On Abscesses

I haven't posted here in an age but I saw a post from someone on a Facebook page about dealing with a hoof abscess. As is common with most issues relating to horse health and management, there were many opinions.

An abscess is a cavity in the body that is filled with pus – a liquid consisting of serum, living and dead white blood cells, tissue debris and, in a septic abscess, a pathogen, most commonly some sort of bacteria.

Septic abscesses are caused by an external pathogen such as bacteria being introduced into the vascular tissue in the horse’s hoof, ie the solar, wall or frog corium. 

This usually happens as a result of a penetrating injury, including a farrier breaching the white line with a nail. Sometimes pathogens gain access to the vascular tissue via damage to the white line, or frog etc. 

There are also sterile abscesses which occur when the body is attempting to expel inert tissue, such as its own dead tissue, for example, a region of corium (aka the hoof quick) that has died as a result of persistent pressure or bruising from such things as long under-run heels or bars that run across the sole.

 

In both types of abscess, inflammation results as the body walls off what its immune system has registered as a foreign body. Serum, containing white blood cells, floods the area and because the hoof (like our nails) cannot stretch in the way that skin can, the pressure from the build of fluid is extremely painful.

 

The white blood cells attempt to break down the invader, and as wound serum dissolves protein, it gradually eats through the skin and is expelled as pus. (This capacity can be seen in the scalding of skin around a wound that is oozing wound serum.)

 

If an abscess breaks internally, ie through the tissue walling it off from the wider body rather than out via the skin, eg by bacteria multiplying faster than white blood cells can kill them, or by the abscess being lanced incorrectly, the infection can become systemic.

 

A hoof abscess typically finds an exit point between horn and skin, at the coronet or at the heel bulb. Occasionally it is resorbed.

The pain from a hoof abscess can be severe enough to mimic a major traumatic injury, or be so mild it is not noticed. 

Fore limb hoof abscesses are always more painful than hind because of the differences in load bearing between fore and hind limbs.

 

Sometimes, in a sub solar abscess, opening the sole can release the pus which usually provides immediate pain relief. 

However, breaching the sole also exposes the blood rich corium to external pathogens, and if the abscess was not “ripe” it can reoccur.

 

Abscesses that are tracking up through the wall are not as easily located or relieved by excision, and the possibility of faster relief from pain has to be weighed against the degree of longer term damage to the hoof.  

On the other hand, a large abscess exit at the coronet in the toe region can take 6 months or more to reach ground level.

 

Swelling in the lower leg may be due to a septic abscess in in the hoof having become systemic but it may equally be due to oedema, “stocking up”, as a result of the horse not moving or loading the hoof.

 

Antibiotics are are unwarranted if the abscess is sterile; and as an abscess is “walled off” from the rest of the body, they may not be effective if a septic abscess.

However, if it is known or suspected that the abscess is septic, prophylactic use of antibiotics may be advisable in case it becomes systemic. (There will be wider clinical signs such as elevated temperature, inappetence etc.)

 

Vets may prescribe nonsteroidal anti-inflammatories (NSAIDs) as analgesia on the grounds that not relieving the severe pain of an abscess or injury is inhumane, but as the inflammatory response which is causing the pain is part of the body’s defences and healing processes, their use can be counter-productive. 

Typically NSAIDs are no longer used in human medicine where wound healing is a priority. 

 

The pain relief used for horses is most typically phenylbutazone, or ‘bute, as it is much cheaper than new generation NSAIDs. 

 

It’s important to know that NSAIDS work by affecting the prostaglandin synthesis that is involved in inflammation, but even modern NSAIDs can also affect prostaglandin synthesis that is involved in vital functions, such as the maintenance of the lining of the gut, blood vessels and the tubules of the kidneys.

 

There is good reason to be wary of NSAID use (especially the old type of NSAID), in horses which are blood volume depleted because of blood loss and/or dehydration, or which have known to have gut problems or kidney issues.

 

Soaking in cold water, padding the hoof, and encouraging the horse to move are important for resolution of the abscess, and for wider health. 

 

On conformation and the connection with hoof form

We know that the conformational ideal for the equine athlete is for the two pairs of hooves to be evenly matched.  We also know the adage – ‘no hoof no horse’ is as true as it is ancient.  Logically, given the different ways the fore and rear hooves are loaded at rest and in movement, and the critical importance of straightness in the equine athlete, vets, farriers and other equestrian experts ought to be alert to any significant deviation in the size and/or orientation to the ground of the hooves, and be equipped to prevent, halt or reverse adverse changes.

