The weight-bearing phase of a horse’s stride sequence is extremely complicated.

It is three-dimensional at the least and four-dimensional when interjecting the ground reaction force (GRF) vector. Each dimension’s role in the overall mechanics of weight bearing is often hotly debated. That GRF is dynamically changing (as is the leg) throughout the weight-bearing phase, making that portion even more difficult to understand. 

As farriers, we have our ideas of what modifications to the hoof/shoe dynamics can be done to augment and support that weight-bearing portion of the stride.

Dr. Olin Balch, DVM, PhD, wrote his thesis on that subject, “The Effects of Changes in Hoof Angle, Mediolateral Balance, and Toe Length on Kinetic and Temporal Parameters of Horses Walking, Trotting, and Cantering on a High-Speed Treadmill” of the weight-bearing phase. It’s interesting reading for sure. The thesis identifies that farriers affect more than impact and breakover while practicing their craft.

Undoubtedly, the most lip service is paid to the aspect of breakover. I doubt it deserves all the attention it’s given, but I will give it its due diligence. What does breakover mean to you? What does easing the breakover mean to you? When does breakover start? When does breakover stop? How much propulsion is the normal horse getting from that breakover? Does easing the breakover reduce the force in the hoof?

We all should be asking these questions to those who advocate doing everything you can to the toe of the hoof to “ease the breakover.” Keep asking more questions and you will gain a greater appreciation for the complexity (and subsequently the misunderstanding) of the distal limb’s weight bearing and breakover. There are a lot more questions that could be asked without throwing “breakover” under the bus completely.

Here are a few questions I can answer.

What portion of weight bearing is the peak total limb load occurring? Mid-stance.

Where do the peak loads to the superficial digital flexor tendon and the suspensory ligament occur? Mid-stance.

Where does the peak load to the deep digital flexor tendon (DDFT) occur? About 75% of the weight-bearing phase, roughly halfway from mid-stance to heel off. Strain from this point on is decreasing until the knee opens and unloads the DDFT from strain.

Does “easing the breakover” reduce the force in the hoof? No, it doesn’t. The forces in the hoof are a result of the body mass applying force down the leg. If you don’t remove body mass and the potential energy of movement from the formula, you don’t remove force. How much weight bearing is in the hoof when it rolls out of the ground if the knee is open? Hardly any, just the weight of the distal limb below the carpus (knee) or tarsus (hock).

I’m sure that last one has you stirring in your seat a little bit because that’s surely not what most people think. In 1990, H.C. Shamhardt of Utrecht University implanted strain gauges in a horse’s soft tissue (DDFT, superficial digital flexor tendon, suspensory ligament and inferior check ligament) and published a paper with the results of that research.

Shamhardt’s work showed that the DDFT goes to 0 strain before the hoof has heel lift. That 0 strain is a result of the knee opening before heel lift (Figures 1 and 2). Figure 1 is from Dr. Jeff Thomason’s work showing the position of the leg with center of mass (CM) force still going down the leg and the GRF vector no longer aligned and going up the leg, so a shear is formed at the carpus causing it to mechanically collapse. 

The deep digital flexor tendon goes to 0 strain before the hoof has heel lift, which is the result of the knee opening. The position of the leg with center of mass force continues down the leg and the ground reaction force vector no longer aligns and goes up the leg. A shear is formed at the carpus, causing it to mechanically collapse. FIGURE 1: Jeff Thomason; FIGURE 2: Randy Luikart

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Shamhardt’s research is the gold standard and certainly would disqualify or at least question all the information subsequently gathered on force plates, which isn’t gold-standard research.

Surely that should make sense as the body of the horse is moving along the ground from the major muscles of the shoulder and pelvis. The small muscles attached at the tops of the soft tissue in the distal legs are certainly not large enough to provide a significant amount of propulsion for the horse. Their purpose is to provide impact stability (shock absorption and resistance to loading forces), weight-bearing stability (steady acceptance of weight-bearing forces) and swing phase function to the distal limb as the leg unloads and swings. The upper leg (think shoulder and hips) propels the body and lifts the leg as the body passes over the hoof. The carpus should open before the heel comes off the ground. When the knee opens, the distal limb becomes essentially non-weight-bearing.

