Physics of Equine Locomotion

Image showing the structures of the ligaments in a horse's neck at different head positions.

Head


The head in most gaits acts as a counterbalance. The movement of the head tends to be rather energy efficient as it relies on some passive mechanisms. The nuchal ligament2 is highly elastic and acts like a spring alternating between potential and kinetic energy. This ligament provides between 30% and 50% of the energy needed to move the head in each gait2. As the head swings down, the ligament generates a restoring force pulling the head back up. The force of gravity allows the head to be lowered again without much effort. There is a frequency at which the head is stabilized mostly by inertia: 1-3 Hz2. Motions of the head also act to generate torque at the point between the neck and body, which can shift center of mass backward2 and aids in moving the front end.

Image source:http://www.sustainabledressage.net/rollkur/why_not.php
Diagram of the parts of a horse's spine

Shoulders (& The Body)


The shoulders and trunk of the horse play a part in the balance and momentum of equine locomotion. Horses hold a disproportionate amount of their weight on their front end. The trunk in all gaits has the least movement in the up-down and left-right direction compared to other parts of the body such as the head and neck2. The compact and relatively stiff nature of the horses spine may play a role in allowing this.

Image source:https://www.horsetalk.co.nz/2015/02/04/help-horse-kissing-spine-goodbye/
Image of the bottom of a horse hoof

Knees and Toes

Arguably the most important aspect of the horse's ability to run is the anatomy of their limbs. As a horse's hoof contacts the ground, it can hit with great force and acceleration, causing vibrations that travel though the hoof and up the leg as they dissipate. At the fastest gaits, this force can be up to 2.5 times the horse's body weight4. As Newton's Third Law states, for every force an object applies there is an equal and opposite force applied back.
So, assuming a horse weighs about 455kg (~1,000lbs), it would experience 455kg*9.8m/s2=4,455N if the force was equal to its body weight, so the maximum force a horse can experience is 4,455N*2.5=11,136N!
Structures of the leg and hoof that help combat these forces are:
  • Digital cushion5: an internal structure that takes up a large portion of the rear part of the hoof; its flexible structure allows it to be one of the main shock absorbers in the hoof.
  • Frog5: Can be seen in the image to the left as the triangular structure in the rear center of the hoof bottom; it is an elastic structure that is the initial area of contact when the hoof hits the ground. It transfers force to the digital cushion and bars of the hoof wall.
  • Navicular bone and bursa5: A small bone and fluid capsule toward the back of the foot. The bursa reduces friction between main hoof tendons and the bones of the hoof.
  • Pastern3: The first part of the leg above the hoof. The joints of this part of the leg are elastic and bend as the hoof hits the ground. The restoring force then helps push the hoof back off the ground.
As the hoof hits the ground, forces are transmitted from more solid structures such as the bones and walls of the hoof to the more flexible structures like the digital cushion. This allows the force of the impact to be distributed over a larger distance as these soft structures can move and give.

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