If the dorsal angle of a left hoof is 5 degrees steeper than that of its partner, something is seriously amiss because when the dorsal angles of a pair of hooves are significantly different, the way the bones are loaded and the balance between the muscle systems which support and act across joints will be different.

Similarly, given the different primary roles of the two sets of hooves which is reflected in their shape, i.e. the fores carry upwards of 60% of body weight and are rounder, and the hinds are vital for propulsion and are narrower and more concave, the length of the toe in the hooves is critical.  

All too often, in addition to imbalances in the pairs of hooves, we see hinds with long, under run heels and long toes, and fores that have long but more vertical heels and shorter toes.

When you consider the roles of the hooves at rest and in movement, having hind hooves with long, shallow angled toe, and collapsed heel, and fores with a short toe and long vertical heel  is anatomical nonsense and is setting the horse up for all manner of musculoskeletal and metabolic problems.

The hoof is a ‘cast’ around P3, the distal phalanx or pedal bone. The hoof is a truncated, slanted cone in shape, open at the heel region where the hoof wall is attached to thick pads of cartilage which effectively extend the pedal bone rearward and allow the heel region to deform both laterally and vertically under load.

The front limb typically takes about 60% of the horse’s weight. In any given horse the dorsal angles of the pedal bones, when their distal margin is ground parallel, are between 2 and 5 degrees steeper in the hinds than the fore. The front pedal bone is typically 45-48  degrees and the hind around 48-50 degrees. 

The distal (ground most) margins of the pedal bone are fine and because bone yields to persistent pressure, the margins are susceptible to erosion when P3 is exposed to unphysiological pressure and/or impact. Too much load taken to one side or to the front can, and often does, lead to loss of bone. Incorrect loading of the coffin joint can and often does lead to damage to the coffin bone and the ligamentous structures of the joints.

Another conformational ideal that is widely spoken about but woefully misunderstood, is for the cannons to be vertical to the ground when the horse is in halt or,  more importantly for the horse’s long term health and wellbeing, when it is at rest.  

How often do we see horses standing completely square when they are sleeping / resting?  How many horses end up with short toes and high contracted heels in the front and long collapsed heels and long toes in the hinds because they habitually elevate the front heels by steepening the pastern, or pointing a toe, or by standing under in front or behind or both? 

How many farriers, vets and equine experts understand the stay apparatus, not just the mechanics of the patella locking of the hind limbs, but the way the front limb SA operates, and the consequences to the horse of a failure of that vital system?

My advice to ALL horse owners is to ask their farrier, vet and trainer, the people they pay a great deal of money to and in whose hands they place the well being of their horse, to explain the equine stay apparatus to them.

I will lay odds that many of them will not have a clue. Many will also not understand how vital the neutral position of the coffin joint is to the correct loading of the limb.

The adult horse sleeps mostly upright. It is unable to rest lying down without suffering severe system damage. Its ability to rest and repair its tissue optimally whilst upright depends on its ability to fully relax the bulk of its skeletal muscle which it can only do by means of a certain alignment of its skeleton and the passive weight of, and balance between certain muscle groups. A key element in this are the deep digital flexor muscles, and their check ligaments.

The reason the deep digital flexor tendon is so huge and has such a massive insertion point on the underside of the pedal bone is not because of its dynamic function of flexing the coffin joint, but because of its passive function of backing up the suspensory apparatus of the fetlock joint, without which the stay apparatus cannot function properly. And if the SA isn’t functioning, the horse is being set up for musculo-skeletal and metabolic harm. The progress of that harm and the speed with which it progresses is significantly increased in the equine athlete, for obvious reasons.

As an illustration of part of this complex issue,  think about what is happening during the stance phase of the stride.  

Taking the front limb only, the hoof comes to a rapid halt in early stance (initial touchdown) which should be fractionally heel first as the limb is being retracted, and with the coffin joint in or very close to neutral. 

Gravitational force means body weight keeps coming down but the ground reaction forces are dampened and spread by the lateral deformation of the heels which tightens the fibrocartlaginous bands in the digital cushion.  

