What's a lever and why do we care?!
Updated: Mar 23, 2023
A fellow Facebook page has recently caused some stir by pretty much dismissing quite a few topics in hoof care and saying that they know best. This usually results in a bit of verbal debate (and worse) as any extreme position causes angst and anger in others. However, after the initial "What the hell are they on?!" I tend to start asking myself questions about the controversial topic(s). I suppose this is not surprising since I was first and foremost trained as a scientist, and the scientific method is all about questioning and researching to find out what's actually known before forming an opinion or hypothesis.
Soooo, what topic got me first annoyed and then thinking? Quite a few, but the complete dismissal of levers as being important or playing any role at all started this newest obsession to try and find out what knowledge is actually out there... is it me who got it wrong, or 'them'? Or both? Let's find out!
What the heck is a lever, and what does it have to do with hooves?
Well, basically, the hoof IS a lever! Let's Talk Science define a lever this way:
A lever is a simple machine made of a rigid beam and a fulcrum. [...] The fulcrum is the point on which the beam pivots.
Yes, I know, this reminds you of high school physics, but bear with me here. There are lots of levers in every-day life: door handles, hammers, tweezers, wheelbarrows, scissors, clothes pegs, crow bars, the Weymouth bit in double bridles, gags, pelhams and hackamores are horsey examples of levers.
There are three types of levers and the hoof is of the third type (third degree levers): the fulcrum, or pivot point, is at the end, and the 'beam' is the rest of the lever, just like the handle of a door handle, or the shanks of tweezers or the handle of a hammer.
Still, you ask, what the heck does that have to do with hooves?? Well, each time the horse wants to move a leg forward, it has to lift the hoof's heels and roll over the hoof's toe (at the breakover). The tip of the toe is the pivot or fulcrum, and the 'beam' is the rest of the hoof.
If you have ever walked in flippers (also a third degree lever), you will know that long flippers make it very hard to walk. You end up having to lift your leg really high and resort to a waddling, duck-like gait; or give up and walk backwards. The tip of the flipper is the pivot or fulcrum. And the longer the beam, the higher you have to lift it to pivot around the fulcrum (the tip).
So back to horses: if the tip of the toe is the fulcrum the hoof has to pivot around, and the rest of the hoof is the 'beam', then having a very long toe makes it more difficult to pivot the hoof forward. The longer the toe, the longer the beam of this third degree lever, the more force/energy is required to lift it. And the shorter the toe (e.g. club foot), the easier it is to roll the hoof.
Photo above: An example of a horse with a very long toe prior to trimming (left). The red dots are the pivot points (breakover) of the toes. The solid lines shows the distance of the fulcrum from the coronet band (chosen purely for visualisation). On the right, after trimming, the toe was shortened and now the fulcrum is closer to the coronet band (the blue line is shorter than the red line). Less force/energy is needed to lift and roll the hoof forward.
Image above left: a hoof has to pivot up and around the fulcrum/pivot, and break over the tip of the toe (the breakover) to move forward. The yellow dot is roughly where the centre of rotation of a hoof is - it moves aorund the fulcrum on a cirlce. A hoof with a long toe (top right) does the same; however, because the toe is longer, the lever is longer, so the hoof (i.e. centre of rotation) has to lift up higher to pivot around and move forward.
You might have heard of a farrier or trimmer "squaring the toe" or "bringing back the breakover". This pretty much means that they shorten the beam (the hoof behind the toe) to make it easier for the horse to pivot on its toe and roll the hoof forward (see images above).
So, the hoof IS a lever! So what?! Long levers require more energy to lift, as we have seen. So a horse with longer toes will have to use more energy for lifting the leg/hoof high enough to pivot over the tip of the toe. This changes its gait as well as the energy muscles have to excert and the way tendons act/pull.
Also, while the horse's hoof is on the ground (landing, stance or breakover), it is squeezed both by downward forces, upward forces (ground reaction force) as well as strong elastic forces (tendons). This all works well in healthy, balanced hooves. However, if the hoof is unhealthy, unbalanced, the lever is extremely long or in an awkward spot (think flares), or the structure that holds the bone and hoof capsule together (lamellar corium) is weak or damaged (e.g. laminitic horses), these forces acting on the hoof can have negative effects.
More on that in my next post, where I want to explore what happens to the forces inside a hoof!
Bibliography
DEJARDIN, L. M., ARNOCZKY, S. P., & CLOUD, G. L. (1999). A method for determination of equine hoof strain patterns using photoelasticity: an in vitro study. Equine Veterinary Journal, 31(3), 232–237. doi:10.1111/j.2042-3306.1999.tb03178.x
O'GRADY, S. E. (2018). Equine Hoof Capsule Distortions: An Overview. 2018 AAEP Convention Proceedings. https://aaep.org/sites/default/files/issues/proceedings-12proceedings-In-depth_the_Foot_from_Every_Angle-Parks%20(1).pdf
O'GRADY, S. E. (2013). How to Evaluate the Equine Hoof Capsule. 2013 AAEP Proceedings. 59, 54-61.
PARKS, A. H. (2012). Aspects of Functional Anatomy of the Distal Limb. 2012 AAEP Proceedings. 58, 132-137. https://aaep.org/sites/default/files/issues/proceedings-12proceedings-In-depth_the_Foot_from_Every_Angle-Parks%20(1).pdf
THOMASON, J. J., McCLINCHEY, H. L., & JOFRIET, J. C. (2010). Analysis of strain and stress in the equine hoof capsule using finite element methods: comparison with principal strains recorded in vivo. Equine Veterinary Journal, 34(7), 719–725. Doi:10.2746/042516402776250388
JOHNSTON, C., & BACK, W. (2006). Hoof ground interaction: when biomechanical stimuli challenge the tissues of the distal limb. Equine Veterinary Journal, 38(7), 634–641. doi:10.2746/042516406x158341
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