July 2016 - A Day With Dr. Hilary Clayton

Understanding equine biomechanics gives rapt audience in Petaluma an edge in their training and riding.

by Patti Schofler

As a renowned expert on equine biomechanics, Dr. Hilary Clayton’s world is about bones and muscles. In May this equestrian, veterinarian, author, researcher and clinician shared her extensive knowledge of what lies under the skin of a horse and how that influences performance and conditioning with nearly 70 rapt listeners at a day-long seminar in Petaluma, hosted by Equinology, Inc. equine bodywork school, at Flying Cloud Farm in Petaluma. The talk covered the mechanics of equine sports, core training exercises for horses, symmetry and purity of gaits.

Dr. Hilary Clayton discusses equine biomechanics at Flying Cloud Farm in Petaluma. The day-long seminar was sponsored by Equinology, Inc., the equine body work school.

The day struck off with conformation analysis as determined by lengths and angulations of the bones, overall bone proportions, body balance, symmetry/straightness and strength of horse. The audience came to understand that what creates good posture in horses is when the body parts are aligned to fight the effects of gravity and allow horses to support themselves with minimum of effort. Poor posture may be conformational or acquired, for example, from pain.

Through the morning, Dr. Clayton explained the uniqueness of the horse’s body, beginning with an explanation of the horse’s core, or all the parts that aren’t legs: axial skeleton (skull, spine, ribcage and pelvis), back muscles, abdominal, sublumbar and extrinsic limb muscles.

Axial skeleton includes 56 vertebrae. “It is amazing that you can take these little bones and create something that holds together to let us ride,” she said, as she described the aspects of the vertebrae and how they function.

The vertebrae, dividing into cervical, thoracic, lumbar, sacral and coccygeal sections, do not follow the topline of the horse as we see it, but instead form a curve that allows ample room for attachment of muscles and tendons and ligaments that create the shape for the horse’s neck and trunk. Horses do not have a clavicle or collar bone, as humans have, to connect the forelimbs to the body. Instead, the thoracic sling muscles form a springy attachment between the forelimb and the trunk. Tension in the left and right sling muscles affects the position of the withers and ribcage relative to the forelimbs. When the forelimbs are grounded, bilateral action of the thoracic sling muscles raises the sternum and withers and assist in rocking the horse’s weight back onto the haunches. Unilateral action affects straightness and the position of the shoulders.

When the sling muscles develop more tone, the horse can travel with the wither and ribcage lifted in self-carriage. Traction of the sling muscles elevates the withers, providing the horse with the most effective posture in which to carry a rider.

With sling muscles development, you will see the withers rise up. “This is why you don’t invest in an expensive saddle the first few months that a horse is ridden,” she advised.

The Mid-Section

Looking toward the back of the horse, Dr. Clayton discussed the ribcage that protects the body’s organs, drives respiration and contributes to the forelimb attachment. “When we want him to turn, we want him to yield in the ribcage and give you a place for inside leg. Horses have to learn to shorten the ribcage on the inside and lengthen it on the outside. And the less spring (or projection outwards) in the ribs, the easier the ride. There is limited lateral, or sideways, movement in the trunk.

She pointed to the different possibilities in back shape. Measured from the withers to the croup, the back has a natural curvature. As riders, we seek a neutral spine, or the position of an individual’s spine where every joint is held in an optimal position to allow a favorable distribution of force through the entire structure. In a horse, a sway back (lordosis) has too much curvature downward. Roach back, curvature upward (kyphosis), can be often associated with pain in stifle or hind leg or is formed from birth.

The Back

As a competitive dressage rider herself, Dr. Clayton chooses short-backed horse every time. Horses with long backs usually have the same number of vertebrae, but the vertebral bodies are longer. However, about 30 percent of horses have the sixth lumbar vertebra fused to the sacrum.

A long back is weaker and easier to bend, so it shows more lateral displacement than a short back, but requires greater muscular force to stabilize it and prevent hollowing. Therefore, long back tends to sag more than a short back, and horses with this conformation require strong core muscles to minimize the amount of hollowing.

The short back muscles have easier time rounding and lifting the back muscles. However, a short back takes more time and training to bend.

