighten or loosen a nut. The moments acting on the foot of the standing horse are shown in Figure 3, equation 2.

  • DFb-(Fa+CEc)=0

As there is equilibrium of linear forces, there is equilibrium of moments about the center of rotation in the distal end of the middle phalanx (syns.: short pastern bone, P2). The linear force, F, acting at right angles to the moment arm, a, generates a clockwise moment, -Fa. (By convention in mechanics counterclockwise moments are positive and clockwise moments are negative.) The common extensor and the extensor branches of the suspensory ligament also exert a moment, here lumped as -CEc. These moments are equilibrated (balanced) by the linear force, DF, of the deep flexor acting around the moment arm, b, the distance of the deep flexor tendon from the center of rotation, the moment being +DFb. Note that capital letters are used for the linear force and small letters are used for the moment arms - the perpendicular distances of the linear forces from the center of rotation.

It is important to emphasize that in the foot there is only one center of rotation . Once the shoe is nailed or otherwise fastened to the hoof, the shoe becomes mechanically a part of the hoof, and moments affecting the shoe operate around the center of rotation in the distal end of the middle phalanx.

Figure 4: The resultant R defining
the position of the force F.

It is necessary to locate the position of the reaction force (vector) F since its position determines the value of a and, thus, the value of the moment Fa. In order to determine the position of F we must know the position of the resultant R, Figs.1 and 4. Fortunately we can take R as identical to the anatomical axis of the pastern (proximal and middle phalanges), Figure 4. Remember that R is normally parallel to the horn tubules of the hoof wall and remains parallel no matter the position of the pastern during movement.(1) F for the normal digit is dorsal to (in front of) the center of rotation, so that the moment Fa is clockwise - negative.

The situation with moments is somewhat more complex than with linear forces. As discussed by Rooney (1984,1997), one can cause the pastern to become more upright(at a larger angle with the surface) if the hoof angle measured at the toe is decreased and more sloping if the angle is increased. One can accomplish that by trimming heels or toes or by the use of wedges or wedge-shaped shoes. While there has been disagreement in the past about this, I believe most now recognize that this is what happens. It is of interest that the change of pastern orientation with change in hoof angle was well know to Lungwitz among others in Europe at least as early as the 1880s.

The Egg Bar Shoe and “Support”

The bar shoe and, specifically, the egg bar shoe are frequently employed to provide “support.” In both conversation and the literature the nature of this “support” is at best vague and at worst completely undefined. In mechanical terms support must be force (or forces) which operates either as a linear force or a moment or both. One thing to be achieved in this paper is to erase the term “support” in favor of the more precise terms: linear force and moment.

As already noted the bar shoe, per se, can only decrease the value of S, the linear force per unit area of the bearing surface, if it is in contact with the frog. The egg bar shoe, on the other hand, extends behind the heels and is not usually in contact with the frog. What function, then, does the egg bar subserve?

Figure 5: The position of the foot when
the deep flexor tendon is severed
and in the “flaccid tendon” foal.

In order to answer that question I must go around the barn to a certain extent. If the deep flexor tendon is cut in half, the toe of the hoof will come off the ground, Figure 5. Examination of equation 2 and Figure3 will show why this happens. DF is no longer present, and there is only Fa and CEc both of which are clockwise. Obviously Fa cannot raise the toe from the ground, and this is done by CEc.

The foot is now in a position like that of Figure 6 with F, the point of contact of the hoof with the ground having moved toward the heels. In order to correct this an egg bar shoe is applied, so that F is in the position of Figure 7 and Fa is now counterclockwise and presses the toe back unto the surface.

An analogous situation pertains with the foal born with so-called flaccid tendons. The cause of that condition is not known but the tendons will shorten with time in many foals. The foot of a leg with flaccid tendons looks like Figure5 since a loose or flaccid deep flexor tendon is not unlike one with the tendon cut in half. Applying the egg bar to such a foot, then, would be expected to pull the foot unto the surface. Doing so tightens (increases the tension in) the common extensor tendon and the extensor branches of the suspensory ligament, both of which will tend to pull the pastern into a more upright configuration.(2)

Figure 6: F palmar to (behind) the
center of rotation, so that Fa
is counterclockwise.

In both of these situations, severed deep flexor tendon and flaccid flexor tendons, there has been loss of equilibrium of equation 2. There is insufficient DF (at least) in both cases. The egg bar places F in such a position, behind the center of rotation, that F acts as if it were DF.

  • DFb+Fa-CEc=0

The egg bar shoe has a place, then, in helping the foal with flaccid tendons achieve a more normal conformation until the tendons shorten appropriately.

The egg bar has been suggested and used for many other conditions such as bowed tendons and putative suspensory “strain”, among others. The mechanics strongly suggests that the egg bar can only function as discussed above, that is, when equation 2 is not in equilibrium. With a bowed tendon or strained (torn) suspensory there is no loss of equilibrium and the egg bar will accomplish nothing.

(A note for the exasperated reader: You can’t remember what equation 2 is all about. Go back and study it!)

Egg bar shoes, and the closely related shoes with trailers, do have an effect on the moving foot . Specifically, the trailer or egg bar will contact the surface first if the horse is moving fast enough for the normal heel-quarter-toe impact sequence. In the case of the trailer the foot will tend to yaw(3) around its axis (the axis of the pastern). It might appear that the trailer in contact with the surface is acting as a center of rotation. In fact, the center of rotation, as already emphasized, is always at the coffin joint in the distal end of the middle phalanx. At the point of contact of the trailer with the surface, the surface is exerting an upward linear force on the trailer which acts around the moment arm perpendicular to that force from the center of rotation, Figures 7 and 8.

mechanics_of_foot_and_shoe_7 mechanics_of_foot_and_shoe_8
Figures 7 and 8: The action of a trailer or egg bar shoe. The force F is acting at the end of the trailer which first comes into contact with the surface. The equation DFb+Fa-CEc=0 applies to Figure 8.

The bending of the trailer will absorb and dissipate energy (see egg bar, below). The trailer is usually on the outside (lateral) branch of the shoe as an aid to preventing or minimizing cross-firing by pacers. Since the trailer contacts the ground first, the foot will yaw laterally - the toe pointing more to the outside at impact. Pacers are predisposed to cross-firing by toed-in and/or toe-narrow conformation and turning the toe of the hoof outward at impact tends to counteract the toed-in, toe-narrow condition(4).

The egg bar, and, to a lesser extent the straight bar shoe, also contacts the surface first, bending the bar and so absorbing and dissipating energy with little tendency to yaw. The value of such a shoe is in reducing the energy of impact and thus reducing any pain being experienced by the foot as a result of impact.

Toe extensions are used for animals with so-called contracted tendon or tendons. The situation here is nearly a mirror image of that encountered with the egg bar and the flaccid tendon foal. Extending the toe of the shoe beyond the toe moves F and, so, increases a. The clockwise moment Fa is increased and tends to counter the increased counterclockwise moment of the flexor tendons. There are, of course, other measures to be taken to counteract shortening of the flexor tendon, Rooney (1999). Note that the value of the extended toe is dependent upon the continuing shortening of the flexor tendons. That is, the foot is continuously “trying” to rotate - move - as the tendons shorten. If this shortening ceased, there would be no effect of the toe extension. In other words equation 2 would be in equilibrium.

We can now restate the initial and major proposition of this paper: The effect of the horseshoe on the movement or stance of the horse is completely determined at the moment of impact of the foot (and shoe) with the surface and at the moment of lift-off of the foot from the surface. When the foot is still upon the surface, the horseshoe has no effect whatsoever. What this says can now be said more succinctly: the horseshoe has an effect only when the foot is moving, i.e., when the equilibrium of moments equation 2 is not in equilibrium.


(1) As the vertical force F increases from impact of the foot with the surface to midsupport the force DF increases and, similarly from midsupport to lift-off both F and DF decrease. These two forces increase and decrease in phase with each other, so that the resultant, R, although increasing and decreasing in amount, does not change its direction of action. R, in other words, is a vector force and vectors have two characteristics: amount (scalar) and direction (vector). The scalar of R changes but its vector does not. It remains parallel to the horn tubules.

(2) The common extensor tendon is closely adhered to the periosteum on the front (dorsal) surfaces of the proximal and middle phalanges and, so, acts in a passive, check ligament-like fashion.

(3) The foot can move in three directions (Figure 7): 1. Tipping forward and backward is called pitching; 2. Rolling from side to side is rolling; 3. Spinning around an axis drawn through the long axis of the pastern, the movement being parallel to the surface, is called yaw.

(4)There is considerable variation in the literature about base narrow and base wide conformation. I don’t intend to become involved in the resulting confusion. In this paper base narrow and base wide refers to the space between the legs at the level of the chest and the thigh. Foot-narrow means the feet are closer together than usual when standing and foot-wide means they are farther apart than usual. Toed-in and toed-out have the usual meanings.


Butler, K.D. (1985) The Principles of Horseshoing. Doug Butler. Maryville, Missouri.

Rooney, J R (1984) The angulation of the forefoot and pastern of the horse. Journal of Equine Veterinary Science 4: 138-143.

