The "Physiological Trim"

What it is and why it could be good for your horse

This is the foot of a 6 yr. old Welsh cross who came to me with badly overgrown bare feet. Note the shrivelled, poor condition of the frog, as well as the underslung heels and horribly long toes. Despite all that, with just a few months of good trimming, she became the best "rock crunching" trail horse I've ever ridden. Not a whole lot of concavity in her feet, but even so, she could walk over the worst trails with tons of awful rocks and not ever flinch, despite carrying a rider who was really too big for her (me!) Note how nice her frogs became!

It is easy to get confused these days when it comes to what constitutes good hoof care. Various farriers and barefoot practitioners advocate an array of methods that generally sound logical enough when you listen to each individual, yet their techniques often stand in complete contradiction to one another. What is the average horse owner to do, and who are we to believe? While there will always be people ready to dispute any assertions, a few intrepid souls trying to come up with definitive answers through hard science – and their findings are starting to find their way into mainstream hoof care. One such man is Robert Bowker, VMD, director of the Equine Foot Laboratory at the College of Veterinary Medicine at Michigan State University. Bowker’s groundbreaking research on the physiological function of the equine foot has led him to define what he calls the “physiological trim”, a set of trimming guidelines that is being utilized with great success by hoof care professionals in a variety of “camps”. Still, Bowker’s approach may seem strange to some, though much of what he advocates is not really new at all. What is new is that his work is helping to illuminate why and how these trimming parameters may be helpful in the prevention and treatment of hoof-related lameness.


The goal of the physiological trim, Bowker explains, is to “allow the foot to dissipate energy with maximum efficiency, while at the same time providing maximum support.” To achieve this, Bowker outlines these basic principles:

- The frog should be in contact with the ground. This usually means that very little if any frog tissue is removed during a trim.
- The bars should be weight-bearing and just a millimeter or two below the level of the hoof wall. Most horses will require little if any trimming of the bars.
- The live sole should not be “lowered” or removed. The sole should share in weight bearing, along with the frog and bars, when the horse is standing or moving on a conformable surface.
- The toe should be short, with approximately 1/3 of the hoof in front of the true apex of the frog, and 2/3 behind the apex.
- The toe should be bevelled to facilitate breakover.
- The coffin bone should be slightly higher (2-5 degrees) in the back.
- Horses should be kept barefoot whenever possible; if shoes are necessary for certain events or activities, remove them afterwards.
- All changes aimed at achieving this or any trim should be made gradually.

In a healthy hoof, you will see that there is more mass behind the widest part of the hoof (drawn here in red) than in front of it. In the distorted hoof on the left, you can see that the backs of the heel buttresses (drawn here in blue) are well forward of the heel bulbs, and the toe has also been "pulled" forward. The heel has contracted, and the frog is narrow and would likely be prone to thrush and other problems due to poor circulation and lack of stimulation. Also note the unhealthy, "C" shaped curve of the bars, vs. the much straighter bars on the healthy hoof on the right. This distortion can be the result of the heels pushing forward, as in this illustration, but "layed over" bars can also result in somewhat of a  C shaped bar.

In addition, Bowker recommends that horses be kept on a surface that has enough give to conform to the solar surface of the foot (engaging all structures in weight-bearing), but enough firmness to provide beneficial pressure/stimulation to the frog, bars and internal structures of the foot. Many find pea gravel very suitable to this purpose, and even adding it to small areas of a large pasture or paddock can make an appreciable difference.

According to Bowker, the combination of the physiological trim with plenty of movement over the right kind of footing is the best way to create a foot with optimal biomechanical function, a foot that protects the bones and soft tissues from injury while enduring tremendous thrust and load-bearing forces. Though Bowker’s studies have led him to conclude that the greatest benefits are achieved when horses are left barefoot, he feels that even horses that are regularly shod would likely benefit from the other aspects of this trimming protocol.


Whether a horse is shod or not, Bowker emphasizes the importance of the frog making ground contact and the horse landing heel first. A horse with well-balanced, healthy feet will land slightly heel first most of the time (the exceptions possibly being when travelling uphill or at a slow walk), something easily observed in feral horses and virtually all sound domestic horses. The physiological trim facilitates heel-first landing and good frog-to-ground contact, both of which are important for several reasons.