The bars, frog and sole share load with the hoof wall, and critically, the suspensory apparatus of the fetlock joint allows a hyperextension of that joint which counterbalances the flexion of the coffin and pastern joints.   

The suspensory ligaments, which contain muscle fibres so are more extensible than normal ligament, are backed up by the digital flexor system. As the limb accepts load the direction of loading forces on the insertion point of the deep digital flexor tendon becomes horizontal. 

The toe region of the hoof is stablised by the tightening of the extensor branches of the suspensory ligaments. The check ligaments are ready to cut the muscle belly of the flexor muscles off should the strain on the digital flexor tendons become too great. 

The balance between the deep digital flexor muscle and the extensor system keeps the carpal joint stable; the deep digital flexor muscle, which is under tension, keeps the elbow in extension which in turn ensures the angle of the scapula is optimal as the horse's body weight passes over the planted limb prior to late stance when break over is initiated and the limb is flexed, i.e. the point at which the body has to overcome the resistance of the ground and when too long a toe can massively increase lever forces on the coffin joint. 

The coffin joint is the first joint to experience the forces of deceleration and concussion and also lever forces on break over.  In a very high heeled hoof and/or one with inadequate toe height, the horse starts with a coffin joint that is permanently out of neutral range, so it cannot land with the joint in neutral.  As a result, the critical balance between the various systems is thrown out: ground reaction forces are increased; lateral expansion of heels is reduced (worse case scenario, heels actually narrow under load), or in a splayed hoof, lateral expansion may be too great.  

In both a narrowed and a splayed hoof form, the frog, digital cushion and lateral cartilages are deformed and weak;  the balance between muscle systems – their ability to stretch or play out and to contract in synchrony with their opposites is reduced, leading to increased potential for soft tissue or bone injury.

Nor can such a horse stand with its joints in neutral when it rests which means that in order to remain upright, it  must be in a state of persistent muscle contraction.  Even the highly tendinous biceps and the superficial digital flexor muscle in the hind limb will break down if subjected to a persistent, unphysiological strain, i.e. too much and / or for too long.

Straightness is impossible to achieve or maintain for any length of time and a rider’s efforts to achieve it may lead to greater discomfort and fatigue which increases the risk of injury and anxiety.

I cannot comprehend how anyone could lecture people about the need for balance in diet, schooling, saddle fit, rein contact – whatever – and not comprehend the obvious fact that all of that is negated if the essential balance in the musculoskeletal system is out. As with all things, the problems manifest in different ways and at different times in different horses. 

It's why I bang on about the stay apparatus; given its fundamental importance to the horse, the fact that it remains the greatest area of ignorance is an indictment of the equestrian world. 

A horse with a long contracted heel and/or too short a toe has a persistently misaligned pedal bone and coffin joint. Because the coffin joint rotates back and forth (in the sagittal plane), it is in effect rotated out of its normal, neutral position, even if it is still tightly connected to the hoof wall.  Anatomical fact.  

The optimal balance between the coffin joint and the other joints of the limb exists within fairly narrow parameters and if those parameters are exceeded, harm will result. How much harm, when and where it occurs will vary but harm will be done.

The bars and other complicated stuff

All parts of the equine hoof grow groundward and, because the form of the hoof is a slanted, truncated cone, it also grows slightly forward.  The mysteries of the conveyor belt arrangement - how the hoof wall and sole stay attached as the horn grows, and importantly, why the system fails, are only just being unravelled.

The slanted, truncated cone of the hoof capsule should reduce evenly in its length (linear distance from hair bearing skin to distal margin) and its height (vertical distance from the ground to hair bearing skin) from the toe region to the heel buttress.

You can mark the lines of growth by drawing a line in a contrasting pen down a tubule from coronet to distal margin. Do that all round the hoof at regular intervals and you get a clear picture of the way the hoof is growing and whether there are areas that are growing at significantly different angles to the ground.

The hoof wall is open at the heels.  At the heel buttress, the wall turns inwards and becomes the bars - so the bars are, in effect, a continuation of the wall and share many of the same characteristics eg there is corium at the top of the bar which produces the bar horn and a laminar corium which produces interdigitating laminae that bind bar and sole horn together.