Rooney, J R (1997) Plantigrade to digitigrade evolution of equids. Journal of Equine Veterinary Science 17: 340-345.

Rooney, J R (1999) Surfaces, friction, and the shape of the equine hoof, Online Journal of Veterinary Research 3:137-149.

Rooney, J R (1999) So-called clubfoot of horses. Anvil Magazine, In Press .

Lungwitz, M (1910) Leisering u. Hartmann, Der Fuss des Pferdes. 11th Ed., Schaper, Hannover.

Lungwitz, A (1913) Horseshoeing. Facsimile Edition. Oregon State University Press. Corvallis.

]]> (James Rooney, D.V.M.) Hoof Mechanics and Physics Thu, 30 Jul 2009 07:02:50 +0000
Heel Wedges: Their effects on Tendon and Ligament Strains

An introduction to basic mechanical properties which govern movement in the lower limb, story primarily those which are exerted by the tendinous/ligamentous structures.

Moment equations for the coffin and fetlock joints:
Coffin: DFb-F1-EBa=0
Fetlock: DFr+SF2r+SL.5r-Ff+EB.5r=0

To understand the mechanical functioning of the lower limb, we must first discuss the forces which initiate or prevent movement. Several structures are involved in the function of the lower limb including the deep digital flexor tendon, superficial digital flexor tendon, suspensory ligament, digital extensor tendon, extensor branch of the suspensory ligament, laminae and the ground reaction force. To model the lower limb, forces are generated by muscle contraction and relaxation which are transferred by the elastic tissue of tendon and ligament.

Each tendon or ligament generates a turning force (moment) about one joint or several joints. One example of this is the coffin joint which is acted upon by the ground reaction force, deep digital flexor tendon, extensor branches of the suspensory ligament and common extensor tendon. In the standing horse, the following mathematical relationship can serve to define the equilibrium which exists. However, during movement, the equation would change to represent different parts of the stride and it would not be in equilibrium. To simplify the present mechanical definition of the lower limb, we will consider only the forces as they are in the standing horse. If at any point part of the equation reaches its threshold level, then damage to the associated structure will likely occur. For example, if the deep digital flexor tendon component increases by a change in the toe angle, then that structure or related structures may be prone to damage under normal loading conditions.

An illustration is a case where the toe angle has changed to increase the force present in the deep digital flexor tendon. In order to maintain equilibrium, the other tendinous/ligamentous components must change their relative distribution of forces.

Once a mathematical relationship like this is formulated, empirical or derived data must be collected which will either support or disprove the hypothesis. To do this, it is now possible to measure the contribution of each tendon, or ligament structure in the lower limb with devices which measure strain. From these we can calculate the direction and magnitude of the forces which can be used to build a mathematical model of the lower limb. One use of this model is to compare the effect that different farriery techniques have on the relative loads that associated structures must carry.

Hoof balance is a term frequently used, yet the question still remains regarding how to precisely define a properly balanced foot. Balance in the sagittal plane has been associated with the shoulder and pastern angle, yet medio-lateral balance is somewhat more difficult to quantify. Alterations in hoof balance through the addition of wedge pads have been used for many years for therapeutic and performance reasons. In recent years, largedegree wedge pads have been used successfully in the treatment of laminitic horses. However, limited studies are available that quantify the influence of toe angle on the normal kinetic forces of the lower limb.

Equine tendon and ligament injury occurs in many horses which are in race training. It has been reported that injuries due to tendon or ligament damage occur in 10-15% of the horses in training. The etiology of tendon damage is not well defined and likely is multi-factorial. The damage to the tendon or ligament results from a separation or tearing of collagen fibers due to a load applied to the tendon which is above its point of failure. Hoof angle is frequently implicated as one cause of lameness in horses. A large toe angle (>55 degrees) is not generally associated with an increased incidence of musculo-skeletal damage. However, a low toe angle has been implicated as a factor in the onset of lower limb disorders including degenerative bone disease, navicular disease, bone chips, tendon tearing and ligament strains. Once the tendon is damaged, the treatment administered is the subject of debate. However, one part of the treatment should include the removal of as much of the load borne by the damaged structures as possible. This can be done by a combination of splints, bandaging and changes in the toe angle.

The study which I will discuss here was designed to determine the influence that changes in toe angle had on strain in the suspensory ligament, extensor branch of the suspensory ligament, deep digital flexor tendon, superficial digital flexor tendon and the surface of the hoof wall. This information will help us to determine normal mechanical function of the lower limb as well as to determine treatment of lameness disorders including laminitis and various tendon injuries. For this study, each leg was trimmed to a constant angle (55 degrees). Heel wedge pads were then added to create the following toe angles: 58 degrees, 61 degrees, 64 degrees, 67 degrees, 70 degrees, and 78 degrees.

Strain on the deep flexor tendon was reduced with increases in toe angle. The reduction in strain followed a linear pattern with the increasing toe angle at two locations on the deep flexor tendon. Deep digital flexor tendon strains decreased from 2.49 for the 55 degree toe angle to 1.42% and 1.02% for the 70 degree and 78 degree toe angles, respectively. This represented a 59% decrease in tendon strain between the 55 degree and 78 degree treatments. Strain decreased in a similar fashion at the interphalangeal site, as there was a 64% decrease between the 55 degree and 78 degree angles. Strain measured in the superficial flexor tendon was not affected by an increase in toe angle. Within the range of toe angle used in this study, there was no change in strain on the superficial flexor tendon. Initial strain for the control treatment was 2.90% and varied very little with the increase in toe angle. This is similar to that observed by others. The suspensory ligament acted very similarly to the superficial flexor tendon; there was not any change in suspensory ligament strain with the addition of the toe wedges. Strain in the suspensory ligament had a small numerical increase in strain as the toe angle was increased, but the change was not significant. However, a large increase was noted for the extensor branch of the suspensory ligament. There was very little strain present on the extensor branches with the 55 degree, 58 degree, 61 degree and 64 degree toe angles. However, when the toe angle was increased to 67 degrees, extensor branch strain increased dramatically. At the 55 degree toe angle, strain on the extensor branches was .02% and when toe angle was increased to 78 degrees, strain increased to 1.40%.

Strains on the surface of the hoof wall remained in compression during loading and the magnitude of compression increased on the medial and lateral walls with increases in toe angle, and decreased on the dorsal hoof wall. A similar but opposite trend was observed for the dorsal hoof wall as strain decreased 52.8% as toe angle increased. Strain on the medial hoof wall followed the same pattern as the lateral hoof wall. The trend in this case was for a higher strain on the hoof wall as toe angle increased. The difference was noted between the 78 degree toe angle and the 58 degree and 55 degree toe angles.

Heel wedges had no effect on reducing strain in the superficial flexor tendon and suspensory ligament. However, heel wedges are recommended in cases where reduction of strain on the deep flexor tendon is needed, such as laminitis and tendon flexor tendon injury. One other effect noted with the addition of a heel wedge is the large increase in strain of the extensor branch of the suspensory ligament. This may be of particular interest in the treatment of laminitic horses, as reduction of deep flexor tendon strain and increased extensor branch strain both occur with heel wedges and will work concurrently to stabilize the coffin bone in normal and laminitic horses. Heel wedges also raised compressive strains on the surface of the hoof wall at both the medial and lateral quarters, which may affect the occurrence and treatment of quarter cracks.

Thus, heel wedges have a great influence on redirecting the forces that are generated by the tendon and ligament structures in the lower limb. The use of heel wedges will be of great benefit in treating laminitic horses, due to their effect on reducing strain in the deep flexor tendon and increasing strain in the extensor branch of the suspensory ligament.

First published in ANVIL Magazine, February 1998.

]]> (Kent M. Thompson, PhD) Hoof Mechanics and Physics Wed, 27 May 2009 07:47:04 +0000
Heel Expansion - Says Who?

I believed it myself for years, but like the flat earth theory, the theory of heel expansion, though useful in most instances, is not accurate. This is most apparent when considering run-under heels, under-shot, wiry or wired-on, or simply contracted heels. It is a common problem, apparent by a sudden turning of the heels when viewed from the ground surface of the hoof -- or when viewed from the side, the angle of the heel wall will be lower than that of the toe.

As the affected hoof grows between shoeings, the platform of support shortens and moves forward while the hoof angle decreases; this results in increased stress of the suspensory ligament and flexor tendons, and a predisposition to overreaching, lost shoes, sidebone, corns, strains, bows and navicular disease.

According to the heel expansion theory, weight bearing causes a compression of the plantar cushion which, in turn, pushes out against the lateral cartilage, resulting in expansion of the posterior hoof. Therefore, increasing the vertical compression of the plantar cushion by increasing frog pressure, and/or making the posterior wall more yielding by moisturizing or thinning should correct heel contraction. In the case of contraction of the hoof(with a high angle and a shrunken frog) such treatment usually works well, as theorized. However, in run under heels, such treatments are far from beneficial. Obviously, run-under heels, though a form of contraction, are quite different from the upright (atrophy) type. Dienlin (1973) makes a nice distinction by calling the atrophy type "hoof contraction" because the entire side of the hoof wall draws in, and calling the run-under type "heel contraction" because just the heels sharply turn in, typically while the quarters are flaring, though both types can appear together.

