Breakover question

[Skinfaxi]

This one is simpleish. I had a breakover epiphany that left me scratching my head.
Breakover occurs at the point at which the hoof begins to pivot forward, that is, at the time the heels leave the ground. When that breakover occurs I assumed that that could equally be phrased as the point at which the DDFT begins to have an effective pull on the P3. Correct?
Well, That seems like a big jump from loaded to motion. If when the hoof is loaded the p3 moves down and back to be supported by the frog/plantar cushion, what do you call the point where the p3 is no longer down/back/receiving support, but before the DDFT actually begins to rotate the joint?

[Ronald Aalders]

This one is simpleish. I had a breakover epiphany that left me scratching my head.
Breakover occurs at the point at which the hoof begins to pivot forward, that is, at the time the heels leave the ground. When that breakover occurs I assumed that that could equally be phrased as the point at which the DDFT begins to have an effective pull on the P3. Correct?

Wrong Sarah.
DDFT pulls all the time, just the amount of pull varies. DDFT has been pulling pretty hard way before heels leave the ground. Breakover in my book starts when the toe starts to roll, the hoofcapsule being pretty rigid, that would be the time when heels start to leave the ground.

Well, That seems like a big jump from loaded to motion. If when the hoof is loaded the p3 moves down and back to be supported by the frog/plantar cushion, what do you call the point where the p3 is no longer down/back/receiving support, but before the DDFT actually begins to rotate the joint?

GETTING READY FOR THE BIG JUMP! :D


Ronald Aalders

[Rick Burten]

If when the hoof is loaded the p3 moves down and back to be supported by the frog/plantar cushion,

The concept that p3 moves is a bit misleading. for if it actually moved, it would tear laminae, strain ligaments and in effect, partially dislocate from its stable position with regard to the other bones of the coffin joint(DIPJ), much as what would happen if any bone moved from its proper orientation within its joint capsule. This does not mean that the bone does not articulate as it is supposed to, and in that sense there is movment.

The idea that p3 moves downwards and rearwards stems I think from the fact that as the process of deceleration, load acceptance, weightbearing and breakover occur, the dynamics within the hoof are changing. The quarters are descending and expanding, the heel bulbs are pushing rearwards the coronary band expands, the sole descends , all leading to what appears to be p3 moving down and back, then forward and up.

In essence, this is a change of spatial location, not internal disruptive movement. Internal disruptive movement occurs in two basic situations. Founder and overzealous removal of supporting sole which allows p3 to drop. Both of these situations are demonstrably bad for the horse.

[calshoer]

To add to Ron and Ricks , as the leg moves forward over the foot,(front foot here) with the knee *locked*, the inferior check ligament increases tension on the DDFT, with the navicular bone acting as a pulley to direct the force into the attachment under the bone. When the tendsion (force) of the DDFT pull exceeds the leverage on the end of the toe (Ground Reaction Force) then the heels lift. That is the "moment of breakover".
The more GRF, (ie longer toe, ) the further forward the leg has to go to create enough tension to overcome the GRF at the toe and lift the heel. So if the GRF is too great, then the horse may have to engage muscle he would not ordinarily use to lift the foot in time with the position of the whole body ,which is still moving forward. Then he hasn't enough time to get the leg all the way out forward in the air and land heel first. So the result is a shorted stride and a toe first landing. And sore coffin joint.
One other factor in a delayed heel lift can be a too tall heel, which again caues delay in the tension of the DDFT exceeding the GRF.
Does that help?
Paty

[Stephen E. O'Grady, DVM]

Breakover is defined as the phase of the stride that begins when the heels lift off the ground and ends when the toe lifts off the ground. The GRF is a vertical force that is located approximately in the center of the foot slightly medial to the dorsal third of the frog. It is not the leverage at the end of the toe. The leverage at the end of the toe is a moment arm or lever. For a nice description of function, see Form and Function of the Equine Digit by Andy Parks in my book - Veterinary Clinics of North America - podiatry.

