Unfortunately as a community, farriers have been repeatedly force-fed information that is presented as scientific, but in fact bears little or no resemblance to good science or fact. However, there is science known about locomotion and the linear relationship between farrier techniques and altering that locomotion.

For example, Idaho veterinarian Olin Balch’s PhD thesis was based entirely on how farrier techniques alter the temporal parameters of the weight-bearing phase. I would suggest that every hoof-care
practitioner and researcher obtain a copy and read it.

Gold-standard studies published in the early ‘90s with implanted strain gauges in the tendons and suspensory ligament in the front leg explain misconceptions clearly that run rampant in this modern era of misinformation. For example, all the soft tissue around the back of the fetlock is loaded (fetlock drop) by the weight of the standing horse and the potential energy of movement.

The superficial digital flexor (SDF) and suspensory ligament (SL) are congruent anatomy functioning similarly (even though the SL loads 2:1 of the SDF). The superficial digital flexor (SDF) tendon and suspensory ligament (SL) load during fetlock drop from the horse’s weight and the length of the pastern as this anatomy inserts at the pastern joint. The SDF and SL are single-phase loading during weight bearing. The SL simply stores energy for later recoil back to its vertical assist while the SDF assists the shoulder in lifting the leg.

Two factors make changes in that load: the body mass and its movement and a pastern angle change.

Since farriers don’t change the weight of the horse, increasing or decreasing that load is dependent on whether you raised or lowered the angle of the pastern by changing the hoof angle. When you raise the angle of the hoof, the pastern angle goes down and the fetlock extends, increasing the torque centered on the fetlock joint. That change has been found to be about 1 degree of fetlock angle change per 3 degrees of hoof angle change. In spite of what is being taught, toe length has no effect on that loading.

These loads can be calculated, and subsequently, the changes made by farrier techniques can then be calculated to quantify those changes.

The deep digital flexor tendon (DDFT) also passes on the palmar aspect of the fetlock (first fulcrum) and continues distally to the second fulcrum provided by the navicular bone and inserts on the solar region of the coffin bone. The DDFT is larger than the superficial, and acquires and decreases load as the fetlock drops, because the fetlock is extending (increasing strain to DDFT).

Concurrently, the coffin joint is flexing, which decreases that strain, but does not completely remove it. This requires the distal limb portion of the DDFT to function bi-phasic in manner during force application.

The angles and lever arms that provide that torque to tense this soft tissue will vary between horses, but nonetheless the function between horses is the same.

Need an Antagonist

Horses require an antagonist (support to resist vertical forces, friction, traction to resist horizontal forces) for propulsion. Have you ever seen movies of a horse swimming? They don’t swim well or fast because there is little antagonist to the muscles application.

As ambulatory mammals, we recognize the difference between walking on ice and dirt even with adequate support. That proprioceptive information tells us how we apply force to the ground. We recognize the need and value in shoe design and placement to provide additional support to the limb to allow normal function and antagonist for those vertical forces. Do we really appreciate the hoof’s antagonist requirement for the horizontal application and in shoe design continuing that application?

When there is more loading force in the leg, there must be more antagonist …

Conformations vary among breeds of horses and within a breed. For example, Thoroughbreds commonly have long pasterns in comparison to other breeds. That statement should inform that the torque developed at the fetlock is greater than the torque developed on a shorter pastern horse. In fact, if you have two 1,000-pound horses, one with a 4-inch pastern and the other with a 6-inch pastern with the same pastern angle, the horse with the longer pastern has 50% greater fetlock torque (and subsequently a higher tension in the soft tissue). Do the math: Torque = force (weight) x radius (pastern length) x sine of the angle. (You might have to do trigonometry to get the pastern angle if you have no way of measuring it.)

Antagonist (provided by the soft tissue at the fetlock) to the body mass loads the fetlock. That same bodyweight becomes the antagonist when the horse leaves mid-stance. Those loaded muscles continue to function and assist in leg lift. The muscles in the forearm do not assume lifting totally, if they did then there would be no need for shoulder or rump muscles. The limb muscles assist by stabilizing the extremity so that propulsion provided by the large muscles of the shoulder and rump may be accomplished. Forearm muscles continue functioning during the swing phase to provide limb folding.