The bars'  outer zone, like the wall, is pigmented (most obvious in a dark hoof) and more tubule dense than the bright white and moister inner zone of horn which interfaces with the sole. At the top of the bars (i.e. at the top of the collateral groove) the corium that produces the bar horn merges with the frog corium. 

The bars both allow and control lateral expansion of the heels; they share load, and they help in deceleration if the wall and bar sit slightly proud of the sole. As well as expanding laterally under load, the heels can also deform vertically when the hoof loads on uneven ground.

This flexibility is achieved because the rearward third or so of the hoof grows from dense cartilage - not bone. The wall in the heel region is a little thinner than the toe region (or should be but often isn't because the toe is too weak) but the wall and bar work together to add substance and stability to the heel region.

Just as the wall proper reduces in length/height and width from the toe region to the heel buttress, the bars - also should reduce in length/height and width from the heel to the bar apex where bar horn merges seamlessly with sole horn.

This region, forward of the apex of the bar and about half way along the frog, where bar horn merges seamlessly with sole horn is a zone unlike any other in the hoof.  The two types of bar corium (coronary and laminar) merge with each other and merge with the sole corium.

In the hoof several different types of horn and other tissue meet: hair bearing skin and capsular horn at the coronet; hair bearing skin and the bulbs and frog; sole and wall horn; sole and frog; frog and bar…. and each junction has properties which ensure it's not subjected to too much strain. 

Eg. the wall and sole junction is buffered both by the white line - which is formed by terminal papillae at the distal margins of the coffin bone - AND by the change in the structure of the wall to a less tubule dense, more  moist horn that makes the inner zone of the wall more like  sole horn.   The frog/bar junction is buffered by the interdigitation of laminae and by the elastic and dampening properties of the frog (the region of the bar corium is aligned with the twin folds of the frog in its rearward third). The sole/frog junction is buffered by the elastic frog and by solar concavity - and so on.

We don't actually know much about how these coalescing zones function - what is obvious is that they can become problematic if they are exposed to severe or long term unphysiological stresses.

As a slanted cone, the hoof should diverge towards the ground, i.e. have a smaller circumference at the coronet than at the distal margin. In order to be able to deform laterally under load, the heels have to diverge to some degree. 

If the heels are vertical, ground reaction forces will result in them being deformed upwards. 

If they are beyond the vertical i.e. they slant towards the midline of the hoof, they will tend to narrow under load which compresses the bulbs and the frog. 

If they are too shallow (under-run), impact forces will result in too much lateral or forward spread and the heel horn may completely collapse and lie over the sole.

Each region of the hoof has a certain optimal angle of growth.  This may be a range and it can change according to environment etc, but it is a fairly narrow one. If hoof form is too steep or too shallow, or there is too great a difference between sections of the same hoof, problems will arise.

For example, it's often not understood that, in a splayed or 'forward running'  hoof, where wall and bar sit at too shallow an angle, the sole also will have too shallow an angle of growth.  If the hoof is too steep and convex, the angle of growth of the sole will be too steep.

The angle of growth of the bar is always matched by the angle of growth of the heel. The steeper the heel is, the steeper the bar; the more sloping the heel is, the more shallow the growth angle of the bar is.  If a heel is bent inwards beyond the vertical, the bars will also be bent beyond the vertical and / or it will form a curve. If the heel has completely compressed and is lying flat, so is the bar. 

As with the wall, the angle of growth of the bar determines how the different zones of the bar meet the ground and how loading and abrasive forces affect it.

A feral horse can cope with all manner of pathology in its hooves if it lives in a benign environment but if its environment is harsh and it cannot adapt its hooves to cope with the demands on it, the horse will not survive. 

In domesticity, people often ask horses with severe hoof pathology and all manner of related conformational issues to cope with being ridden or driven and they use shoes and boots to allow them to push the horse past the point it would be able to go to if it had bare hooves. Sometimes the demands people make are too much and the domestic horse also does not survive - because it is put it down. 

Some specific issues:

'The bars help form the sole' 

In a hoof with an 'white' wall and sole, it's not always easy to distinguish the inner zones of the wall from the sole but in a dark hoof, the unpigmented region of the bar is very obvious. If the bar does contribute to the production of the sole in the way that Bowker suggests,  given the bar has an inner bright white zone, all hooves with a dark sole should have areas of bright white in them running forward from the bar apex towards the toe. They don't. 

