Dr. Hilary Clayton: “It’s All About the Forelimb”

Presented at the USDF Convention 2014: Cambridge, MA

On December 5, 2014, I had the opportunity to attend my first ever USDF Convention, held in Cambridge, MA; my primary motivation for taking on the Friday AM commuter traffic to Boston was to hear a lecture being presented by Dr. Hilary Clayton.  Clayton (BVMS, PhD, Dipl ACVSMR, MRCVS) is truly a pioneer in the field of equine biomechanics and I have heard over and over that her lectures are not to be missed; it seemed silly to allow an opportunity to finally attend one to slip away.

Clayton has written several books and is a frequent contributor to the USDF Connection; I enjoy reading her articles but I have always felt that some of her concepts go over my head.  Hearing her articulate and clarify her research was incredibly enlightening.  Here, I will attempt to summarize her remarks presented at the convention this year.  The section headings here mimic those of her talk; why reinvent them when her own words do such a good job?

The Limbs In General Terms

Clayton began her talk by explaining that the limbs of any species are made up of a series of rigid bones which articulate at moveable joints; these joints are stabilized and moved by muscles.  The length of the bones, combined with the angles of the joints, affect each limb’s ability to support body weight and/or provide propulsion to its owner.

If you think about a heavy species, such as an elephant, and you look at the skeletal structure of the limbs, you will see that they are strong, straight and vertical.  This design is excellent for bearing weight, but not so good for athletic endeavors.  Due to this structure, Clayton says that elephants are actually not capable of a moment of suspension and can’t jump, which is why a small moat will contain them at a zoo.  This is an example of a “limb as a supporting pillar”, according to Clayton.

Species with small body weight, such as the cat, tend to have limbs with bones that are highly angled and joints which are compressed.  This allows a great deal of athleticism but sacrifices the ability to bear weight.  By opening up the angles of these compressed joints, these species are able to produce large amounts of propulsive force.  In addition, the spines of these species are usually more flexible, allowing two moments of suspension per stride in the gallop—once in flexion, and once in extension.  Clayton calls this anatomy “limbs as a propulsive lever”.

The anatomy of the horse actually combines these two extremes:  the forelimb is more elephant- like and straight, while the hind limb is more cat-like and angled.

In dressage and jumping training, the focus is correctly very much on the action and use of the horse’s hind limb, which ultimately controls the horse’s ability to leave the ground.  However, Clayton points out that “a fabulous hind leg is no good without an equally fabulous front limb.”  This is because the less angled joints and more upright posture of the horse’s front limbs allow them to act as struts for the horse; the forelimbs ultimately control the position of the forehand and most importantly, control the horse’s speeding and turning ability.

Ground Reaction Force

One of the coolest aspects of Clayton’s presentation was reviewing computerized footage of actual horses moving over the force plates at her former research facility at Michigan State University (Clayton became “emeritus” in April of 2014).   These videos showed how and where the force of movement translated itself through the horse’s body, and also how those vectors moved throughout the course of a stride.

Clayton explained the concept of Ground Reaction Force (GRF) as being the force which actually makes the horse move.  When the horse’s hoof is on the ground, it is automatically pushing against the ground; the GRF is the reaction of the ground pushing back against the hoof.  Because the front limbs bear more weight, the GRF is always higher on them.

The relative sizes and directions of the GRF’s of the forelimbs and hind limbs affect the horse’s balance.  Basically, it is the job of the hind limb to create propulsion, while the forelimb stops the horse’s balance from going wholly onto the forehand.  By changing the angle of the GRF, the horse controls his speed and direction.

To help us to understand the relationship between the GRF and the roles of the front and hind limb, Clayton used a video of a horse jumping a fence.  At take-off, the hindlimb forces cause the jumping horse to rotate forward, towards their center of balance.  The forward rotation is necessary for the horse to be able to take off from the hind limb and land on the fore limb.  At landing, the GRF of the forelimb causes a reversal in the direction of rotation, allowing the horse to shift back towards their center and land the hind limbs.

Finding the Balance

So basically, the horse’s body in movement is a set of opposing forces—one set from the hind limb which propels the horse forward, and one set from the fore limbs which prevent that force from pushing the horse down.  The conundrum is that the harder the hind limbs push and the longer they stay on the ground (so, increased engagement), the greater the tendency is for this force to rotate the horse onto the forehand.  When the hind limbs trail behind the horse, the force pushes the horse onto the forehand.  The role of the forelimbs becomes to maintain an uphill balance and counteract the tendency to fall forward.

Clayton reminded us that horses as a species have adapted to be “cursorial” (aka runners).  Cursorial species have limbs with certain qualities; in particular, the weight of the limb is concentrated in the upper section, with heavy muscles around the hips and shoulders to control their movement.  Cursorial species have lightweight tendons in the lower limb, which is supported on a single digit (toe).  The length of a horse’slimbs is extended by being “unguligrade”, which simply means that they stand on their tip toes, as opposed to their flat toes (or digits, hence digitgrade, like cats and dogs) or plantigrade, like humans.  Our heel is roughly equivalent to the horse’s hock; therefore, we humans walk on the equivalent of the back of the cannon bone.

Finally, cursorial animals have limbs which are “long in stance”, meaning that the body moves further forward over the grounded hoof, while also being “short in swing”, creating less inertia and making it easier to swing the leg forward.

The Forelimb Attachment and Support

As horses do not have a clavicle, there is no bony connection between their forelimbs and trunk.  Instead, the limbs are attached and supported by a network of muscles, tendons and ligaments.  In addition, the horse’s scapula is highly mobile and can rotate and move up, down, forward and backwards across the rib cage.

