Approximately 60% of the weight of a horse is carried on his forward limbs. During locomotion, the forelimbs must be able to take a significant amount of pounding.
The pastern functions as a shock absorber. As the weight of a horse comes down on his forehand, the pastern flexes, dropping the fetlock. The pastern slants from a nearly upright position to a deep slant and back again. However, when the pasterns are too long or sloping it does not support the fetlock enough, and the fetlock may hyper-extend, possibly to the point of dropping the fetlock all the way to the ground. This stresses the soft tissues that run under the fetlock because they are stretched longer.
I have had experience with horses with hyper-extended pasterns to the point that the horse was no longer able to stand or walk comfortably and had to be euthanized. I believe this is more common in thoroughbreds and some riding horses who have been bred either without regard to this anatomical feature or for more sloping pasterns to smooth their gate.
The angle of a standing pastern is important not only for his comfort and ability to function, but also depending on the work the horse will do.
A nicely-sloped pastern is the best for a riding horse (approximately 45 degrees to the ground). It is sloped adequately to absorb the concussion of the horse’s gait, but not so sloped that it will break down, ending his riding career.
A longer, more sloping pastern increases the comfortable ride due to its ability to absorb shock. However, it is inherently weaker than a moderately upright pastern, and increases the risk of break-down and lameness.
A short upright pastern will increase the concussion transmitting up the leg via bones instead of tendons. This type of horse will have a much rougher gate as it will be more jarring. Additionally it decreases his stride length. Short strides decrease the efficiency of a horse’s movement, calling for more strides (more energy) in a given distance. It also increases the likelihood of arthritis and other concussion bone problems.
Draft horses, who are not selected for smooth riding but for pulling, have more upright pasterns (approximately 65 degrees to the ground). However, because they work at slower speeds, there is less chance of damage from the upright position of the pastern. Care should be taken that draft breeds bred for pulling but now used as performance horses don’t suffer concussion damage because of the new stress put on bones from an upright pastern in uncharacteristic gaits.
Ponies often have upright pasterns. When training a child to ride, it would be much easier to use a Caspian Horse (small like ponies but built like horses) because they have a conformation similar to a horse, including an adequately slanted pastern for a smoother ride.
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