I don't think it's as simple as 'bars help form the sole' - and there is the question of which bit of the bar, given the complexity of the junction at a cellular level. We just don't know enough about the bar-sole junction to be able to say for certain what is going on in any given hoof. 

I have here at the moment, one horse with all black hooves, two with one white hoof and one with one black hoof. The one with all black hooves has no white horn forward of the bar apex - although when I first started trimming him he did have pools of white horn that spread out from the bar apex. The horse with one black hoof has areas of white sole in his black hoof but their location never alters and it forms discrete patches towards the toe.  In the two with one white hoof, the black hooves have no white at all.  In the white hooves of course the outer horn is not pigmented or not heavily so. (See post on pigmentation)  I have seen bicolored hooves with white soles that have dark bars and dark horn running from the bar apex all the way round the tip of the frog. Is this bar? Some would say yes, some no. 

The effects of ground reaction forces

The hoof and the structures it protects have to be able to deal with massive deceleration and impact forces - and the persistent pressure of rest stance. 

When a horse stands on a level, hard surface, the toeward region of the anatomically normal bar in a normal hoof is passive when the horse is in rest stance. The buttress region of the bar shares load with the wall, which shares load with the rearward third (folded) region of the frog and the sole adjacent to the wall.  The concave nature of the hoof means the sole under the sharp distal margin of the pedal bone is well protected. 

But the world is not usually completely level and smooth and on a conforming surface the load is more evenly shared, and under maximum impact on any surface the entire volar surface shares load. 

However, the outer regions always impact first and carry most load; maximum impact is fleeting on any given hoof, and concussion should be damped/dissipated by many different mechanisms.

Problems for the horse arise when any of those mechanisms is impaired and any given part of the hoof receives too much load at a given point or over time.

For example, in a severely contracted hoof the bars may not touch the ground on a level surface, or a shoe will lift them out of ground contact even further  but - the internal structures of the digit - specifically the region of the navicular bone - can be impacted by the bars even if they never actually touch the ground, even on a flat surface where they are sitting above the ground because gravity ensures that body weight pushes downwards onto them.  If the digital cushion is weak and suspensory apparatus is compromised,  the situation is worsened.

Photographs of a dissection of a foundered hoof with ingrown bars are shown in this blog post. I have seen far worse impacted bars in foundered hooves. The worst I have seen were in a pony, whose bars  were arched up into the hoof past the level of the coronet and curved inwards under the frog. The pony had spent years denied grazing and movement because she was prone to laminitis. No-one responsible for her care understood the connection between her metabolic and endocrine issues and her persistent attempts to avoid loading her increasingly painful long and contracted heels and bars.

The bottom line is that the bars are a critically important part of the hoof - in horses which cannot grow, load and wear their hooves optimally or have them trimmed optimally, the bars can grow in ways and into places which impact on sensitive structures and affect the correct functioning of the hooves and associated muscle-skeletal structures. The long term effects can be profound.

Dancing horses.

I haven't posted in this blog for a long time but I have not lost my interest in or commitment to equine welfare - so here's a link a horse post on my other blog :

The extensibility or otherwise of tendon ....

Most studies of muscle/tendon extensibility in live subjects consider the muscle and tendon as a unit - for obvious reasons.  Histological studies of cadaver tissue have to replicate the complex biomechanics of the living animal - which is always difficult. 

Plus there's the fact that, just because something can stretch in certain situations, doesn't mean it should. 

When considered as a unit - the horse's digital flexors are highly extensible and it is agued that the digital flexor tendons themselves stretch by up to 10% of their resting length. 

Most skeletal muscle has a voluntary action - ligament doesn't and nor does tendon in isolation from its muscle; their action is automatic.  Joints have a certain range and direction of motion - if they go too far away from that, damage occurs.  To avoid excess strain (to the joint and other bones and muscle/tendon units), the voluntary control of the muscles which act across a given joint needs to be precise. 

This is why - as I always understood it - tendon is practically (in the sense of essentially) inextensible. Any significant degree of involuntary stretch in the tendon would reduce precise control.

Muscle both effects and controls movement by contracting (pulling) and playing out (lengthening).  It also can have a passive, stabilising (semi-automatic) action - and that's nowhere more obvious and important than in the role of the digital flexors in the horse. 

