Google defines 'fulcrum' as:
1. The point on which a lever rests or is supported.
So my little brain translates that as the middle of the see-saw - the point of which the see saw levers around.
How on earth does this apply to the horse's limb? (Other than when a horse steps on a big rock!).
"Fulcrum represents the relationship of the ground weight bearing foot surface with the weight bearing axis of the limb" - The Barefoot Blacksmith advanced trimming workshop booklet.
The lower limb needs to be able to absorb impact - the pastern flexes to absorb force (along with a host of other shock absorbing features in the leg). This can look quite scary - have you ever taken photos of your horse at the gallop?
|Here you can see the down force through the pastern, absorbing the impact of the stride.|
|Red = weight bearing axis of the limb. Pink = weight bearing foot surface. Blue = pastern angle.|
The weight bearing foot surface (pink) needs to be as close to the weight bearing axis of the limb (red) as possible - preferably underneath it. Above is Allie with a fairly fresh trim (maybe a week old). You can see that the red line bisects her heels, but that the weight bearing foot surface (pink) doesn't quite come close enough. Below is a photo of Allie when she is way over due for a trim (maybe at 5 or 6 weeks?). You can see that the pink line has moved forwards to the point where it is coming nowhere near the red line. The weight bearing foot surface has grown forwards so much that it has taken her out of balance.
|Here is the full photo of the above - see how being out of balance just gives her lower fore limb a weak look? See how the pastern/hoof angle is broken forward, and how the lever forces are working against her?|
|Compare to this photo with a fairly fresh trim. The lower limb in front looks far more stable.|
The fulcrum can be found normally by drawing a line across the sole of the hoof at the widest point.
Easy peasy, if you have hooves that are near perfect.
|Red = fulcrum. Pink lines show the edges of the weight bearing foot surface. Blue line shows that the fulcrum should be in the centre of the hoof, bisecting it in two. Half of the foot in front, half behind. Voila!|
Then there are the special feet.
|WTF is the fulcrum?|
In a deformed foot finding the fulcrum can help you envision where the foot is meant to be. When you know where you want it to go, you can trim to encourage it.
|Pink = weight bearing foot surface. Red = fulcrum. Orange is where the toe should be, relative to the fulcrum.|