For starters, researchers including Bowker have demonstrated that repeated heel-first landing with the frog in contact with the ground is necessary for the development of the interior structures of the foot, including the digital cushions and lateral cartilages. These structures are essential for support of the bones and ligaments in the foot and lower limbs. Lack of development of these structures, which is seen in many domestic horses, leaves the horse more vulnerable to injury and progressive problems like navicular disease.

Having the back parts of the foot working as Bowker describes has the additional function of allowing the horse to fully sense the ground and the position of its foot in relation to the ground, which also aids in the prevention of injury. This is made possible by the presence of special sensors called proprioceptors, which are only present in the back part of the foot. “The proprioceptors in the back of the foot tell the horse which muscles it needs to use when, and allows for correct extension of the limb,” says Bowker. “There are other kinds of sensors in other parts of the foot that perform different functions, but I believe Mother Nature put the proprioceptors where they are because it was intended for the foot to hit the ground heel first. If the back part of the foot is not the first thing to make contact with the ground, the horse will have less ability to adjust in response to the presence of potential hazards such as rocks or uneven surfaces.”

Perhaps the most important reason why Bowker feels it is so important to have the frog on the ground has to do with its role in energy dissipation. As he explains, “It is critical to have the frog on the ground, not only because this stimulates the development of the structures inside the foot designed to support and protect the bones, but also because the frog itself dissipates part of the energy of impact through its fibrocartilaginous inner mass, if the frog has been allowed to develop properly. That fibrocartilage contains a lot of proteoglycans – which are compounds that bind water – and it’s that fluid that dissipates the energy.” Proteoglycans are complexes of giant molecules that ‘grab’ large amounts of water to form a hydrated gel which fills the gaps between the tiny fibers of the functional parts of connective tissue, cartilage and bone. “Think of the gel pads used in many high-end running shoes,” Bowker says, “and you’ll get some idea of how this helps the frog to dissipate energy.”

In the photos above, notice the difference in the quality if the frog between the April and July shots. In April, the frogs were somewhat shriveled from lack of ground contact due to overgrown walls, but they are more robust and healthier in July, after only three months of physiologically correct trimming.


The frog, with its fibrocartilage and proteoglycans, however, is only part of the story. Another key factor in the efficient dissipation of energy is perfusion – the delivery of arterial blood to the vast network of microvessels in the hoof. Good perfusion is crucial to the dissipation of energy, for as Bowker says, “Ask anyone in biomechanics and they will tell you that passing energy through fluids that are moving is the best way to dissipate energy.” Bowkers studies on blood flow in the equine foot have demonstrated that the best perfusion is achieved in barefoot horses with the back structures of the foot on the ground and the solar surface sharing in weight bearing on a conformable surface.

Perfusion decreases markedly with peripheral loading (loading the walls) that lifts the frog, bars and sole off the ground. Common shoeing practices often create peripheral loading, but as Bowker points out, “It can occur in barefoot horses as well if you leave the walls too long, so just leaving a horse barefoot does not mean you will necessarily have a properly functioning foot.” Shoeing may also decrease perfusion by reducing the natural expansion and contraction of the foot, a process that helps blood move from the arteries to the microvessels in an unshod foot. Says Bowker, “We don’t entirely understand how shoeing affects the biomechanics of the foot at this time, but we do know that perfusion is different in a shod vs. a barefoot horse, even on the same surface, and we know that peripheral loading decreases perfusion.”

Ultimately, a foot lacking good perfusion and a properly functioning frog is at greater risk of injury. States Bowker, “When the foot hits the ground, whatever energy is there is going to be dissipated. It’s just a question of whether it’s being done by the correct tissues or not. Say you’re in an automobile accident and you hit the window with your face and thus stop moving forward. That means all the energy of your momentum has been absorbed. The problem is that the face is not designed to dissipate energy.

“When the foot is hitting the ground correctly with all the structures doing their job, you get maximal efficiency for the dissipation of energy. However, if we don’t dissipate the energy, what we get is high frequency energy moving into structures that are not designed to deal with it. Human biomechanical studies have shown that it is the high frequency energies that are so deleterious to bone and connective tissues. That’s why we like sneakers for running, because they help dissipate that high frequency energy.” Metal horse shoes, by contrast, appear to raise impact energies to higher frequencies. Exactly what effects this may have on the bones and other interior structures of the foot have yet to be determined. “There is still much that we don’t know,” says Bowker, “but we do know that peripheral loading devices create the most pressure per square inch of weight-bearing surface, due to smaller surface area, whereas spreading the load over the entire solar surface means less pressure per square inch.”