The front limb digital flexor muscles are part of the fetlock suspensory apparatus and, although they are normal contractile tissue and their action is voluntary,  the DDF muscle in particular has an important passive action and, because of the check ligaments, both muscle systems can act automatically - to a degree.

The digital flexor muscles have check ligaments that run from tendon to bone (cannon and radius) which allows the muscles to back up the suspensory ligaments by cutting off the muscle belly from excess strain -ie diverting it to bone. 

The suspensory ligaments can stretch massively because they are not true ligament - ie they contain muscle fibres - but their action is completely automatic i.e. the horse doesn't voluntarily lengthen or shorten them - which is why they are backed up by the flexor muscles. Without strong and balanced muscle systems, the suspensory ligaments will receive too much strain - acute or chronic - and break down. If the suspensory ligaments are too slack or are injured, the digital flexors receive too much strain.

If the digital flexor tendons were capable of a significant degree of automatic stretch, in addition to the extensibility of the muscle itself, why would the check ligaments be needed? And wouldn't an automatic stretch of the flexor tendons place the check ligaments at greater risk of strain? Ditto the suspensory ligaments?

In addition to aiding the automatic function of the suspensory ligaments in the fetlock suspensory apparatus, and the dynamic function of flexing the coffin joint, the deep digital flexor has a passive role in keeping the elbow joint in optimal extension in rest stance.  A certain tension of the DDF tendon and the weight of the DDF muscle (plus correct operation of the SDF system) hold the elbow joint in an optimal extension in rest stance - without which the horse cannot maintain optimal shoulder joint and scapula angles - and as a result cannot fully relax its skeletal muscle whilst upright. The consequences should be self evident but sadly are not.

The extensibility (the ability to lengthen and shorten) of the flexor unit  is (or should be) in the muscle - and a 10% automatic stretch of the tendon itself would reduce the flexor muscle / tendon unit's efficiency and precision in all its critically important functions.

This does not mean that the tendons are just passive cords - they store and release energy but not in any simplistic sense. Also - is the energy store and release of a compressed and released 'spring' different from that of a stretched and released spring?

We always need to remember that the horse has evolutionary imperatives which do not always fit with our ideas of what it should be able to do.  Left to its own devices it will seek to conserve energy and avoid injury and it will not normally choose to place the excessive stress on the suspensory apparatus of the fetlock joints which we impose on it by asking it to 'bounce' up and down with a rider on its back, gallop long distances or leap huge obstacles.

Have we bred greater tendon elasticity into horses as we have bred for longer legs, greater size? 

The greater the elasticity in a tendon - the less precise the control over the joint it attaches to. For the horse, the added force that might be gained from elastic tendons may be paid for in the greater potential for injury of a reduced control of position.

The much neglected stay apparatus

The stay apparatus (SA)  is a woefully neglected aspect of the horse's physiology. Ask even specialist vets about the stay apparatus and they are likely to describe the arrangements in the hind leg. I have even heard an equine vet state there is no such thing as a forelimb SA. When asked if he had any comment about a racehorse which was standing like a goat on a rock he said that was how the horse liked to stand.

Put simply - if a horse cannot engage its fore and hind limb SA, it is in trouble. The form that trouble takes, how bad it is and when it strikes are all dependent on the individual horse but trouble there will be.

Whether the trouble will be attributed to a failure of the SA is another matter  and therein lies the BIG issue for me.

A horse which falls down when sedated, a horse which buckles at the knees when it doses off, a horse which persistently stumbles, or which buckles when one leg is lifted off the ground - all are likely to be suffering from muscular weakness / discomfort  originating in a inability to engage the SA optimally. And the origin of that is very likely to be in the hoof. Even if it hasn't originated in the hoof, chances are the hoof will be a major part of the problem.

The horse is a prey animal; its defence is flight. It has evolved the incredible evolutionary advantage of long light weight limbs which grant speed and conserve energy. It also evolved the ability to rest mostly upright, which gives it a standing start. It achieves this feat by placing its limbs in a certain arrangement that utilises body weight, tendinous muscle, ligaments and fascia to hold the skeleton in balance - thereby allowing the bulk of skeletal muscle to fully relax without the animal falling in a heap.