The theory of heel expansion looks questionable because, not only does frog pressure, exercise, and more flexible walls not help correct, but are known to help cause, heel contraction! Dollar (1898) said, "The greatest tendency to contraction is seen in weak feet, which naturally possess ... low heels ... The more oblique (low angled) the hoof, the more rapidly does contraction proceed. " Reeks (1906) agreed,". . . low heels and abnormally sloping walls predisposes to contraction no one will deny.. . the tendency to contraction already there is aggravated by careless shoeing and the effects of work Even Lungwitz: (1913) noted, "Contraction affects front feet, especially those of the acute-angled form more often than hind feet," and may be caused by weakening the posterior half of the hoof. .." How can it be that low angle (which would favor frog pressure), weak walls (which would yield easily) and work (which should increase blood flow) cause heel contraction? Although Lungwitz' conclusions on the physiological movements of the hoof have been unquestioned since the turn of the century, they are more dependent on his prejudices than his measurements. According to Dollar, earlier experimenters Lechner, Gierth, and Dominik each measured contraction at the heels occurring naturally under weight bearing. Though a larger number, including Lungwitz, claimed to find expansion of the heels, Dollar, apparently unable to prove or disprove the conflicting reports, settled for the consensus, with some additional weight given to experiments with live hooves -- of which, Lungwitz did the greatest number.

In Horseshoeing, Lungwitz describes the hoofs response to weight bearing as: " A lateral expansion ... of the quarters ... between one-fiftieth and one-twelfth of an inch. A decrease in height of the hoof, with a slight sinking of the heels . . . As an outward visible indication of the mobility of the quarters upon the shoe we may point to the conspicuous, brightly polished, and often sunken spots, or grooves, upon the ends of the branches. They are produced partly by an in-and-out motion of the walls at the quarters, and partly by a forward and backward gliding of the quarters upon the shoe."

He made no mention here of any motion at the heels, but certainly, if the toe is attached to the shoe, the quarters can't glide forward and backward, but only in and out. If the quarters are moving out, and the bulbs of the heels sinking, then the heels must be gliding forward. Holmes (1949) makes note of this when discussing the heels contraction caused by expansion of the quarter as sidebone appears 'You cannot have length and breadth in the horse's foot: if you are going to push out the wall you must shorten the length."

You can prove this to yourself when shoeing by examining shoes that you had close fit to the wall your previous visit. Before removing the shoes you will see that typically the quarters have spread over the edge of the shoe, but the heel has not. After removing the shoe, observe that on the hoof surface at the quarter the worn area slopes gradually out, but at the heel, there is a lip at the outer edge and the worn area slopes in toward the frog. Actually any evidence of wear between the shoe and hoof may indicate a weak wall, becoming run-under and, as Dollar quotes him, Lungwitz agrees this occurs in hoofs already with run-under heels: "In hoofs with wired-in heels and compressed bars, dilation under the body weight may still occur, but the most posterior part of the bearing surface of the heel does not take part in it -- rather the contrary."

While run-under heels are not natural or desirable, they are more the rule than the exception. Dollar says ",.. with the exception of pronounced upright hooves, all show moderate convergence of the posterior parts of the heel walls." Axe (1906), as edited by Jones (1972), supports this too: "It is not far from the truth to state that there are few horses in active work whose feet are not more or less contracted."

According to Dollar, Lungwitz' measurements of normal hoof motion were dependent upon "Flexibility of the horn, and a well developed but untrimmed frog." Evidently, his findings were also dependent upon measuring these hoofs before they became run-under, as their condition would tend to make them.

What about the "pronounced upright" hoofs that are not run-under? According to Reeks, in the absence of frog pressure Lungwitz found, "Contraction of the solar edge of the heels occurs at the moment of greatest over-extension of the fetlock joint -- that is, in a foot with pressure from below absent. On the face of it, this appears impossible. Lungwitz, however, has perfectly demonstrated it ... "is but a simple and natural result of foot dynamics. The movement of the plantar cushion will now be downwards, as well as backwards, and, seeing that it is attached to the inner aspect of each lateral cartilage, we shall expect these latter, by the downward movement of the plantar cushion, to be drawn inwards. This Lungwitz has shown to occur."

In light of recent refutation of frog pressure by Emery, et. al (1977), and Stashak (1986), this would appear to be natural motion of the hoof during weight bearing. Why Lungwitz felt frog pressure was essential to proper functioning of the hoof, and why that remained unquestioned for the best part of a century is hard to understand. The authors of the earliest writings on horses; Xenophon, Vergil, Columella, and Pelagonius, centuries before Christ, agreed that hard hoofs with high heels and a hollow sound when hitting the ground (without frog pressure) were the most desirable (Morgan, 1962). Axe related that in the period between 1800 and 1830, "Frog pressure and short shoes were tried and discarded" (Jones, 1972).

Logically, a plastic structure such as the frog, bordered on either side by the empty space of the lateral sulci would function so as to do everything but bear weight, or transmit pressure laterally.

Consider what would result if the internal mechanisms of the hoof functioned to produce expansion. Because of its slope, the hoof wall by itself would expand during weight bearing -- the more weight and the more often it bore weight the more it would expand. If the internal structure increased that effect then one should expect to see collapsed or exploded hoofs from running or jumping. Clearly, the hoof must function to retain its integrity by counteracting external forces, not amplifying them. The action of the hoof with little or no frog pressure as described by Lungwitz should do just that -- internally producing contracting force simultaneously and in direct proportion to the external forces of expansion, or possibly slightly greater than the external forces of expansion, as that would result in a slightly more upright wall in the quarters, better able to withstand the load.

Hunting (1941) largely accepted Lungwitz's conclusions but kept some reservations: '"the heels and quarters may be pressed together to some extent but they are prevented from being forced asunder by the fibrous connections of the frog-pad (plantar cushion) . . . expansion . . . may be practically disregarded in considering the best methods of shoeing sound feet."

Now, run-under or contracted heels make sense: They are simply an exaggeration of normal hoof motion from an overloading of the hoof -- in particular the heel. Dollar clearly observed the process, but either didn't see, or chose to ignore its contradiction of heel expansion: "In upright hoofs, on the other hand, even when this part of the frog is lost, contraction does not occur. The cause of contraction is therefore, not thrush (or the absence of frog pressure) but the pressure of the body-weight, which forces the walls of the heel downwards, forwards and inwards." He further supports this by experience with several pairs of similarly used and shed horses -- which, in each case, heel contraction was worse in the horse with the lower angles: ". . . the reason appearing to be simply that in upright hoofs the heels bear less weight. . . " The heels are overloaded, and collapse, when subjected to excessive weight (i.e., broken back alignment of the phalanges, already run-under heels, compensating for other lame legs, and overwork and or when the wall is weakened (i.e., moist, thin, shelly, flared or low angle).

A few more things pop into perspective: Lightweight shoes are sprung (become wider) when, during weight-bearing, the hoof contracts concentrating pressure at the inside web, causing the shoe to squirt out (or did you think that, without even being attached at the heel with nails, that the heels expanded dragging the shoe with it?).

Now, sidebones make sense too. Though Rooney (1974) and anyone who believes heel expansion cannot explain why a waterbed-like planter cushion would consistently cause the lateral cartilage to ossify and find it even more baffling to explain why occasionally there are large antler-like projections of calcification curving up from the sidebone toward the long pastern: "The cause of sidebone is not exactly understood." However, when you consider that the plantar cushion is pulling on the lateral cartilage and the ligament that suspends it from the long pastern, tearing and resultant calcification can be expected. Tearing of the lateral cartilage's would be most apt to occur when the wall at the quarters is expanding in opposition to the pull of the plantar cushion.

Flexibility of the wall, as Lungwitz specified, is required to obtain even minute movement of the wall, which he theorized was essential to the proper functioning of the internal structures. However, it is apparent that flexibility of the wall encourages heel contraction, which Lungwitz recognized as "the parent of nearly all diseases of the hoof' . .(corns, quarter-cracks, barracks, thrush of the frog) and others mentioned at the beginning of this article.

In addition to heel contraction, flexibility of the wall is responsible for the damage done as the heel of the hoof and shoe wear against each other, changing the anterior-posterior hoof balance and prematurely loosening the shoe. Flexibility and support are not compatible. As the wall's most important function is the support of the entire animal, flexibility of the wall is not desirable. Flexibility of similar horny structures in other animals is not necessary (i.e., human nails, cat and dog claws, cattle horns, etc.) there is no reason to suspect that it is in horses. While previously it had been thought, but now disproved, that such flexibility was essential for the hoof to function as a blood pump, the internal movements of the hoof must be responsible for the flow of lymph up the lower limb, considering the location of the lymph plexuses in the frog and sole, and the inability of muscular contraction or respiration (the usual determinate of lymph flow) to affect lower limbs.

In conclusion, while the internal physiological movements of the hoof are important, in sound feet they do not require and are not helped by movement of the wall, nor ground pressure against the frog. Shoeing, far from being a "necessary evil" can be extremely beneficial, not only for traction and protection of the hoof, but also to stabilize and reduce unhealthy movement of the wall.