Some people argue that point of knee opening before heel lift. Unfortunately, the correct function for gait efficiency and what performance horse owners want does not align. “Flat-kneed” motion is inefficient for the horse. Try that yourself by wrapping your knee so it doesn’t flex and walk off. It isn’t easy to lift your foot without your knee opening. Figures 3 and 4 show a dressage horse with knee opening before heel lift. Those two pictures are from 240 fps video and are separated by 4 milliseconds. Figure 5 is a Saddlebred with the same position.

A dressage horse (Figures 3 and 4) with knee opening at heel lift. The images are captured from 240 fps video and separated by 4 milliseconds. A Saddlebred (Figure 5) opens its knees before heel lift. Randy Luikart

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Remembering that the peak load to the DDFT is about 75% of the weight-bearing phase, the DDFT is rapidly unloading from that point until heel lift (0% strain by heel lift). Since the load in the limb is also rapidly unloading (the horse’s body is going up, not down at this point) that leaves little to no weight in the hoof just before heel lift. This allows the hoof to provide follow-through application while in contact with the ground. Shamhardt showed that in his research, as well. That fact was shown again by Balch in his research and published papers.

Other researchers also have reported that the knee should open before heel lift. It’s possible to gather pictures of a leg lifting with the knee flat. All that is telling you is the hoof is rolling over (breakover) before the leg is done applying force and being dragged across the ground at the toe. The hoof should not do that. 

Altering the mechanics of the hoof so that it must break over earlier puts that breakover when the leg has more load in it. Certainly not reducing any load. Easing the breakover also removes the traction necessary for a smooth follow-through. Did you ever wonder why easy breakover shoes wear excessively at the toe? Now you know.

One more important feature of this fictitious reduction of DDFT strain during breakover is this — cut the DDFT and walk the horse off, and the hoof will breakover. So, what is doing that now? Certainly not the DDFT as it was taken out of the mechanics. The rest of the distal limb’s anatomy, joint limitations and functional anatomy continue to breakover the hoof. 

Metaphorically, the breakover event is like a group of people picking up a casket, 1-2-3 lift. If one in the group goes early, that person has all the lift. Shoeing procedures that facilitate early action of the DDFT give it all the load. If it goes late, then it has little to do. That hoof rollover is all about timing. So, the DDFT is a group participant in breakover but not the only one.

The DDFT muscle unit’s early firing and accepting a larger portion of the load explains to me that the mechanics of DDFT tension on navicular disease pathology (see Dr. Doug Leach’s work) is true, and making the foot shorter is not the answer. It further identifies why the incidence of navicular disease is higher in short-footed performance horses rather than long-footed ones.

The horse community (farriers included) has been force-fed the misinformation gathered on force plates that there is a huge amount of strain at breakover. After all, those force plates say so. 

Force plates have been the primary means used to gather information (other than Shamhardt) on weight bearing. Research using implanted strain gauges would not be allowed now due to the animal cruelty oversight in our research environment. That’s OK, but we need to realize that force plates have two variables, measured in pounds per square inch.

The design of these projects normally uses the same horses to reduce variability in the data gathering. If the horse weighs the same, the force applied should be the same. However, when the shoe’s surface area in contact with the force plate has changed, it produces what the researchers want to see. 

In other words, if you put a flat-toed shoe on a foot and measure the compression force on breakover at the toe, you have a statistical amount of force on a small surface area. When the shoe is removed and a rolled toe is added on the same shoe and hoof, replace it and measure the force, it will be less. That difference is not because the force has reduced, but because the surface area in contact with the force plate has been increased. The horse is still applying the same force.