Skeletal muscle is, for the most part, about movement; muscles act across joints to move the limbs. Muscle is not efficient at carrying direct persistent load. You can try this yourself by standing up and leaning forward as far as you can without falling over. See how long you can stay like that. Or lift your shoulders up towards your ears - and hold them there. Stress posture is used by some unpleasant humans because it is a very effective form of torture that doesn't leave visble scars.

The horse that is unable to fully relax the bulk of its skeletal muscle whilst upright cannot have healthy muscle. Asking it to then carry the load of a rider and tack and perform athletically is going to cause harm - of some sort, to some degree, at some point.

If horse owners and riders don't know this, there is some excuse. If they do know and they persist - there is no excuse.

Finally, to that vast array of professionals out there who make their living out of horses, if you don't understand this, may I respectfully suggest you take up golf.

Some more thoughts on the ground parallel pedal bone

The coffin joint is the first joint in the body to experience ground reaction forces (GRF).  The light weight pedal bone (P3) with its fine distal margins, is especially vulnerable to damage from both impact and unphysiological pressure.  The palmar/plantar processes of P3 are finer than the main body of the bone.  Everything about the way the hoof and limb should respond to load suggests the need for protection of the back of the foot from too much load; so why would or should the coffin joint extend further on touch down or as peak strain is experienced?

The common argument is that, to protect the coffin joint from extending below ground parallel under load (tilting backward), P3 must sit at a positive angle in rest stance.  There is no consensus on what this angle should be but, because the front limb carries about 60% of the horse's body weight at rest and is most prone to damage, the assumption is the fore limb P3 should sit at anywhere between 3 and 10 degrees above ground parallel. This often results in the hind hooves having the same or an even shallower angle than the fores  and in fact, long heeled, steep angled fores and low heeled shallow angled hinds are disturbingly common.

Because the horse gets most of its rest upright, and as a prey species its key evolutionary imperative is to conserve energy and avoid injury, its skeletal balance is critical to its overall health.  The further away from ground parallel the pedal bone is, the greater the impacts are on this essential balancing act.  High heels with a steepened P3 throw the balance out; heels that are too low and/or the presence of heel pain which the horse alleviates by for example elevating its pastern, will also throw the the balance out. 

The evolutionary advantage of a long limb with one hoof attached to it and no weighty muscle below the knee/hock, has the potential disadvantage of the loss of the shock absorption that is conferred by the pads and muscles of the multi toed limb. This is why the horse has a range of impact dampening and energy absorbing mechanisms - including the suspensory apparatus - to  dissipate the potentially harmful effects of concussion on bone and of excessive torque and lever forces on soft tissue. 

When in neutral (rest stance) the fetlock joint (FJ) is already extended. It is held in permanent extension by the suspensory ligaments (SL) and by the passive action of the digital flexors which have check ligaments that run from tendon to bone to cut off the muscle belly from excessive strain experienced as the fetlock hyper-extends. When the FJ is at full extension - the pastern can be at, or even beyond parallel to the ground, but, like many other things, just because it can, some of the time doesn't mean it should, all of the time.

The leaf spring action of the pastern is facilitated by the loading surface of the FJ being increased by the proximal sesamoids, to which the SLs attach. The SLs contain muscle fibres which allows a far greater elasticity than normal ligament. 

Ligaments in the coffin joint run from the distal sesamoid (navicular bone) to the back of the short pastern bone (P2) and the long pastern bone (P1). These are described as a suspensory apparatus also, BUT they are normal ligament and it seems to me that they do not facilitate extension of the coffin joint but limit over extension.   

The coffin joint is not capable of anything remotely like the extension enabled by the suspensory apparatus of the FJ. It's greatest range of motion beyond neutral is flexural; the fetlock's greatest range of motion beyond neutral is extension.  And the coffin joint's action is the first to be affected by the impact and resistance of the ground.  

Taking the front limb only in order to simplify things, the stance phase of the stride starts when the hoof touches down - which it does as the limb is being retracted and is travelling forward and downward.  The hoof touches down heel first but only fractionally. The faster the pace, the greater the load but the more fleeting any given part of the phase is. 

There is a very rapid deceleration as the hoof is planted - it goes from movement to planted in milliseconds and huge concussive and torque forces are at work. The faster the pace the more rapid the deceleration is.  Initial impact is dampened by several elements: the lateral expansion of the heels, the soft tissue of LCs and frog/DC complex, the blood in the hoof etc. 

