Horses generate ground reaction forces (GRF) when they press their hooves against the ground to produce locomotion. One world-renowned researcher in equine locomotion and biomechanics says recognizing these invisible forces is key to understanding how horses move and how they compensate for lameness.
As one of the keynote speakers at the 2019 Equine Science Society Symposium, held June 3-6 in Asheville, North Carolina, Hilary Clayton BVMS, PhD, Dipl. ACVSMR, FRCVS, of Sport Horse Science, in Mason, Michigan, described GRFs and how they change with different movement patterns and lameness.
Ground reaction forces act in various directions and can be affected by the horse’s body weight, gait, speed, shoeing, and footing, said Clayton. Researchers study the vertical, longitudinal, and transverse components of GRFs to understand how they affect movement and performance. You see vertical GRFs, for instance, when horses jump over obstacles; longitudinal GRFs when horses gallop; and transverse GRFs when horses pivot and turn sharply, she explained.
Vertical GRFs represent weight-bearing forces; the heavier the horse, the higher the vertical GRF. They increase to project the horse’s body into the air, such as when jumping over obstacles.
Longitudinal GRFs are responsible for speed and acceleration. They intensify when horses change speed, such as when accelerating out of a starting gate. The hind limbs are the horse’s motor, providing most of the propulsion, while the forelimbs control speed and direction, said Clayton. Their orientation indicates the type of forces they are generating. For example, if a horse needs to stop abruptly, he braces his forelimbs out in front of his body so the GRF acts in a backward direction.
Transverse GRFs act from side to side to turn the horse. The forelimbs are largely responsible for this turning motion. The simplest way for horses to turn, said Clayton, is to lean to the inside, making it easier for the legs to push outward to generate a turning force. Her research has shown that, on average, horses lean in by 15 degrees on a 6-meter circle.
“The two horizontal (longitudinal and transverse) GRFs are generated as a result of friction between the hoof and the ground,” Clayton said. “That’s what stabilizes the hoof and allows the leg to develop forces that push the horse forward, backward, or sideways.”
At the moment of ground contact, the horse’s hoof moves downward and forward. The hoof first decelerates in a downward direction; hard surfaces cause rapid deceleration and high concussion, while softer surfaces allow gradual deceleration with less concussion, she said. The hoof then slows in a forward direction due to the friction between it and the ground.
Ideally, said Clayton, there should be enough friction between the hoof and the surface to slow the hoof’s forward motion gradually. On sand surfaces, the foot sinks in a little, then plows forward through the sand for a short distance, slowing gradually. Hard surfaces such as blacktop often have low friction and allow the hoof to slide uncontrollably. Grippy surfaces, including many of popular synthetic footings, have high friction and might stop the hoof too quickly, which increases concussion. One of the challenges in finding good footing is having enough, but not too much, friction, Clayton said.
She went on to explain how ground reaction forces change during circling. When trotting on a circle, the fore and hind limbs on the outside carry more weight than those on the inside. On small circles and/or at fast speeds, this asymmetry gives the appearance of lameness in the limbs on the inside of the circle.
It’s important for veterinarians to be aware of this when conducting a lameness or prepurchase exam that includes trotting on a circle, Clayton said. Otherwise, they might wrongly diagnose the horse with a bilateral (affecting both sides) lameness.
In summary, she said, horses use their muscles to adjust the magnitude and direction of ground reaction forces between their hooves and the ground to adjust the speed or direction at which they’re traveling. Each gait has its own fore- and hind-limb GRF pattern, and lame horses modify these patterns to unload the painful limb, leading to the characteristic appearance of lameness.