Stephen E. O'Grady, DVM, MRCVS

[calshoer]

DrOgrady wrote:
Breakover is defined as the phase of the stride that begins when the heels lift off the ground and ends when the toe lifts off the ground. The GRF is a vertical force that is located approximately in the center of the foot slightly medial to the dorsal third of the frog. It is not the leverage at the end of the toe. The leverage at the end of the toe is a moment arm or lever. For a nice description of function, see Form and Function of the Equine Digit by Andy Parks in my book - Veterinary Clinics of North America - podiatry.
Respectfully Dr. O'Grady, I disagree based on Hillary Clayton's descriptions of GRF and David Hood's force plate studies.
The GRF locatuion under the foot is hardly static. It is moveable, as the forces directing the foot into the ground constantly change. The GRF therefore can located be anywhere under the foot at any moment, as it is a simple REACTION to the force from above.
If the force of the pull of the DDFT is pushing the toe of the bone down,(trying to lift the heel) the GRF is going to move toward the toe at the same time. Force resistance is force resistance. More force into one area= more GRF at the same point at that moment.
The location of greater pressure in the toe as the limb moves forward is creating greater GRF at the the toe... iE equal RESISTANCE.

Thereby, through the stride, as the heel strikes the GRF is located at the heel. As the foot centers the load, the GRFmoves forward under the foot. . As the DDFT pulls on the bottom of P3 to create heel lift, the GRF moves forward toward the toe.
Patty

[Stephen E. O'Grady, DVM]

Nobody said it was static. Ground reaction force moves toward the toe during breakover but it is less of a force as it moves dorsally.

Off to Seattle...

Stephen E. O'Grady, DVM

[Skinfaxi]

Thank you everyone.
I was overly simple..it is taking me time to learn the proper phrasing for the things I am learning.

I did say "effective" pull, I intended to mean the exact moment that the DDTF is allowed to rotate the joint with easily seen effect on the hoof capsule, lifting heels. Ideally the piviot point lies behind the toe, and the DDFT has been pulling hard for a time in order to lift the heel. I also understand that saying the P3 moves, in this context, is not to say that it moves within the hoof capsule, but that as the foot is loaded and expansion occurs the parts of the hoof come to do thier respective jobs. But if one were only looking at the hoof capsule, isn't there a point when the hoof is no longer loaded but not yet in motion? Or is it unfair to look at the capsule separate from the rest of the leg. Is it too minor to matter/notice when you look at the phases of movment, or is it non existant?

May I understand what you are saying Patti and doctor O'Grady, as describing a sort of springboard. Rather the tension driving the loaded hoof down is equal to the tension in the DDFT. So as the heel lands, hoof levels, weight becomes born directly over the hoof, and continues over. The tension level of the DDFT and the tension in the GRF move in an (matching)arc. So the GRF tension would have to reduce as motion proceeds and reaches the Breakover point otherwise the DDTF could not lift the heel right???

[Diana Margarita]

Hey Dr. O'G...would you check out the Epona people when you're there?
I'd like to give their shoes another try...without packing and glue, just nailed on. I would most appreciate your advice.
Thank you in advance,
Diana (and Chico)

[calshoer]

I did say "effective" pull, I intended to mean the exact moment that the DDTF is allowed to rotate the joint with easily seen effect on the hoof capsule, lifting heels.
The DDFT does not rotate the joint in the flexed direction at all to lift the heels...
Rather ,the joint is in full "extension*, (ie a momentary broken-back alignment) with the bottom of the foot firmly planted on the ground and the pastern rotating forward until finally the DDFT gets tight enough to overcome the lever arm at the toe and the heels lift.
Only *after* the foot is all the way off the ground, (completing it's pivot over the toe, can the DDFT begin to pull the joint into the flexed direction. In other words, The DDFT does NOT initially lift the heels by flexing the coffin joint!
The joint is extended at that moment. That should explain why the navicular bone and impar ligament takes such a beating if the toe is too long.
Patty