One interesting fact from Balch’s work is that the knee must break before the foot comes off the ground. This fact supports earlier work reported by Dr. Doug Leach. Another study Balch did on toe length (sponsored by the American Farrier’s Association Research Committee) found that time to be approximately 30
milli-seconds and that additional toe length made that time differential longer, so conversely, shorter toe length would shorten that separation of time.

More importantly, the found that additional ½, 1 and 2 inches of toe length (same weight) made no difference in leg function parameters. Once that knee breaks open, the leg is essentially non-weight bearing.

Any basic horsemanship book indicates that horses move with a combination of pulleys and levers. All length (or radius of pulleys) represents leverage in mechanics, no matter how short or long. The ensuing discussion should then center on that leverage being an advantage or not. Varying lever lengths and pulley diameters provide a means to load required soft tissue for load acceptance and subsequent advantage for application. Ground interaction provides stability and resistance (antagonist) to those applied forces required to support maneuverability. Efficient movement and maneuverability requires the resistance to equal or exceed the application for equilibrium of movement. Lack of adequate resistance will result in application failure. (Think accelerating the motor on your truck and spinning your tires, you get little forward motion.)

How many farriers were taught to map the foot, back up the toe, shorten the toe, rock the toe and do everything in their means to make the toe as short as possible to “reduce breakover strain.” Put the shoe in the same position from the center to the outer edge of the hoof. As a result of these practices those long pastern horses will essentially have the same length and shape of hoof as short pastern horses. That makes the long pastern horse have the same antagonist as the short pastern horse. When there is more loading force in the leg, there must be more antagonist.

A Marvel of Nature

There is no mechanical need or benefit for farriers to abuse that toe or veterinarian to prescribe farriers to do so. That hoof is an evolutionary marvel and I am not smart enough to overcome or claim to completely understand Mother Nature’s design of it. Is it really mechanically correct to make the feet the same on the end of those two pasterns? No, the same size hoof on the end of the long pastern is lacking even more resistance than the hoof on the short pastern.

Gathering up the toe, uniform wall thickness, rasping flares and taking back the hoof are implemented as standard practice, often without any consideration of the antagonistic requirements of the hoof and horse. Reducing that resistance to below the minimum amount required by the horse is contraindicated.

Remember those tendon strain studies? They actually found that the DDFT at maximum strain is about 70% of the weight-bearing phase and is rapidly declining to heel off. Think about that. What is actually happening is that when the pastern is lifting and passes through matching pastern foot axis, it achieves maximum tension (as a result of the two fulcrums of fetlock and navicular bone). The rapidly dynamic conformation of the distal limb has now put the tendon muscle(s) belly. Maximum tendon strain (DDFT) is not at heel lift or toe off as you have been lead to believe.

We have been fed misinformation for so long that we now believe it …

Any weight-bearing center you want to plot all migrate towards the medial heel during weight-bearing mid-stance (maximum load of the fetlock) and to the diagonal toe during application. This makes the hoof base become a class two lever and the farther those centers get from the navicular bone to the toe, the “easier” it becomes to pick the hoof up.

Try it. Put a weight on a bar where you will pick it up, the other end will become the fulcrum. Then move the weight closer to the fulcrum, it becomes easier to lift that weight. That leverage is now a mechanical advantage for the DDFT to provide application to the ground from the tension it has developed earlier in the weight-bearing phase.

If ground interaction has not provided the hoof with the proper amount of antagonist then the foot will spin out of the ground, eliminating follow through of weight bearing.

Once you realize that antagonist (resistance, traction) is required, how much is required? How do you quantify that clinically? Have you even considered that?

There are mathematical methods to assess the antagonistic hoof area and shape for each particular horse, but making the hoof as short as possible and the “breakover” as short as possible doesn’t provide that necessary antagonist.