Extrinstic muscles attach the limbs to the body and move them relative to the body; the extrinsic muscles move the limbs forwards, backwards and sideways.

Intrinsic muscles provide attachments between limb bones and help to bend the joints.

The thoracic (sling) muscles suspend the ribcage between the forelimbs.  These muscles attach behind the scapula as well as to the cervical vertebrae and to the ribcage.  The Serratus ventralis thoracis muscle is the most important sling muscle; its contraction raises the ribcage.  When the sling muscles are engaged, the horse’s withers lift.  When they are relaxed, the withers are low and the horse rolls onto his forehand.  Therefore, to develop uphill balance, a rider must work to develop the ability of these muscles to engage and lift.

It is the equal activity of the left and right sling muscles which holds the ribcage centrally between the forelimbs.  However, horses must also learn to use these muscles unilaterally to raise and stabilize the rib cage when one of the front limbs is lifted.  In most horses, the sling muscles are weaker or less active on one side.

Riders will experience the effects of this asymmetry under saddle in particular when turning.  Horses tend to prefer to collapse their weight onto the inside shoulder and to push off of that limb, rather than taking the weight onto the outside forelimb, particularly on their weaker/less developed side.  Therefore, they actually have to learn to use the outer forelimb to support and lift the inner forelimb when turning.  This is opposite of their natural tendency when moving without a rider.

The Forelimb in Motion

As was mentioned earlier, the horse’s scapula is highly mobile due to a lack of a clavicle.  When it slides upwards, the withers are down; when the scapula slides downwards, the withers are raised, and when it slides backwards, the shoulder is tucked in.  To see the full range of the scapula was truly impressive, and for me it drove home the importance of ensuring that the horse’s saddle is not impeding this movement.

The entire forelimb rotates around the upper scapula, at the insertion point of the S. ventralis thoracis muscle.  As the limb rotates forward, the point of shoulder moves up and the scapula rotates backwards.  The elbow muscles drive the movements of the distal limb in the swing phase of the stride.

When seen from the side, you can observe the degree of protraction, or how much the leg swings forward, and retraction, how much it swings back.  From the front, you can observe adduction, how much the horse’s limb swings towards and across the midline, and abduction, how much the limb moves away from the midline.

Throughout the stance phase of the stride, the horse’s forelimb is retracted.  The trunk is pulled forward over the grounded hoof.  In the swing phase of the stride, the forelimb is protracted, then retracted; the retraction during the swing phase helps to reduce the forces of concussion on the limb.

As the degree of collection increases, the legs move through a smaller range of motion and become more vertical at contact and lift off, which facilitates the elevation of the horse’s forehand. The trapezius, pectoral muscles and the bones of the shoulder and forelimb work together to help turn the horse, as well as to execute lateral movements like leg yield or half pass.

Horses naturally lean into their turns and use their forelimbs to push outward to generate a turning force.   This is why horses will feel as though they are leaning in on their stiffer side.  Through training, dressage horses are taught to maintain a vertical position while turning.

No wonder lateral movements take so much practice—think of the coordination involved and also how much the horse must resist his natural tendency to lean in!

More on the Sling Muscles

The sling muscles which support the horse’s shoulders and forelimb must be developed in the equine athlete.  Bilateral activity of these muscles contributes to good posture in the horse, and allows him to elevate his withers.  Unilateral activity develops the strength required to create straightness.

Due to the importance of the sling muscles, Clayton actually co-wrote a book with colleague Narelle Stubbs, which is full of exercises aimed at increasing the strength and coordination of this area, called Activate Your Horse’s Core: Unmounted Exercises for Dynamic Mobility, Strength and Balance, published in 2008 by Sport Horse Publications.

Clayton referred specific questions about exercises to this book, but mentioned that downhill slope training was a great way to increase the strength of a horse’s sling.  She says that walking, halting, and performing exercises like rein back up hill, half steps and lateral movements, on a downhill slope, all while preventing the horse from leaning on the bit, will help to activate the critical muscles.

Additional categories of helpful exercise include those which challenge coordination and balance; those which elevate the point of the shoulder, and those which increase the loading of the forehand.  Two specific examples Clayton provided were jumping and teaching the Spanish walk.

A Few Words on Asymmetry

Clayton concluded her lecture with some interesting comments on forelimb strength asymmetry, and how it reveals itself to the rider.

One of her first comments was that unequal strength between the right/left forelimbs can manifest itself as a head nod, especially in highly collected movements.  The head and neck of the horse will be raised as the weaker limb pushes off, due to the horse using the strength of their neck to lift the limb up.  This is the same mechanism a horse uses in the case of lameness, so it takes a careful eye to distinguish the difference.  This weakness is most obvious in piaffe and pirouettes.

Another way that this asymmetry is detectable is through uneven rein tension.  Most horses take an uneven contact on the left versus the right rein, but the position of the head/neck, as well as small amounts of bending or twisting at the poll, can further affect the difference in rein tension.    This is particularly notable when the horse’s shoulders are not straight, as when they fail to lift the inside shoulder when turning.  Typically the rider will feel the heavier weight on the weaker side.

Conclusion

Clayton wrapped her talk with a brief summary of some ideal conformation points of the shoulder and humerus, specifically relating these body components to how the conformation will affect the mechanics of the forelimb. She also discussed what her research has shown regarding diagonal dissociation—basically, it is more common that we thought, and not a bad thing in most cases—and entertained a question and answer session for the audience.

All in all, quite an enlightening lecture.  Understanding a bit more about the biomechanics of the forelimb really helps to highlight the critical significance of correct conformation as well as the constant stresses placed upon these important structures.