More objective research as well as practical experience with an open mind may lead us to readopt valuable methods which have become unfashionable -- quarter clips' and bar clips, for example, and to invent better methods. (The new Glu-Strider, if fit close at the quarters and cut out in the center of the pad, may prove very beneficial by preventing harmful expansion at the quarters, while allowing slight contraction of the heels. And, since the shoe can flex slightly with the hoof instead of rubbing against it, the hoof and shoe would not wear and damage each other.) In the meantime, question authority, and good luck on your journeyman test.


Adams, O. R., D.V. M. 1974, Lameness in Horses, Philadelphia: Lea & Ferbiger.

Dienlin, John A. 1973, Balance of the Equine Foot and Gait and Therapeutic Shoeing, Auburn: Aubum University.

Dollar, J.A.W., M.R.C.V.S. 1898, A Handbook of Horseshoeing, New York: Wm. R. Jenkins.

Emery, Miller and VanHoosen, D.V.M. 1977, Horseshoeing Theory and Hoof Care, Philadelphia: Lea & Ferbiger

Holmes, Charles M., F.W.C.F. 1949, The Principles and Practice of Horseshoeing, Leeds: Farriers' Journal Pub.

Hunting, Wm., F.R.C.V.S. 1 949, The Art of Horseshoeing, Chicago: Alexander Eger, Inc.

Jones, Wm. E., D.V.M. Ph.D. 1972, Horseshoeing, E. Lansing: Caballus.

Lungwitz, A. and Adams, J.W. 1966, A Textbook of Horseshoeing, Corvallas: Oregon State University Press.

Morgan, M.H., Ph.D. 1962, Xenophon; The Art of Horsemanship, London: J.A. Allen & Co., Ltd.

Reeks, H. Caulton, F.R.C.V.S. 1906, Diseases of the Horse's Foot, Chicago: Alex. Eger.

Rooney, J.R. 1974, The Lame Horse, New York: A. S. Barnes & Co.

Stashak, Tad S., D.V.M., M. S. Adams' Lameness in Horses, 4th Ed., Philadelphia: Lea & Ferbiger.

]]> (Henry Heymering, RJF, CJF) Hoof Mechanics and Physics Wed, 27 May 2009 07:42:33 +0000
Gait Transitions

The question of how and why horses change gaits has been debated in the literature for a number of years. That literature which was reviewed in Rooney 1999. I do not wish to discourage the reader, medicine but the material in this essay is not trivial and will require attention and thought.

I blundered into this area with some work published in the Online Journal of Veterinary Research in 1999. Since then, pharmacy further work has been done on the same data with some interesting results and suggestions.

The thrust of that earlier paper was that the trot to gallop transition in horses occurs when the actual (damped) frequency of the gait is at or near the natural (undamped) frequency. When actual and natural frequencies correspond in this way, ask resonance occurs. A characteristic of resonance is large amplitude vibrations which can be destructive. My thesis was, and is, that the “appreciation” of approaching, possibly dangerous, resonance is the trigger for the change of gait. This was put in physical terms by measuring the actual (damped) frequency and calculating the natural frequency. That calculation was based on changes in coffin joint angles as the leg was loaded. The two frequencies were then compared and fundamental resonance was identified when the two frequencies were the same or nearly so. The method is summarized next below, and the original paper can be consulted for the full story, Rooney 1999.


The dynamic responses of a stationary or moving mechanical system may be evaluated in terms of the undamped natural frequencies (Fn) and the damped natural frequencies (Fd). The damped natural frequencies are those frequencies actually measurable in the standing and moving horse. The undamped natural frequencies cannot be measured in the horse, and we resort to the following calculations to approximate Fn.

The undamped natural frequencies are obtained at each of a series of static positions, and this series of statically obtained frequencies are considered approximations to the dynamic undamped natural frequencies. "When a machine is mounted on springs, there will be an initial deflection in the springs caused by the weight of the machine." Grosjean and Longmore 1974. Because the static deflection and the natural frequency both depend on the mass and spring stiffness, they can be related by:


where Fn is the undamped natural frequency, g is the acceleration of gravity and ds is the deflection. We take the horse as analogous to such a machine mounted on springs, Rooney 1969. When the leg is not loaded, the coffin bone (distal phalanx) is approximately at right angles to the axis of the middle and proximal phalanges. When the foot is on the ground and bearing weight, the angle formed by the axis of the phalanges and the ground will be an angle less than 90°; on average in the standing horse about 50°.

The rotation from 90 to 50° was calculated as linear displacement (deflection) of the fetlock joint toward the ground.


where s is the linear distance from fetlock to ground, a is the palmar angle of the coffin joint, and r is the linear distance from the center of rotation of the fetlock joint to the center of the coffin joint, ~15 cm. (Figure 1)

Figure 1: The measurements used to
determine undamped natural frequencies.

The deflection then is:


The value of ds is entered into the first equation to obtain Fn for the standing horse.

The deflection of the fetlock is taken at 1° intervals from 50 to 10° coffin angle. The assumption, as indicated, is that the undamped natural frequencies at each position can be taken as approximations to the dynamic undamped natural frequencies.

The two sets of data, Fd and Fn were compared and the two frequencies were nearly identical at 1.8 Hz which is at the trot/gallop transition as found, for example, by McMahon 1975,1984. (Table, below)

It is important to note that gait transitions are not carved in stone but can be modified by fatigue, bumping, and size of the animal. This is dealt with at length in the original paper.

New Studies

The present study is based on the average gait characteristics of the same four Thoroughbred horses moving steadily from standstill to full gallop on a treadmill. Different ways of plotting the same data from Thompson et al 1989 are helpful in both locating and better understanding the sites of gait transitions. Reexamination of this data suggests that a resonance condition could be related to the walk/trot transition as well as to the trot/gallop transition.

In Figure 2 the damped frequency is plotted on the horizontal axis and the stride length on the vertical axis. The areas of walk/trot and trot/gallop transition are indicated. In Fig.2 the damped frequency on the horizontal axis is plotted versus the first derivative of that frequency on the vertical axis, i.e., in the phase plane. The transition areas are clearly evident. The large variation of the amplitude of the trajectory in the phase plane near and at the transitions is notable and suggests that frequency is a major determinant of gait transition.

Figure 2: Damped frequency and stride length for average of 4 horses. The walk is to the left of the left vertical bar, the trot between bars, and the gallop to the right of the right bar.

Figure 3: The phase plane representation shows large amplitude changes in the rate of change of damped frequency related to the transition from walk to trot, left arrow (~0.75 Hz), and trot to gallop (~1.75 Hz), right arrow..

Velocity and stride length were plotted versus time as well as in the phase plane, but none of these plots provided the information of Figures 2 and 3.

Walk/Trot Transition

In my 1999 paper only the trot/gallop transition was considered. As shown in Figures 2 and 3 it is apparent that the area of the walk/trot transition can also be found. It is not immediately clear, however, that frequency resonance can be related to that transition as it was for the trot/gallop transition.

According to Burton 1994, superharmonic resonance of order 3 occurs when the driving (damped) frequency is near 1/3 the natural frequency. This is different, of course, from the fundamental resonance when the damped and natural frequencies are the same or nearly so. In the Table the natural frequency is in the left column, the damped frequency in the middle, and 3 times the damped frequency is in the right column.. This is done, of course, to compare the damped frequency with the natural frequency at order 3.


In the yellow highlighted row Fn and the superharmonic frequency Fd *3 are almost the same, and this appears to be superharmonic resonance order 3 at the walk/trot transition. The green highlights the fundamental resonance at the trot/gallop transition. It is rewarding to observe that the value of 0.79Hz for Fd found here for superharmonic resonance is almost the same as that shown for the actual data in Figures 1and 2 for the walk/trot transition. The third column will be discussed below.


The overall concept of resonance as a significant factor in gait transitions is compatible with the sophisticated analyses of Schöner et al 1990. Resonance can imply loss of stability, and the detection of resonance permits the central gait processor to change the gait in order to avoid macroscopic instability.

Standardbred race horses provide interesting examples of instability. These horses are said to "make a break" when they shift suddenly from trot or pace to the gallop. There are several reasons for this transition, including interference of one leg with another and bumping particularly on turns. The animal becomes macroscopically unstable and switches to the gallop, the more stable gait at higher velocities. There, of course, needs to be a reason why the gallop is more stable than the trot or pace at higher velocities. While not worked out in detail, the following may be suggested. There is one fly period (no feet on the ground) per stride at the usual diagonal or round gallop while there are two fly periods per stride at the trot or pace. It is reasonable that the fly periods are at least potentially unstable, so that the gallop has less opportunity for instability per stride than the trot or pace.

There is no data available to directly relate such "breaks" to a resonance transition. While it may be simply a matter of over-ride, one might speculate that the misstep (bump or interference) overloads one or two legs, so that the coffin joint(s) rotates into the trot/gallop frequency transition area. This assumes that coffin rotation at trot/pace racing speeds is less than at the trot/gallop transition and gallop as can be seen by examination of the plates in Muybridge 1957.

