My stuff from earlier post...
Horses don't really "break-over" on the hind feet! They break over at the fronts.
It is more correct to describe the final stage of the hind landing phase as unloading versus breakover. The "mustang roll" has little effect on a hind hoof presenting serious toe or quarter cracks.
Please explain? I obviously have a different perception of the term to you. Breakover to me basically means the point around the toe(assuming the horse is travelling straight) in a stride, when the heel has lifted off the ground. Based on that, I can't work out how a horse wouldn't have a 'breakover' on his backs unless he has some weird gait that causes him to lift his toes off the ground first, or levelly?
Breakover is a somewhat poorly defined term in farriery. Same holds true for the term "balance".
Some will define breakover as the first point at which the heel leaves the ground as the horse's center of mass passes over the front, supporting limb.
Still others will define breakover as that moment just before the toe leaves the ground, well after the center of mass has passed over the axis of the limb.
The later is the more common use of the term and tends to include discussion surrounding the phalangeal lever length as it pertains to ground reaction force at the dorsal aspect of the hoof wall. I am more inclined to agree with this camp but the argument is an exercise in academics.
The debate is arguing points on the arc formed by the distal interphalangeal joint. Since these are dynamic versus static vector torque forces, the discussion should better focus on scalar length measured from the center of articulation of the dip joint to the ground. That lever length is typically greatest at the toe hence our point of greatest ground reaction force during the breakover phase of stride.
The hinds present a completely different set of dynamic and static mechanics. The reason is the reciprocating mechanics of the SI/Femorotibial/femoropatellar/tarsus joint relationship. You simply cannot compare this to the hinged joint arrangement of the supporting forelimbs.
The forelimb carpus/fetlock/pastern joints are hinged joints with limited condyliod movement. Movement is largely limited to flexion and extension. There is a slight allowance for abduction and adduction, particularly at the DIPJ.
Breakover applies because the forelimb is essentially a support only device with movement limited to a single axial plane. Best to think of the forelimb as spokes in a wagon wheel. The horse just "rolls" over that planted foot. As the body mass passes over the center of the limb, ground reaction force increases at the dorsal toe with the maximum force value occurring when the mass is directly over the center of articulation. The shorter the phalangeal lever, the smaller the ground reaction force; hence, the oft cited need to "shorten the toe". Of course, doing so also reduces the resultant kinetic (spring) force stored in the suspensories. That is why the "long toe is faster" story still prevails on the backside of so many parimutuels.
The hind limb is all about propulsion with the stay apparatus largely responsible for static support. That propulsion comes from a reciprocating pivot joint arrangement versus the hinged joint arrangement of the forelimbs. The center of mass is always forward of the limb axis so breakover ground reaction force is largely irrelevant. The horse doesn't so much "roll" over the limb axis as he simply "picks the foot up" or... unloads it.
The wear and distortion we see at the front toe is a consequence of friction, load and breakover ground reaction force. The wear we see at the hind toe is a consequence of the horse pushing off. Have you noticed that you rarely see a "dishy" toe on the hinds? If anything, you're more likely to see either a dubbed toe on the hind or simply an overly long toe but not necessarily dished. Yes, let it get long enough and it will eventually curl up, but it's usually not the same "ski-slope" distortion we see on the fronts.
Interesting experiment. Using a horse with excess wall length (we don't want to leave him like this!), trim conservatively to medial/lateral imbalance in the fronts and move the horse. What happens? Depending on the imbalance, the horse will land unevenly and wing or paddle through the stride. He'll land on the high side, drop to the low side, then breakover at a diagonal to the higher heel.
Now do the same thing on the hinds. What happens? The hoof will twist medial/lateral during the load/unload phase of the stride! You can see the reciprocating movement at the hocks.
Front legs are wagon wheel spokes that "breakover" and serve mainly to keep the horses nose out of the dirt.
Hinds are piston rods on a reciprocating crankshaft that "unload" and define the horse as a rear wheel drive animal. Any breakover of the hinds is really a directional force vector at the toe as the horse pushes then unloads the hoof. We can encourage that "breakover" direction (and stride) using square toed, blunt toed or offset toe hind shoes
This dynamic biomechanical description is best suited to discussing the horse moving at speed on a relatively level surface. Things change somewhat on hills, particularly going uphill when the animal will stronly engage the forelimb flexors to assist in locomotion. The discussion is largely irrelevant to static mechanics.
There are still a lot of track platers and trainers that put much stock in the idea of the horse pulling itself along with the fronts as a significant part of locomotion. That's why you still see the toe grabs on a lot of racing plates. The resultant injuries associated with the increased traction of those grabs is also why most tracks have either banned toe grabs or limited them to 2mm in height. From their perspective, increased breakover length in the forelimb is a good thing.
So... given this explanation of breakover as relates phalangeal length in the supporting limb, shall we talk about big lick walkers?