Lower Limb Research--Bluegrass Laminitis Symposium

Probably the foremost biomechanics researcher in the country, Hilary Clayton, BVMS, PhD, MRCVS, McPhail Dressage Chair in Equine Sports Medicine, McPhail Equine Performance Center, discussed recent lower limb research during the 16th annual Bluegrass Laminitis Symposium. Some of the studies she described were performed in collaboration with researchers at California State Polytechnic University.

“We use a motion analysis system to illustrate movement of different body parts and joint angles,” she began. “We also calculate a lot of things we can’t actually measure.” She initially focused on kinematics (motion) of the fore and hind limbs. “The maximum velocity of the horse’s hoof is about double that of the horse’s maximum velocity,” she noted. “So for a horse that’s running at about 40 mph, the maximal speed of the hoof as it swings forward is about 80 mph…That’s pretty darn fast.”

She went on to compare durations of the swing and stance phases of the fore and hind limbs during trotting. “Stride duration, which is the sum of stance duration and swing duration, decreases as speed increases. In the forelimb, the swing phase doesn’t change much with increased speed,” she explained. “so the reduction in stride duration is from a decreased duration of the stance phase--the leg spends the same amount of time in the air but less on the ground.” In the hind limb, both the swing phase and the stance phase durations decrease with increased speed.

“The limbs push against the ground during the stance phase to provide propulsion. When the stance phase shortens with increased speed, the horse must generate higher forces to compensate for the decrease in contact time,” she said. “With humans, and likely also with horses, the ability to achieve high sprinting speeds is influenced more by the ability to generate large forces in the stance phase than the ability to move the limb rapidly during swing.”

Equine Evolution and Physiology

Clayton also discussed the evolution of horses’ limbs and their adaptation for faster running speed. She noted that the long limbs of the horse (particularly the long cannon bone, which is analogous to a human’s hand or foot bone) help him cover more ground, as the distance his body rotates forward over the grounded limb is proportional to limb length. Additionally, the lower limbs are very light in weight, with the large, heavy locomotor muscle groups concentrated around the shoulder and hip and no muscle below the knee or hock. This reduces the moment of inertia of the limb, making it easier to swing the limb back and forth. The lower limb is also lightened by the decreased number of digits compared to species with multiple toes/fingers.

“The joints flex during protraction, bringing the limb mass closer to the point of rotation, which requires less effort to swing them,” she added. “There is an effective shortening of the limb in the swing.”

She showed the audience that the hoof flight arc is not the simple single arc like many believe for either front or hind limbs; rather, it is biphasic with the first peak higher than the second. Also, she noted that with increased speed, the early peak in flight arc for front and hind limbs became higher with increased velocity, reflecting more flexion of the joints.

Heavy Shoes

Since this symposium’s attendees are mostly farriers, they were quite interested in shoe research. Clayton discussed research on the effect of using shoes or increasing the weight of the shoes, noting that there was no apparent difference during the stance phase, but the addition of weight to the hoof increased animation during the swing phase. She described increased carpal (knee) and fetlock flexion, resulting in a higher peak flight arc.

“The proximal (upper) limb is decelerated and retracted prior to ground contact, resulting in a whiplash effect in the distal limb,” she added. “With heavy shoes, the whiplash effect is exaggerated. As a result, the pastern tends to be more horizontal during late swing, and the horse has more exaggerated heel-first ground contact. What we found is that you can change the end of the stance phase and early swing phase (with heavier shoes), but the horse can reestablish normal conditions at impact unless the speeds are very high or the shoes are very heavy.”

The muscles were of course affected as well--Clayton explained that the weight of the shoes caused the elbow flexors work harder in early swing to pull the limb forward, and the elbow extensors to work harder late in the swing phase to initiate retraction. This principle can be used as a training method for increasing muscle strength. For a short time following removal of heavy shoes, the horse is likely to continue to show the same animation as he did with the heavier shoes due to muscle memory.


Torque, or turning force, is required to flex and extend the joints. Measuring normal forces is essential to understanding how different factors can change those forces for better and worse.

“Muscles move or stabilize joints by creating torque,” Clayton said. “The net torque at the weight-bearing joints of the limbs opposes the torque due to the ground reaction force. It counteracts the tendency of the limbs to collapse during the stance phase. Breakover at the end of stance involves quite a bit of leverage, or torque at the coffin joint, which acts as the fulcrum during breakover. Changes in hoof angle and toe length affect coffin joint torque.

“It’s not a simple relationship between hoof conformation and coffin joint torque; the more I know, the less I know,” she continued. “With an increase in the hoof angle, tension decreases in the deep digital flexor tendon (DDFT) and breakover is delayed. Because the breakover is delayed, the ground reaction force is lower when breakover starts and there is less torque at the coffin joint. Conversely, with a decreased hoof angle, DDFT tension increases, which might hasten breakover, but with an increased torque at the coffin joint.

“If the shape of the bearing surface of the hoof changes, the force that resists breakover is distributed differently and the point of force application moves relative to the coffin joint. This changes the length of the moment arm (leverage) on the joint. Coffin joint torque during breakover is affected by both the magnitude and distribution of the DDFT force and the ground reaction force. The components cannot be considered in isolation, however.

“In the next year or so, I’m hoping that we can do some major projects on the hoof,” Clayton said. “Now I have a combination of analytic techniques that gather all the information I need simultaneously in a coordinated fashion, rather than focusing only on kinematics in one study, force plate in another, etc.

“The effects of farriery manipulations are not always intuitive,” she concluded. “What we need now is enough money to really study these concepts and prove or disprove them.”

About the Author

Christy M. West

Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.

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