Harness horses may break for other reasons such as inadequate training and fatigue. Fatigue has been discussed in my earlier paper. A basic premise is that training involves teaching a horse to allow the human brain to supercede the horse’s brain, to allow the human to determine what the horse does. The training of a Standardbred to race at the trot or pace, then, includes convincing an already genetically selected horse to override the usual response of the trot/gallop transition mechanism described here. With inadequate training that mechanism is inadequately or incompletely over-ridden and breaks occur: the trot to gallop transition occurs.


The fourth column is the damping constant , E, and is clear evidence for considering the horse as a self-excited system. The complete series for E is not shown since it continues negative as the damped frequency (velocity) increases to the limit. As described in the earlier report, positive damping means energy is being dissipated from a self-excited system while negative damping means energy is begin supplied to that system. The energy supply is, of course, active muscle work and passive recovery of tendon/ligament/fascial strain. The energy loss is of several types, but we are concerned here primarily with the loss as a result of the braking action of the feet on the ground.

There is evidence that braking is greatest at the slow walk and increases with increase in velocity. This is quite plausible, certainly, since the opposite – slowing down – must mean increasing braking. It is well-known (though I need reference) that the efficiency of movement increases with velocity and that is evident in the Table as the positive damping decreases until it becomes negative.


Burton, T D Introduction to Dynamic Systems Analysis. McGraw-Hill. New York. 1994. p.661.

Grosjean, J and Longmore D K Fundamentals of Vibration in Petrusewicz, S A and Longmore, D K. Eds. Noise and Vibration Control for Industrialists, Elek Science, London, 1974.

McMahon, T A. Using body size to understand the structural design of animals: quadrupedal locomotion. Journal of Applied Physiology 39: 619-627, 1975.

McMahon, T A Muscles, Reflexes, and Locomotion. Princeton University Press, Princeton, 1984.

Rooney, J R. Biomechanics of Lameness. Williams and Wilkins, Baltimore, 1969.

Rooney, J R The Lame Horse. Meerdink, 2nd Ed, Neenah, WI, 1998.

Rooney, J R Gait transitions in horses. Online Journal of Veterinary Research. 4: 64-72, 1999.

Schöner, G, Jiang W Y, and Kelso, J A S A synergetic theory of quadrupedal gaits and gait transitions. Journal of Theoretical Biology 142: 359-361, 1990.

Thompson, K N, Rooney, J R and Shapiro R. Equine Locomotor Patterns During Gait Transitions. Proceedings Equine Nutrition and Physiology Symposium 11:27-28, 1989.

]]> (James Rooney, D.V.M.) Hoof Mechanics and Physics Wed, 27 May 2009 02:13:00 +0000
Fractures of Long Bones

Fractures of bone have always been and will always be with us. Fracture can be good problems since they can usually be repaired and sometimes will do the repairing without human intervention. While much has been written about the treatment of fractures, try very little attention seems to have been directed to the actual mechanism of fracture - how and why they occur. Here we shall look at how and why on the gross, pharmacy macro level. The microscopic details of fracture are complex and mathematically difficult and will not be attempted.

We must begin with some basic mechanics (no math required). Bone is a plastic/elastic material which can be compressed, diagnosis bent, and twisted somewhat like rubber and completely recover from such deformation. Since bone is plastic/elastic rather than almost purely elastic like rubber, the recovery from the deformation may be slower than the almost immediate recovery of stretched rubber. If the force or forces causing the compression, bending, twisting exceed the normal range, however, the bone will fail - break - as if it were a brittle material. This is a most useful property helping us to understand the mechanics of failure since many fractures of bones can be mimicked by applying appropriate forces to pieces of chalk. Glass tubing fractures in the same manner, but I cannot recommend it as a study material.

An imperfect analogy for the elastic/plastic - brittle transition of bone could be a piece of rubber or plastic tubing which is bendable and flexable until it is sufficiently chilled or frozen and shatters (brittle failure). Figure1, below, is a generic stress/strain curve illustrating what has been said.


The stress on the vertical axis is the force per unit area being exerted on/experienced by the bone. The strain on the horizontal axis is the deformation of the bone relative to the stress. In the elastic region the stress and strain are directly (linearly) related to each other. In the plastic region the strain increases more rapidly than the stress, and failure of course is fracture.

Next we deal with the types of forces experienced by any bone to one degree or another and shall just use a generic "bone" to start with. If you wish to follow along using pieces of chalk to represent the bone, you will be that much farther ahead. I use the terms simple and complex to describe fractures. These are not the descriptive terms simple, compound, comminuted usually used. As will be seen my terms simple and complex are based on the manner in which the fracture occurred - the pathogenesis.



The block of bone (or chalk) to the left, A, is subjected to a compression force along its long axis - an axial force - which causes a shearing stress in the block as indicated by the diagonal arrow. If that axial force is sufficiently large (you hit the block of chalk with a hammer), the block will break, fail, as shown in, B, to the right. This is shear failure, and when a material fails in a brittle manner because of compression, it fails in shear. This is an important type of failure of bones and will be discussed in more detail below. Please note that while the fracture lines shown in this essay may be drawn as smooth lines; in fact, because of imperfections in chalk and in the microstructure of bone, the fracture lines will always be somewhat irregular and jagged. The main fracture lines will be, however, as shown.

It may be noted that shear failure is theoretically at 45o to the compressive force. In real materials such as bone, however, the angle is more nearly 30o or 60o depending upon the direction you look.



Simply pulling on each end of the chalk will cause it to fail (fracture) across its length (transversely) as shown. Such tensile failure is rather difficult to demonstrate with chalk and is never seen as the only cause of failure of horse bone. We shall see, however, that bending failure always has a tensile component at right angles to the long axis of the bone. Bone is stronger in compression than in tension and, so, fails in tension before compression.


The remaining type of simple failure is torsion, Figure 4, below.


The column is twisted as shown on the left and the fracture plane is as shown both together and apart as on the right. The curved plane of the fracture is theoretically at 45o to the curving lines of the twisting being exerted on the column. As already noted the angle is more nearly 30o or 60o in real materials. This is easily demonstrated with a piece of chalk

This is an important and almost invariable component of fractures of long bones as will be seen. It probably never occurs alone in fractures of horse bones.


Long bones of horses are normally subjected to bending, and bending entails compression, tension, and shear. Figure 5, below. The bending may be present because of the angulation of a bone relative to the ground but is largely caused by muscular and tendinous forces exerted on a bone.


In Figure 5 the beam is bent downward with compression of the upper part and tension of the lower part. Since long bones are more nearly columns than beams, we have Figure 6, below:


In Figure 6 the red lines indicate the column before the slightly eccentric load is applied, causing the bending of the column and compression and tension as indicated.

Failure in bending alone is uncommon in horse bones but does occur, often as the result of an external force such as a kick to the fore or hind cannon bone (Mc3, Mt3). Such fractures are more often seen in foals kicked by older horses. An example is shown in Figure 7, below. The arrow indicates that a force was applied from the left.. Tensile failure occurred on the right side of the bone - the side in tension. - and is at right angles to the long axis of the bone. As the bone failed, the tensile crack progressed across the bone, the bone became weaker and weaker and finally completed the failure on the left side because of shear in compression. This is immediately shown by the near 45o angulation of the fracture on the left side.


It is evident that as the crack progresses (very rapidly) across the bone that compression is increasing on the intact bone remaining at each instant1. As the crack nears the left side, the stress is sufficient to cause shear failure of the bone before the transverse tensile crack reaches the left edge.

Using the chalk model one can cause pure tensile failure, a transverse break without a shear component, by pulling on the ends of the chalk while bending enough to initiate the break. Failing that, simply bending a piece of chalk will cause failure as in Figure 7 as it occurs in real bones.

Complex Failure

Most long bone fractures in horses are complex. Once again the chalk model will be useful. The commonest long bone fractures are of the humerus, tibia, femur, radius, and 3rd metacarpal/metatarsal bone in about that order of occurrence. That is obvious, to say the least - what other long bones are there?

It is evident that long bones always experience three forces:

  1. an axial compressive force along the long axis of the bone which is generated by the body weight on that leg
  2. a bending force which is generated by the axial compressive force and by the muscles and tendons around the particular bone
  3. a twisting or spinning force which is generated by the movement of the bone ends - the joints

The first two forces are easily visualized. The third may not be. That spinning/twisting occurs because the joints are not simple hinge joints but, rather, are cams which direct both the large scale extension/flexion and, at the same time, a spin of the bone around its own long axis, three dimensional movement, in other words.


Figure8, above, is a schematic view looking down on one half of a joint surface. The medial, inside, part of the surface is larger than the lateral, the outside1. We take the straight line, a, to represent the other, mating, half of the joint. When a rotate/slides on the other joint surface as indicated by the small arrows, a goes to b. The arrowhead on a obviously changes direction as it goes to b. That is, as the bone swings into flexion the uneven size of the joint surfaces (cam) insures that the bone spins around its long axis from medial to lateral.

