Equine locomotion seems like a sophisticated thing. All those delicate bones, joints, tendons, and muscles must move in tandem to propel the horse’s large body forward at varying speeds.
But in truth, it’s really quite simple and predictable – so much so that veterinarians can evaluate a horse’s movement pattern and detect abnormalities related to pain or dysfunction.
Last month, Michigan State University professor Hilary Clayton discussed equine locomotion and how the horse’s gaits adapt to lameness at a Focus on Horse forum in Louisville, Kentucky.
Clayton shared her knowledge about the horse’s stride and assessing it for lameness; but first, she gave the veterinary audience a brief physics refresher.
The Forces and Torque on the Equine Limb
“For every force there is an equal and opposite force,” Isaac Newton stated in his Third Law.
This applies even to horses. Think of the hoof’s interaction with the ground as he moves, for example: As the hoof pushes downward and forward into the ground, the ground pushes back equally against the hoof but in the opposite (upward and backward) direction. Researchers call this the ground reaction force (GRF).
“Ground reaction force determines the speed and direction of movement of the horse’s body and pushes the horse in the direction in which the GRF acts,” Clayton explained. More specifically, in the early stance phase of a horse’s stride (when the feet are stationary and in contact with the ground and while the body is moving forward) the GRF vector points toward the horse’s tail, slowing forward motion. In the late stance phase the vector points upward toward the horse’s head, propelling him forward.
“The hind limbs are the horse’s motor, and the forelimbs do the braking and steering,” Clayton said.
If the GRF fails to pass through the joints’ centre of rotation as the horse moves, it creates torque around the joint. The greater the GRF’s magnitude and distance from the joint, the greater the torque.
Tension in the soft tissues on the opposite side of the joint help to stabilize the joint. For example, the GRF acts on the front side of the fetlock joint, and tension in the digital flexor tendons and suspensory ligament support the torque that’s produced. In horses that have long sloping pasterns, the GRF torque is larger and places greater strain on the flexor tendons and the suspensories. This is why long, sloping pasterns are an unfavourable type of conformation.
Gait Adaptations to Lameness
When a horse is forelimb lame, Clayton said he tries to control and reduce GRF by changing his movement. She said the easiest way to accomplish this is by slowing down.
“If a horse is forced to maintain speed (e.g., because of the rider or handler’s urging),” she explained, “he will compromise in other ways,” such as decreasing his suspension to avoid generating the extra propulsion needed to elevate his body into a suspension phase. This is why a mildly lame horse’s trot might lose its springiness and become flat.
Clayton said these horses might also increase the duration of the stance phase of their stride—which allows them to spread the vertical force on their limb over a longer period—and take more steps per minute, spreading the vertical force over more strides. When this happens the horse’s trot looks more earthbound.
“Once a horse exhausts these mechanisms, he needs other options,” she said, such as unloading the painful limb by redistributing force to the other limbs . One of the manifestations of this is the telltale head nod, as the horse attempts to shift weight away from his lame limb.
“The head and neck comprise 10% of the horse’s body weight,” Clayton said. “Rocking these up and down shifts the weight of the lame forelimb to the hind limb. The head is raised to a higher level right before the lame foot touches the ground.”
What about hind-limb lameness?
Clayton said horses with hind-limb lameness will be reluctant to weight or push off with their lame leg, compensating by elevating their haunches to the highest point just before the lame leg contacts the ground, which we recognize as a hip hike.
What you see as the horse trots is that the entire pelvis sinks to its lowest point during the compensating limb’s stance phase and is rises rapidly to reach its highest point at contact of the lame hind limb.
“The compensating limb does all the heavy lifting,” Clayton said.
“Equine locomotion involves consistent patterns of limb movements that change in predictable ways when the horse is lame. The lame horse decreases the peak vertical force on the lame limb by moving with a flatter trajectory (less suspension), spreading the force over a longer period of time, and redistributing the weight among the lame and compensating limbs.”