GRF peak is in the middle of the stance phase when the full body weight passes over the planted  limb. Assuming all other things are equal - cannon and radius are vertical to the ground; the CJ and PJ are flexed (they should work together) and the FJ is hyper extended, with the SLs, backed up by the digital flexors, taking the load. This is the point of greatest strain on the suspensory apparatus of the fetlock joint.

The SLs have extensor branches which merge with the common extensor tendon at the extensor  process of P3.   As the SLs reach peak strain these extensions pull back on the extensor process and have the effect of making the toe more rigid -ie they help to stabilise the CJ.

Late stance is the initiation of break over where the body has to overcome the resistance of the ground on the toe of the hoof. The longer the toe and / or the deeper it has dug into the ground, the greater the effort required and the greater the lever force on the toe. 

How the limb actually meets the ground depends on a number of factors. The  fractionally heel first landing at speed is only visible under slow motion. 

The angle of the scapula is determined in part by conformation but also by the angle of the elbow - which is highly influenced by both the passive and active functions of the digital flexors and the deep digital flexor (DDF) in particular.  The weight of the DDF muscle, held in passive tension by the correctly tensioned DDF tendon, holds the elbow joint in extension. Change the insertion angle of the DDFT in the back of P3 or damage the DDF muscle and a domino effect is created.

For example, a long heel (or a too short toe) results in a steeper hoof-pastern angle. This alters the angle of insertion of the DDFT and with that the degree of elbow extension which results in a steeper angle of the scapula as the shoulder joint changes angle to accommodate.  With a steepened scapula the horse cannot raise its forearm as far; in effect the protracted limb is straighter and lower and the toe flips up - something which some people think is a good thing because it is seen as necessary for the much desired and grossly misunderstood  'heel first landing'.  A toe flip is NOT a good thing.

In other words, if there is too much extension in the knee and fetlock joints during protraction, the horse cannot swing the limb forward as far and the toe flips up, the coffin joint may be too extended when it comes into land. This is as damaging to the joints as the opposite where the CJ is flexed at touch down and the hoof lands toe first, and it is a situation in which the back of the CJ is highly likely to be stressed. 

Everything I know - and I may be missing some vital piece of the puzzle -  says to me that neutral position for the coffin joint is when P3 is at  or very close to ground parallel when the horse is in rest mode - ie that point when the musclo-skeletal system is in an alignment that best permits the relaxation of all major muscle groups.  The coffin joint needs to be in, or very  close to neutral at initiation of the stance phase as either too flexed or too extended risks stressing the CJ which has the capacity to absorb some stress - but nothing like that which the fetlock is structured to absorb. Even that will break down if the peak GRFs are too much for it.

Dynamic action, however extreme, is fleeting compared to the persistent loading in rest stance but it can (and does) result in catastrophic injury if peak strain exceeds the limits of any given part of the system. The ligaments in the horse's joints are so strong that bone fractures are more likely than dislocations.  In rest stance any damage done in movement is repaired. If the rest stance is compromised and as a result  repair of damage is impaired or slowed, the animal is more at risk of traumatic injury when peak load is experienced. There are also a whole constellation of other consequences. 

If the caudal hoof is deep and strong (ie vertical integrity of heels and bars, robustness of lateral cartilages, and of the frog/digital cushion complex); the suspensory apparatus is healthy, and there is a balance between strong healthy flexor and extensor muscle systems - I cannot see why P3 can or should tilt caudally either on initial impact or when the suspensory apparatus is under peak strain.   If it does occasionally do so in a healthy hoof which is being subjected to an extreme load, the structures of the CJ should be able to absorb it.  Problems arise when less than optimal structures are exposed to extreme load and this is made much worse when that load is repeated over and over.

All other things being equal, the horse will seek to conserve energy and avoid injury. WE make it do stuff that runs completely counter to that key imperative. We force it to run faster and further than it would ever choose or need to in nature; to leap obstacles it would prefer to go around, and to maintain display postures past the point even the most testosterone fuelled stallion would choose to - with the added burden of saddle and rider and the effects, on respiration, of a bit in its mouth. 

Too often the demands of utility - our needs - ride rough shod (pun intended) over those of the horse.   And too often healthy debate about these issues founders on the rocks of sectarianism and vested interest.