Mechanically, the horse with the longer pastern needs more antagonists. The tension in the associated soft tissue is greater in the long pastern horse including the DDFT. Allowing the hoof to grow and utilizing that hoof doesn’t increase the peak load of the DDFT (or any soft tissue for that matter) as it is set by body weight and conformational variables at mid-stance. That length of hoof will however extend the time from mid-stance to lift off, allowing more time for that already set force to apply that tension.

Physics equations stipulate that an increase in time applied to accomplish work says the force required will be less. By shortening the toe, which in turn shortens the weight-bearing phase, the horse has to apply more force to the limb to accomplish the same speed since the time of the weight bearing phase has been shortened. Additionally, muscle function is more efficient longer and slower rather than shorter and faster.

Impacting Horses

The lack of this understanding was prevalent when the Thoroughbred industry and the committee assembled decided to take the toe grabs off racehorse shoes. I still question its impact today.

We have been fed misinformation for so long that we now believe it and are transcribing that misconceived thought into practice.

Did you ever wonder why horses with a shorter hoof and higher angle have a higher incidence of navicular disease than long-footed performance horses? Don’t be foolish enough to blame that all on genetics.

Take 100 Western Pleasure horses and 100 American Saddlebred horses. Then radiograph them and this will be revealed. Add to that foolishness when a Quarter Horse is diagnosed with navicular disease and you are told to shorten the toe further and raise the heels to treat the disease process. Really?

Shortening the toe to the point the foot is trying to spin out of the ground before the knee breaks open causes the DDFT to be in a higher tension status at lift off than after the knee breaks. That explains when you take off a perimeter fit shoe and replace it with an “easy breakover shoe” the toe wears excessively.

Efficient movement occurs when the timing of the knee break is before heel lift. At that point, there are essentially no generated fetlock forces within the hoof (other than the bulk weight of the distal limb). Wear of the shoe will be on the outer edge of the shoe rather than through the whole web of the toe.

Are we really shoeing these horses correctly by not shoeing their hoof to their mechanical needs …

The only way to adjust that timing is with toe length. I said it more than 40 years ago and I’ll say it again, “for every pastern angle and length, there is hoof angle and length to allow that digit to function in equilibrium.” That statement should also include adequate antagonist.

Do you think Mother Nature would be foolish enough to design a horse’s protective and propulsion apparatus that would become mechanically inefficient with hoof growth?

Thoroughbred horseshoeing techniques evolved over years of trial and error. Since that horse generates much more torque around its fetlock than the average horse, it requires more traction, not less. Since you obviously cannot convince those farriers to provide that antagonistic support with hoof, you must provide traction. Watch these horses come out of the gate with their leg slightly bent (knee open) so that they can dig that toe and apply that force to the ground for rapid acceleration.

Think about how you would use your toe if you want to accelerate quickly, and then take it away — you won’t have that option any longer.

Racehorses are required to go fast and any inefficient force application to the ground costs the horse speed. Any decrease in DDFT application means that the fetlock and associated soft tissue will have to pick that load up during the weight-bearing phase.

Did you ever wonder why fetlock injuries in racehorses are so high and getting higher since the removal of toe grabs? Did you ever wonder why toe grabs are used on pulling horses and are made so the grab bites dirt better as the heel lifts?

Failure of any mechanical feature during weight-bearing will require the horse to adjust that weight bearing to accommodate that failure in the next step.

Following our example, if the toe is spinning out of the ground, the horse will adjust it’s leg so the DDFT doesn’t do as much, making the SDF and the SL do more. The horse does that just like you would if you were walking on ice.

Somewhere between Quarter Horses with stub feet, high hoof angles and backed up shoes and giant, pulling horses all other performance animals exist. Standard removal or modification of toe mechanics eliminates a correct follow through of the weight-bearing phase and eliminates any use of that toe if the horse desires to use it.

We need to incorporate antagonistic requirements into our thought processes.

Are we really shoeing these horses correctly by not shoeing their hoof to their mechanical needs? Have we really made an effort to understand these mechanical needs? Has our community designed these standards of practice around our lack of mechanical understanding? Have we designed our shoeing practices for appearance rather than function, simplicity vs. correct?

At a bare minimum, we need to leave the hoof alone.



December 2019