We show that again in Figure 9, below, that for the proximal phalanx (long pastern bone) dorsiflexing, and spinning as it does so, from medial to lateral on the distal end of the cannon bone. This is the movement of the fetlock joint as the load comes onto the leg.


Figure 9a

rooney_fracture9bFigure 9b

rooney_fracture9cFigure 9c

rooney_fracture9dFigure 9d

As the proximal phalanx is dorsiflexing from 9a to 9d, the larger medial side of the distal end of Mc3 causes it to spin as shown from medial to lateral, eventually close-packing at 9d.

Having done with that, we can return to fractures per se. Long bone fractures are a combination of axial compression, bending, and twisting. One can with careful examination of the broken bone or radiographs taken from at least two positions determine which parts of the bone were in tension and compression when the fracture occurred. That is clearly shown in Figure 10, below.


This is a so-called screwdriver fracture of the proximal phalanx. The dorsal surface -left picture- has a spiral fracture line while the palmar surface (right picture) has a nearly vertical fracture line. This says that the dorsal surface was in tension and the palmar surface in compression when the fracture occurred. The fracture in this case began with tensile failure because of bending on the dorsal surface combined with twisting . When the fracture had progressed to the palmar surface, the two ends of the spiral crack are joined by a vertical, or nearly so, crack as the bone opens like a book. In non comminuted fractures such as this one the vertical crack has intact periosteum, so that repair is a matter of closing the book again, Figure11, below.


Twisting in the direction of the arrows opens the fracture like a book around the heavier black line "hinge." Obviously, realigning the fracture is accomplished by twisting the two bone ends in the opposite direction.

When the spiral cracks reach the compressed side of the bone, they are "blocked" by the compression. As the crack lines gape open, the compressed area fractures along a vertical, or nearly vertical line, failing in tension on the inner surface, Figure 12, below.


Figure 13, below, is a fractured tibia. The spiral component (the right picture) on the cranial surface was the part in tension when failure occurred. The vertical component (the left picture) on the caudal surface was in compression.


Clearly, the vertical failure in Figure13 is not as obviously vertical as in Figure10. This illustrates the important fact that in complex fractures there are variable contributions by compression, bending, and twisting. One or two components may be more significant than the third and, so, the appearance of the fracture lines can vary somewhat from example to example. In my experience, at least, the spiral component is always clear and obvious while the vertical component may vary. This suggests, of course, that the twisting force is always strongly present while bending and compression are always present but more variable.

Chip Fractures

While fractures of long bones certainly do occur, so-called chip fractures are by far more common in working horses, particularly horses that work at speed. The common sites are the dorsal aspects of the fore and hind fetlock joints, the carpus, and, less commonly, the tarsus. There are two types of fracture and one nonfracture. The first is a single event shear failure as the result of excessive compression force. The second, and most common, is a shear failure which is pathological, occurring after the articular cartilage has been damaged and eroded (arthrosis) and the underlying cancellous bone has become sclerotic (denser). The third, nonfracture, may appear as a fracture on radiographs but is, in fact, new bone formation (osteophytes) formed because of arthrosis. Obviously, the second and third types can overlap. Thus, the articular cartilage is damaged and new bone begins forming in response. The cancellous bone becomes denser, and a shear fracture may occur, so that both osteophytes and fracture are present. All of these conditions cause swelling, pain, and eventual thickening on the dorsal aspect of the fetlock, carpus, and tarsus and, in the case of the fetlock at least, are often called osselet.


Figure 14 is a postmortem specimen of a single event shear fracture of the third carpal bone. Such fractures also occur in the radial carpal bone.


Figure 15 is a radiograph showing a "lump" of bone on the dorsal aspect of the fetlock. This could be either a chip fracture from the dorsal lip of the proximal phalanx or osteophyte formation without obvious attachment, on the x-ray, to bone.


The subject of fracture has hardly been exhausted even on the gross level of mechanics that we have been considering in this essay. I have not dealt with fractures of vertebrae, sesamoid bones, splint bones nor considered in any detail what one can learn of the kind of step or misstep which led to the fracture.

For those who might be interested there is more to be found in my The Lame Horse and in the book Equine Pathology. For those even more seriously interested a search of the internet for fracture mechanics will reveal a wealth of information, much of it highly technical, not say mind-boggling, in nature.

]]> (James Rooney, D.V.M.) Hoof Mechanics and Physics Mon, 18 May 2009 23:12:24 +0000
Basic Mechanics of the Hoof and Horseshoe - Rolling and Sliding

It is the basic premise of this essay that the sensitive and insensitive laminae and the horn tubules of the hoof wall are constructed in parallel with the primary force exerted on the foot since the laminae and tubules are best able to sustain the applied force in that configuration, story

Fig.1 (After Ellenberger and Baum 1927)

This is true whether the horse is standing still or at any gait and velocity when moving. Fig. 2 makes the mechanical case although for many readers it may be sufficient to simply state that the vertical force on the foot combines with the tensile force exerted by the deep flexor tendon to assure – normally – that this parallelism of force and laminae is maintained. For those who wish more please read on.


Figure 2
In Fig. 2 are shown the vector forces on the foot when the horse is standing still or the foot has impacted with and is fully in contact with the ground. R is the actual, cialis resultant force in the foot and is composed of the vertical force, F and the horizontal force, H..

As the vertical force increases from impact to midsupport the F force increases to a maximum as indicated in the figure to the right. At the same time the fetlock is dorsiflexing, increasing the tension in the deep flexor tendon, indicated by the longer horizontal (blue) vector in the right figure. Normally, the increase of F and H are in phase with each other, so that R, while becoming larger, does not change direction. Therefore, R, the force in the foot, remains in parallel with the laminae of the hoof wall.

There are some subtleties involving the change of direction of the deep flexor tendon and its components, but these do not change the argument presented.

]]> (James Rooney, D.V.M.) Hoof Mechanics and Physics Sun, 11 Jan 2009 03:21:03 +0000
Basic Mechanics of the Hoof and Horseshoe

Herein are presented some aspects of the basic mechanics of the horse’s hoof and the horseshoe. In an earlier paper the wearing of the unshod hoof was discussed and this is a follow-up paper (Rooney 1999).

In order to discuss the mechanics of the foot the forces - linear forces and moments - acting on the foot must be defined. This will be done as simply as possible.

I realize from long experience that most farriers and veterinarians do not concern themselves with mathematics. There is no way, here however, to understand how the foot and shoe function without mechanics, and one cannot do mechanics without basic algebra and simple line drawings.

The experienced veterinarian and farrier may say (and many have not hesitated to say) that all this “stuff” is unnecessary, that experienced rack of eye is what is needed to properly shoe a horse. While in part true, one hears and reads often enough that if one thing doesn’t work try the exact opposite which alone says that rack of eye is often no more than guesswork.

I shall not enter into discussion and/or polemics about the several horseshoeing “systems” said to be based on the study of feral horse feet. Consideration of my earlier report (Rooney 1999) on the wearing of such feet will show how incorrect much of the interpretation of the shape and wearing of these feet has been. This misinterpretation has led to some bizarre shoeing systems that demonstrate how wonderfully adaptable the horse is to even the most misguided human interference.


As already noted there are two types of force to be considered: linear forces acting in straight lines and moments that are turning or torque forces.

The linear forces acting on the foot of the standing horse are shown in Figure 1 and equations 1:

F-W=0 [1]



Figure 1: The linear forces acting on the foot.
The symbols are defined in the text.

These are equilibrium equations. W is the portion of body weight, the load, on a given leg. It is a downward force and, by convention, is negative. F is the upward force generated by the surface upon which the horse is standing and, by convention, is positive. -H is that portion of the downward force that tends to slide the foot forward on the ground. It is equilibrated, balanced, by the force H that is the friction between the bearing edge of the hoof wall and the surface. That frictional force is H=µF. µ is the coefficient of friction that is determined empirically.

H and F are vectors. When added together, the result (resultant) is R with -R for -H and -F.

The vertical force, F, is spread over the bearing edge of the hoof wall on a firm surface. It may, also, be spread over varying areas of the sole and frog on soft or yielding surfaces. Stress, S, is force per unit area - the amount of force experienced by some unit area of the bearing surface such as pounds per square inch, kilograms per square centimeter, etc.

In mechanics one may consider F spread over the bearing surface to be concentrated at a single point called the center of pressure. This does not mean that the force is indeed concentrated at that point; rather it means that one can account for the mechanics of the foot by calculating as if the force were so concentrated. If a triangular support were to be placed precisely at the center of pressure, Figure 2, the horse could stand naturally and balanced (granted that is more easily said than done.)

Figure 2: The approximate position of the
center of pressure in the standing horse.

No matter the horseshoe used, the total force, F, and the stress, S, the force per unit area, on the bearing edge of the hoof wall remains constant as long as the bearing edge is the only part of the hoof in contact with the surface. The stress can only be reduced if some or all of the frog and/or sole are in contact with the surface, as on a soft or sandy surface or with, for example, a bar shoe. The total force, F, cannot be changed by any type of shoeing.

For example: one weighs the same whether standing on a bathroom scale or on a truck platform scale.