Simply put, if you place a 100-pound weight on 1 square inch on a force plate you get a 100 psi reading. Put that same amount of weight on 0.5 square inch and the force plate reads 200 psi, 0.25 square inch and it reads 400 psi. The force is the same, but the surface area is not, so in this example, the results change due to surface area rather than force.

That is the way it is in the horse, as well. At a given point in the weight-bearing phase, the forces will be the same in the same horse barring the environmental variabilities of gathering that data. At a constant rate of movement, the weight-bearing phases will be similar in time. Reduce the weight-bearing phase in time (i.e. quicken the breakover, raise the angle of the hoof, set the shoe back, all items we have been instructed to do) while maintaining the same rate, and the horse must apply more muscle/tendon force to the limb, not less. That’s a physics law that simply says if you shorten the application time, the force has to be increased to maintain the velocity. That law certainly flies in the face of what we have been instructed to do by self-appointed experts.

It is essential to appreciate that the horse is not in contact with the ground during all portions of its stride. 

We know and often forget the fact that the horse’s movement is cyclic. Although smooth, it’s not the same as your truck. Your truck has tires in contact with the ground continuously, so the propulsion is a steady application of forces. 

The horse propelling itself is not continuously in contact with the ground. Horse movement is more impulse from the hip and shoulder muscles, while the distal limbs provide dampening to the ground for that impulse. That dampening occurs both at impact and departure. We all understand that during impact, peak loads in the hoof are not achieved until the foot has stabilized in the ground. It’s the same way on departure when heel lift occurs. That’s the end of major propulsion, which was accomplished between 50-75% of the weight-bearing phase and follow-through is all that is left by the hoof. The leg unloads just the opposite of the loading sequence.

I’m sure that you have walked, and at one time, have pushed hard on the toe of your shoe to extend your weight-bearing phase or apply just a little more push to your stride’s weight-bearing phase. The horse can do that, too, if it’s required to and has ample toe to do so. If you try doing that exercise on ice, you “feel” the difference and will need traction. Any force application that exceeds the traction will “feel” slippery to the horse.

Look at a draft-pulling horse and the toe grabs on their shoes. These horses will utilize that toe piece on landing and departure to gain as much propulsion (and stability) from that limb as possible. At landing, the hoof does not hit heel first. The leg is in an arched position so the toe can grab at impact. That position is once again achieved when the knee mechanically collapses and opens before heel lift.

Thoroughbred racehorses are another example that should have toe grabs on shoes. When that racehorse is applying force to the track, it will use that follow-through time from knee opening to heel-off to toe-off to apply a little more force to the ground. It’s foolish to remove toe grabs from those two types of performance horses and expect the same performance.

I find it difficult to understand why our industry has focused on “breakover” as the primary fix for the huge list of mechanical and pathological variables that exist during the weight-bearing phase. 

That list includes variable conformations and performance requirements that these horses endure from their owners. That focus has extended to the veterinary profession, horse owners and farriers who persist in ignoring gold standard science to proclaim that shortening the hoof to address breakover will fix all those maladies. 

Perhaps we are just bull-headed and stuck behind the law of primacy. That law says the first things we learn are harder to change, even when wrong, as we revert to the original thought taught.

This confusion reminds me of a story that I’ll bore you with. When I took my private pilot test, the examiner asked me what made airplanes fly. My mind works on physics, so I began the dissertation on flight mechanics including Borelli’s principle, which says the wind is split at the wings’ leading edge. The wind over the wing passes at a different angle, resulting in a lower pressure on the top and a higher pressure on the bottom, creating lift. 

The examiner (fully aware of the mechanics) said, “No, money makes airplanes fly.” He was right philosophically; it does take a lot of money to keep airplanes flying. However, money has nothing to do with the mechanics of flight. Breakover is that money as it has little to do to address the weight-bearing mechanics of the distal limb. Far less than the importance it is given.

Gold-standard research that doesn’t agree with your perception of what’s happening shouldn’t be ignored. Perhaps your perception needs to be modified by knowledge at best and reconsidered at the least.