Bowker is certainly not alone in his conclusions about the back part of the foot, blood perfusion and energy dissipation. Pete Ramey, one of the world’s leading authorities on natural hoofcare, has been following Bowker’s work for years and could not agree with him more. “Every aspect of the back part of the foot is designed for energy dissipation,” states Ramey. “The foundation is cartilage instead of bone, there are miles of ‘extra’ blood vessels for hydraulic energy dissipation, and the back of the foot is designed to twist, flex and conform to the ground. The frog is a very important, yielding structure that should be the front line shock absorber for the whole system. All this combined creates an easier, safer ‘ride’ for the joints, ligaments, tendons and muscles; much like the difference between rubber tires vs. steel wheels on a truck. Additionally, horses with poor development in the back of the foot (bare or shod) tend to land toe first. Toe first impact is related to navicular disease and is an important contributor to coffin bone rotation, most wall cracks and white line separation. The key point should be PREVENTION. By understanding the importance of the shock absorbing qualities of the back of the foot, you start to realize how many problems horses may actually endure due to excess shock.”


While it is difficult to argue with the logic and science of Bowker’s conclusions, some might quite reasonably argue that the frog and bars don’t need to be “on the ground” if the horse is kept in softer footing, as the foot will sink into the footing enough for the back parts and the sole to make contact. To this Bowker says, “If the frog is not quite on the ground but the horse is always on a conformable surface, it may not be a bad thing, but there is still a great deal to learn here. I’m starting to notice that when the horse’s frogs are deeply recessed, there are differences in the connective tissue inside the frog. There is basically nothing inside of these feet. Even though they have a frog, it’s just the horn there – there is no fibrocartilage or fibrous tissue inside the frog.” Such observations add weight to the “use it or lose it” argument that Bowker makes about the frog, and to the idea that the external development of the frog reflects the robustness – or lack thereof – of the internal structures.

Bowker also points out that while soft footing helps in some ways, it is often too soft to provide the frog pressure necessary to develop the internal structures of the foot. “When you have peripheral loading,” he explains, “the hoof wall is the main thing dissipating the energy, and bone is not being compressed. You can mitigate the problem of energy dissipation to some extent with soft footing, but you still face the problem with the bones, which need the compression that occurs when the entire solar surface is correctly engaged to become dense and strong.” That is why Bowker speaks of a ‘firm yet conformable’ surface like pea gravel as optimal for the promotion of hoof health, rather than a soft one such as spongy grass.


If the principles of the physiological trim make sense to you, but your own horse’s feet are currently trimmed very differently, rest assured that it is never too late to make the change. However, it is important to know that such changes can take time, especially if the horse has severely contracted heels, poorly developed frogs and/or pain in the back of the foot. As Pete Ramey explains, “It can appear to be kind of a circular problem, because you must encourage heel-first landing, but if you cut the heels too low, they’ll just land on their toes because the foot isn’t developed enough in the back to function properly, and that makes it very sensitive.
“The best solution to this conundrum is to pull the shoes at least for a while, and use hoof boots with padded insoles until the foot is fully developed, especially when riding. Horses with sensitivity in the back of the foot will land heel first (and actually use/stimulate the back of the foot) in these padded boots when nothing else will work. Second, the addition of pea gravel to the horse’s environment works wonders. THEN, once the foot is fully developed, you can think about whether or not you want to go back to metal shoes. You might be surprised to find that your horse doesn’t “need” shoes anymore, because the feet are now much healthier and function far better than they did in shoes.