To reiterate: there is no horseshoe that can reduce the total linear force experienced by the bearing surface of the hoof. It is possible, of course, to reduce the stress by using, for example, bar shoes and wide-webbed concave shoes that are in contact with the sole and/or frog. There are problems, of course, with such application of force to the sole and frog.


Moments are turning forces such as used to unscrew bottle caps or tighten and loosen nuts. The moments acting at the coffin and fetlock joints of the standing horse are given in Equations 2, 3 and Figure 3.

DFb-(Fa+CEc)=0 [2]

Td-(Fl±CEe)=0(1) [3]

Figure 3: The moments acting on the digit, specifically at the
coffin and fetlock joints. The symbols are defined in the text.

Just as there is equilibrium of linear forces, Equation 1, so there is equilibrium of moments. This equilibrium is taken around a center of rotation that is in the distal end of the middle phalanx (short pastern bone, P2)(2). The linear force, the vector F, acting at a right angle to the moment arm, a, generates the clockwise moment, -Fa (3).

The common extensor tendon (long extensor tendon in the hind leg) and the extensor branches of the suspensory tendon (4) (5) also exert a clockwise moment, -Cec (6). These clockwise moments are equilibrated by the linear force, DF, acting around the moment arm, b, generating the counterclockwise force, DFb. T represents the total linear force of the suspensory tendon plus the superficial and deep flexor tendons.

It is important to emphasize that there is only one center of rotation (locus of rotation) in the foot. Once a shoe is nailed or glued to the hoof, the shoe becomes mechanically part of the foot.


By inspection it is clear that the moment arms, b and c, are anatomically fixed and unchanging. The moment arms, a, l, and e are not fixed. We first find the line of action of F for the standing horse, stationary foot, Figure 3. The resultant, R, is the vector sum of F and H, the latter the frictional resistance to sliding forward of the hoof on the surface as already noted. R is, as well, the actual line of action of the force coming down the leg (that portion of the body weight borne by that particular leg). As is apparent in Figure3 the intersection of R with the surface sets the position for the line of action of F. R normally is parallel to the horn tubules of the hoof wall no matter the position of the hoof or the stage of movement.


It has been known at least since the late 1800s (Lungwitz) that the angle of the pastern with the surface becomes more upright if the angle of the hoof, as measured at the toe, is decreased while the angle becomes more sloping if the angle at the toe is increased. Such angle changes can occur by trimming, wear, or appropriate wedging.

A recurring question has been the use of changes of hoof angle in the treatment of the several types of tendon damage. The immediate response to decreasing hoof angle is an increase of DF, the tension in the deep flexor tendon. This “pushes” the fetlock up and forward, making the pastern more upright and tending to decrease the tensile forces, SF and SL. As discussed below, however, there is little or no change in SF and only a small decrease of SL. Once the pastern moves up the fetlock joint opens, its dorsal angle increases, and DF tends to decrease. The end result is a modest decrease of tension in all three palmar or plantar tendons.

Rooney (1969) pointed out that the superficial and deep flexor tendons are tightly bound together and to the cannon bone by strong deep fascia. One can, for example, sever the superficial flexor tendon either at the check ligament or below the carpus/tarsus and have no loss of tension in the superficial flexor tendon distal to the site of transection. One can sever the deep flexor between the fetlock and coffin, and the tendon proximal to the cut will remain tense. This means that whatever angular changes of the hoof cause tensile change in the one flexor tendon will cause change in the other tendon. Thus, when the deep flexor tendon tightens with lowering of the hoof angle, the superficial flexor tendon would loosen if the two tendons were not tied together by the deep fascia. The deep fascial ties cause the superficial flexor to tighten as the deep flexor tightens even as the opening of the dorsal angle of the fetlock causes the superficial flexor to loosen. The net result is cancellation and little or no change in SF.

Similarly, with an increase of hoof angle, DF decreases and SL and SF should increase. The superficial flexor tends to tighten with decrease of the dorsal angle of the fetlock, but it tends to loosen because of the deep fascial ties to the loosening deep flexor. Again, the net result is no change in SF and a small increase of SL.

In vitro, at least, these fascial connections remain intact after hours of continuous, cyclical loading and unloading.

These observations are relevant to the erratic and variable results of in vivo and in vitro measurements of tendon force that have been reported in the literature. In none of those reports was the importance of the deep fascial interconnections taken into account.


The question remains as to the desirability of changing hoof angles, and so tendon tension, in the treatment of tearing of the superficial flexor tendon (bowed tendon), the suspensory, or the check ligament of the deep flexor. From what has been presented it seems clear that there is little or nothing to be gained by changing hoof angle in the treatment of bowed tendon or suspensory tendon damage. The decrease of tension in the deep flexor with a larger hoof angle might be of value in the treatment of tearing of the check ligament of the deep flexor. There have been no measurements of the tension in that ligament, but the deep fascial “cancellation” of changes in the superficial flexor suggests that the identical or very similar situation would pertain with the check ligament.


Can different types of shoe assist tendon healing quite apart from changes of hoof angle?

Extended toe shoes are used for animals with so-called contracted tendon or tendons (7).

Figure 4: The extended toe shoe with F in a new position
when there is any pitching (8) of the hoof (increase of hoof angle).

Extending the toe of the shoe moves F forward and, so, increases a, Figure 4. The larger clockwise Fa can, then, resist the counterclockwise moments exerted by the shortening tendons. Obviously, no amount of toe extension can stretch the shortened tendons to normal length unless the extended toe is raised from the surface. While such strategies can be employed as adjunct therapies, it is well known that decreasing the rate of gain of weight of the affected young animal is the immediate and important strategy.

The extended toe shoe can also be helpful during the healing of transected long extensor tendons in the hind leg - a frequent site of traumatic transection. In equation 3 CE is lost, and the extended toe allows F to be farther forward, so that Fa is larger, replacing the lost CE. As is well known, the severed tendon will adhere to the periosteum on the dorsal face of the cannon bone and act in the manner of a check ligament with virtually full restoration of normal function of the long extensor at the fetlock.

It is important to recognize that the effect of the extended toe shoe (and the egg bar as will be discussed) occurs only when there is movement of the foot. That is, the extended toe shoe works only when the hoof moves, so that the angle of the hoof increases, Figure 4. If the foot is absolutely stationary on the surface, neither extended toe nor egg bar shoes have any effect whatsoever.


The straight bar shoe and egg bar shoe have the same mechanical characteristics, but the egg bar extends farther back and, so, exerts more moment.

If the deep flexor tendon is transected, the toe of the hoof comes off the ground. In equation 2 and Figure 3 DF is no longer present, only Fa and CEc. Obviously, Fa cannot lift the toe off the ground, and this is done by CEc, the action of the extensor tendon and extensor branches of the suspensory tendon. The mechanical situation is shown in Figure 5, the line of action of F having moved toward or even at or behind the center of rotation.

Figure 5: With the deep flexor tendon severed, the CEc
moment raises the toe off the ground, and the line of
action of F moves toward the heels.

An analogous situation is the foal born with so-called flaccid flexor tendons of the hind legs. The cause of that condition is not known. The tendons appear to lack tensile strength. The situation corrects itself with time in many foals. To deal with this before natural correction occurs, an egg bar shoe is applied, so that CEc is equilibrated by the counterclockwise moment provided by -Fa, Figure 6, in place of DFb, and the toe is pressed back to the surface. The loose or flaccid tendon is mechanically equivalent to a transected tendon and the toe will be off the ground with the fetlock often resting on the ground. The egg bar shoe acts in the same manner as with a transected tendon and helps the foal with flaccid tendons achieve a more normal conformation until and if the tendons mature appropriately.

Figure 6: The egg bar shoe, extended heel. The reverse of
the extended toe shoe. The position of F when the hoof
pitches backward (hoof angle decreasing).

Suspensory desmitis is a degenerative condition afflicting the hind legs of, particularly but not solely, Paso Fino horses. It can be at least ameliorated by the use of the egg bar shoe. In this case the suspensory tendon is “degenerating”, losing tensile strength and allowing the fetlock to sink toward the ground (9). This is similar to the situation with older multiparous mares in late pregnancy. When the foot tips back, so that the egg bar exerts a counterclockwise moment around the coffin joint, DF is reduced and the deep flexor tendon shortens. It appears then that the beneficial effect of the egg bar is to allow shortening of the deep flexor, so that it is supporting additional load, some of the load that can no longer be borne by the damaged suspensory tendon.

It is apparent that the egg bar moves the line of action of F toward the heels if the toe is tipped up and off the surface. As F moves toward the heels, a decreases until F moves behind the center of rotation when the moment, Fa, reverses, becoming +, counterclockwise.

When the horse is standing still the moments are always present and in equilibrium: DFb -(Fa+CEc)=0. The slightest movement-pitch of the foot-throws the system out of equilibrium. Toe extension or egg bar simply exaggerate such responses to loss of equilibrium by moving the line of action of F when the pitching occurs.

Any value of a shoe in the treatment of acute tearing of tendons - bows, suspensory, check ligament – could only reside in decreasing tension in the involved tendon. We have seen, however, that changing hoof angle is of little or no value in decreasing tension in the superficial flexor and suspensory and doubtful for the check ligament of the deep flexor tendon. The only other possible strategy is to move the line of action of F toward the rear, toward the egg bar, thereby decreasing DF and allowing the deep flexor tendon to shorten.