This was precisely the experience of Hilary Zielke, a horse owner in Langley, British Columbia. As she explains, “We have a Quarter Horse mare named Jackie who had terrible problems with her feet which had been attributed to navicular disease. She was a show horse and had been shod continually for most of her life. She definitely had her frogs way off the ground, her toes long, and everything that we were told was normal and right at the time. Her pain had gotten so bad that the farriers were getting frustrated with her because she couldn’t lift a foot long enough for them to put a shoe on. We were told that she had to have shoes to help her, but she only got worse and worse, even though we were giving her huge amounts of bute.” Zielke had actually scheduled the vet to come put Jackie down when she met Heather O’Brien, AFA, CA, a farrier who has been using methods similar to Bowker’s on both shod and barefoot horses for more than 25 years. The two talked and Zielke decided to give the mare one last chance.

O’Brien pulled the mare’s shoes, and over time worked to get her frogs on the ground and her bars and soles sharing in the weight bearing. This greatly alleviated the mare’s pain and her condition improved dramatically. “Her feet look totally different than they did when she was shod,” says Zielke. “They are much wider and rounder, and her frogs are strong, healthy and on the ground. Although Jackie is retired now, you should see her ripping around in the pasture, rearing and bucking and bossing all the horses. It has been eight years since she was scheduled to die, and she’s still going strong.”

However, according to O’Brien, ‘going physiological’ does not necessarily mean that your horse has to go barefoot. O’Brien firmly believes that shod horses can and should have feet that approximate Bowker’s physiological trim. What she doesn’t understand is why so many people view this as something radical or new. “When I went to farrier college in 1982,” she says, “we were instructed to use the sole as a guide for trimming of the wall, and to remove only the loose, chalky, exfoliating sole, leaving the rest alone. We were taught to trim the heels enough to allow the frog to make contact with the ground – unless doing so would interfere with the live, functional sole. We were to trim only the loose rags from the frog, as little as possible. We were told to provide ‘breakover’ at the toe (usually by rockering or rolling the shoe), if shod, and to bevel the hoof wall around the ground surface if barefoot. So, what Bowker and others such as Pete Ramey and Gene Ovnicek are saying today is basically what I was taught 25 years ago.”

O’Brien, like most farriers, agrees with Bowker’s assertion that it is best to keep horses barefoot whenever possible, but she doesn’t see any reason why shod horses can’t have very good feet. As she states, “Even shod horses can develop and maintain strong walls and soles, wide heels, a prominent, healthy frog and a natural cup to the foot by proper trimming and fitting of the shoe to compliment that trim.” O’Brien points out that to get this kind of success with shod horses, where there is no natural wear to the hoof walls, it is extremely important to reset the shoes frequently. “I have horses that routinely get their shoes reset at 3 or 4 week intervals, because that is how fast their feet are growing,” she says. O’Brien also favors the use of thinner shoes, which makes it easier to get the frog on the ground. “If the farrier uses a thin enough shoe and resets it frequently on a properly balanced foot, the horse can have very healthy feet that function much the same as those of a well-trimmed barefoot horse, with only minimal compromise.”

Equine sports massage therapist P. Ann Turner, of Abbotsford, BC, also believes that shod horses can function very well with the physiological trim, though she likes to see them barefoot for at least part of the year. As she states, “Horses trimmed and shod in this manner have less wear and tear on the joints from incorrect loading, they stay sound longer and are a lot less susceptible to injuries. You will have some limitations in a shoe such as loss of some of the natural expansion of the foot when weight bearing, but we have seen great improvements in many shod horses when they were trimmed to have the frog on the ground, the toe short, and so on.”

In some cases, Turner will use a period of barefootedness to alleviate problems, then put shoes on once the foot is healthy. “If the hoof is badly distorted from an injury, or just years of neglect, the barefoot trim period allows the hoof to regrow into a different shape, so when you do put shoes on, you have a much better hoof to work with, and it does not cause the horse discomfort. You now have the correct amount of heel, thicker walls and correct angles. As long as you keep trimming correctly, the shoes should not undo that progress, especially if you allow the feet some ‘time off’ from shoes for part of the year.”

Bowker concurs that if you are going to shoe a horse, the methods described by O’Brien and Turner would likely be considerably better than techniques that create peripheral loading. “I don’t know where the idea got started that the walls are meant to be the primary weight-bearing structure,” says Bowker, “but all I can say is that the data we have gathered seems to indicate otherwise.”

For more information on the physiological trim, illustrations, and articles on other hoof related subjects, visit:

MSU Equine Foot Laboratory publications page:

Pete Ramey’s website,

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