The immediate question that arises is: why should the line of action of F move when the suspensory or superficial flexor has been damaged and an egg bar shoe is applied? Indeed, does an egg bar have any effect on the normal foot and leg?

We approach this by examining first what a horse does without shoes or with its usual shoes when there is damage to one of the tendons. There are several possible responses to the pain. The immediate response, of course, is to decrease the load on the damaged leg/foot - decrease F. Doing so, the leg tends to straighten at the coffin and fetlock, thereby reducing tension in all tendons. With reference to Figure 3 both a and l, the moment arms for the coffin and fetlock joints, decrease and, so, the equilibrating tendon forces will decrease. After the acute pain subsides, no matter its origin, a horse may either stand normally (but with reduced F) or in the so-called “standing-back” position. When the horse stands-back the pastern becomes more upright, and the line of action of F moves back and the moment arms, a and l, decrease and the moments Fa and Fl decrease just as when F simply decreases. Obviously, the tension in all three tendons decreases.

Now for the egg bar: how can it effect either of the above situations? First, there is simple reduction of F, decreasing the body weight on the affected leg, and the bar shoe adds nothing. The second case, standing back, seems to be more commonly associated with low grade, persisting pain. The line of action of F moves back as the pastern becomes more upright with reduction of tension in the deep flexor. Does the egg bar have an effect in this situation? Not per se. It can have an effect only if the hoof angle decreases (toe up, the foot pitches back).

Figure 7: The standing-back position. The dotted lines
represent the normal standing position and the
solid lines the standing-back.


Egg bar shoes and the closely related trailer shoes have an effect when the horse is moving. The trailer, egg bar, or to a lesser extent the straight bar will contact the surface first with the heel-quarter-toe contact sequence of the faster gaits. With a trailer the foot will tend to yaw (10). While it might appear that the trailer in contact with the surface is acting as a center of rotation, the center of rotation is always at the coffin joint in the distal end of the middle phalanx. At the point of contact of the trailer with the surface, the surface exerts an upwardly directed linear force (in effect F) that acts around the moment arm from the center of rotation perpendicular to that linear force.

The bending of the trailer or egg bar will absorb and dissipate energy. In itself this will help to meliorate the pain of impact of the foot with the surface. The trailer is usually on the outside (lateral) branch of the hind shoe as an aid in preventing or minimizing cross-firing by pacers. Pacers are predisposed to cross-firing by the toed-in and/or toe narrow conformation. The trailer induces yaw, a turning out of the hoof at impact that tends to counteract the inwardly directed toe conformation.


Butler, K D 1985 The Principles of Horseshoeing. Doug Butler. Maryville. Missouri.

Rooney J R 1984 The angulation of the forefoot and pastern of the horse. Journal of Equine Veterinary Science 4:138-143.

Rooney, J R 1999 Surfaces, friction, and the shape of the equine hoof. Online Journal of Veterinary Research 4:73-93.

Lungwitz, A 1913 Horseshoeing. Facsimile Edition. Oregon State University Press. Corvallis.

Lungwitz. A 1910 Leisering u. Hartmann. Der Fuss des Pferdes. 11th Ed. Schaper. Hannover.

Mosier, S M, Pomeroy, F, and Manoli II, A 1999 Pathoanatomy and etiology of posterior tibial tendon dysfunction. Clinical Orthopaedics and Related Research. No. 365, 12-22.


(1) CEe is the moment at the fetlock caused by the extensor branches of the suspensory tendon. If the fetlock dorsiflexes without movement of the coffin joint, the moment is negative. When the coffin joint palmar flexes, followed or accompanied by dorsiflexion of the fetlock, the extensor branches slide distally and the moment becomes positive. Thus the ± in eqn.3.

(2) There is no single center of rotation for any joint. One joint surface rolls on the other forming a locus of points of rotation With little loss of generality it is customary to use a single compromise center.

(3) By convention in mechanics counterclockwise moments are positive and clockwise moments are clockwise.

(4) The suspensory tendon is, in fact, not a ligament but a greatly reduced muscle and enlarged tendon: the interosseous medius. It will be referred to herein as the suspensory tendon.

(5) This study deals with the passive, automatic function of the several tendons. This is valid for the in vitro leg in a testing machine and for the in vivo leg without muscle action. The muscles act with the tendons, but the static equilibrium of the distal part of the leg is primarily a function of the superficial and deep flexor tendons with their check ligaments, and the suspensory tendon. The common and long extensor tendons are tightly bound to the periosteum of the dorsal surfaces of the phalanges, so that they, too, act as if they had check ligaments.

(6) Cec is the moment at the fetlock caused by the extensor branches of the suspensory tendon. If the fetlock dorsiflexes without movement of the coffin joint, the moment is negative. When the coffin joint palmar flexes, followed or accompanied by dorsiflexion of the fetlock, the extensor branches slide distally, and the moment becomes positive. Thus the ± in eqn. [3].

(7) Note that the tendons are shortening and not contracting. Tendons do not and cannot contract.

(8) Pitching, yawing, and rolling of the foot are defined in endnote 10.

(9) There have been no scientific reports on this condition, and pathology and pathogenesis are not known. We deal only with the fact that the fetlock moves down. abnormally far and apparently does not return to normal with time. This condition may be analogous to posterior tibial tendon dysfunction in humans(Mosier et al,1999).

(10) The foot can move in three directions: tipping forward and backward is pitching; rolling from side to side is rolling; and spinning around the axis of the pastern, a movement parallel to the surface, is yaw.

]]> (James Rooney, D.V.M.) Hoof Mechanics and Physics Sun, 11 Jan 2009 03:04:08 +0000
Basic Mechanics of the Digit - Transverse View

In several other essays on this site I have addressed certain aspects of the shape of the hoof primarily from the lateral or side view. That is, order of course, quite insufficient. We should like to have a full dimensional view: sagittal (the lateral view), cross sectional (transverse), and frontal (looking at the foot from directly above or below). These views are shown in Fig. 1. The gray panel indicates the cross section view (perhaps better called the transverse view) and the blue the frontal view. The lateral or sagittal view, Fig. 3, is in the plane of the page, the olive panel.

Figure 1
Figure 2


The material in this essay and in Basic Mechanics of the Digit-2. have been done before, but perhaps a repeat is not a bad thing; besides I like it!.

In the this essay we consider the transverse view.

As everyone surely knows the obvious configuration of the hooves as seen from the front is as shown in Fig. 3; the medial (inside) wall is vertical or very nearly so. The lateral (outside) wall is slanted outward.

Figure 3

Our task here is to explain why this shape. It is quite simple and requires no complicated (or simple) mathematics. A somewhat more difficult analysis is given in the appendix.

Figure 4

Fig.4, a transverse view with the two legs and the eccentric position of the weight, -W, between and exerting a downward and inward pull on the two legs. That action tends to collapse the medial walls of the hoof and slant the lateral walls outward. Since this normal action is always present, we can say that the hoof was “built” in order to have the walls lined up with the forces to be expected. Thus, the medial wall is built in line with the downward compressive force while the lateral wall is built in parallel with the slanting compressive force. In this regard please see, as, well Basic Mechanics of the Digit-1.

Please note, as well, that this configuration assures that the horn tubules in the hoof wall are parallel to the force being experienced by the hoof wall. All this has been expressed in a grievously teleological way, suggesting that the hoof knows what is going to happen and gets ready for it!

We avoid this teleological explanation by examining newborn and otherwise still young animals. These young hooves are nearly symmetrical in the cross section view. The orientation described above is acquired as the animal grows and gains weight and is, therefore, an acquired state, the hoof assuming that shape which best fits the forces applied to it.


This appendix gives a more definitive explanation for the shape of the hoof in the transverse view. The footnote may be of interest as well to the geometrically minded.[1]

Figure 5

This looks horrible but really isn’t so bad. We are looking at the horse from the front with the left fore (lf), right fore (rf), center of mass (cm), body weight (-W), and the reaction of the rf (W) to –W. Now by the trickery of vector mechanics we can show that the down motion of the cm can be represented by two force vectors, in red, one to each foreleg. (We are just doing this for the right fore, the dashed red vector to the left fore only indicating that the same thing is repeated there.)

Actually, we use the red vector to the outer part of the right fore hoof and the green vector to the inside part. The vertical force is the same both outer and inner (and in fact all the way across) as indicated by the vertical red and green vectors. The force directed outward, laterally, the horizontal vectors, is larger on the outside of the foot and smaller on the inside. If we move the two resultant vector [2] forces, red and green, down to the right in the picture, it is apparent that we approach rather well the transverse appearance of the hoof.


[1] The transverse view is mimicked very well by the geometric figure: a trapezium.

[2] A resultant vector is the “result” of adding two vectors together, in this case the vertical vector and the horizontal vector.

]]> (James Rooney, D.V.M.) Hoof Mechanics and Physics Sun, 11 Jan 2009 02:53:38 +0000