The back acts as a beam that is supported at each end by the legs, but tends to sag in the middle due to the weight of the internal organs and the rider’s weight. The hollowing effect is evident when the horse is standing and even more so in motion. How much weight a horse should carry is a complex issue and specifying an amount based on the body weight of a horse is too simplistic. “It depends on rider’s weight, fitness, symmetry, balance, postural control and health issues.”

Hollowing of the back under a rider causes the dorsal spinous processes to become closer to each other and may result in kissing spines, said Dr. Clayton. She emphasized the importance of teaching the young horse to carry a rider with rounded back while allowing time to strengthen the abdominal and sublumbar or pelvic girdle muscles that contribute to the rounded back.

Dr. Clayton explained how the abdominal and sublumbar muscles work with the back muscles to cause the back to round and the hind limb to engage. The long mobilizing back muscles extend and bend laterally the entire back.

If they contract on both sides, they hollow the back. If just one side contracts, the horse bends the back. As one side of the back shortens, the other side must lengthen. “It takes incredible muscle force to bend a horse’s spine.”

Long mobilizing back muscles can raise the back when the hip joint is held in a flexed position and this lightens the forehand in highly collected movements through the use of back and gluteal muscles. This is self-carriage.

“Excessive back mobility during motion is not the goal,” she emphasized. The back shows flexion/extension, bending and rotation during locomotion as a result of the effects of gravity, inertia and forces exerted when the limbs push against the ground. The role of the back muscles is to stabilize the back, rather than actively moving it. Back motion decreases as speed increases due to great muscle activation.

The Neck

The neck and head make up 10 percent of the horse’s weight. The cervical vertebrae have an S-shaped profile that allows for considerable movement in all directions.

Our goal is for the horse to employ the upper neck muscles to support the neck. The elastic nuchal ligament (NL), coming from top of the withers, helps the muscles to support the weight of the neck. When the neck is lowered, the ligament stretches then bounces the head up. Tension in the NL, due to lowering the neck, can raise the back passively and help to keep the back round until the back has developed sufficient muscle strength to take over that function of keeping the back up.

“As the head and neck go down we want the muscles on the top of the neck to turn on and to control downward motion under the influence of gravity so the neck is supported from above. This is referred to in equestrian texts as ‘the falling down neck.’ As the horse becomes stronger and develops better self-carriage, the base of the neck is raised between the forelimbs.”

The atlas and axis are the first two cervical vertebrae. The joint between the skull and the atlas allows the head to flex and extend and also to flex laterally. The second joint between the atlas and axis allows the head to twist (tilt) due to excessive flexion of the joint between the axis and C3. This type of carriage usually is a consequence of the rider using the reins to position the poll.

The entire neck can be turned, raised and lowered from its base. The joints in the middle of the neck show surprisingly little movement The joints just behind the poll adjust head position and provides a stable platform for proprioceptive input.

Carrot Stretches

Dr. Clayton’s diagramming of the workings of the back, abdominals and the neck gave the audience an understanding of the how the body works in order to apply core training exercises to improve performance.

Known familiarly as carrot stretches, the horse follows a controlled movement pattern to supple the intervertebral joints and strengthen the muscles that move and stabilize the horse’s back. Carrots cut lengthways or another type of treat are usually the motivator.

Carrot stretches can be used to stimulate three types of movement: rounding, extension and bending. They are useful for young horses preparing to go under saddle, for rebuilding strength and flexibility after surgery, and for athletic horses to improve performance and reduce risk of injury. Three scientific studies of the effects of carrot stretches have included riding school horses, horses used for therapeutic riding and racehorses. All three studies showed an absolute increase in the size of the multifidi muscles, which play an important part in stabilizing the spine and preventing back pain, in as little as six weeks.

The principle of the carrot stretches is that the horse glues its feet to the floor, stands square, and follows the bait. As the spine is moved and stabilized, it activates and strengthens the muscles. You start with a small amount of movement, and hold the position for several seconds, then allow the muscles to relax. Ideally, each exercise is repeated three to five times daily. Clayton assured the audience, “Don’t beat yourself up if you can’t do the exercises every day or if you don’t have time to do every exercise every day. But do try to do some of the exercises at least four times per week.”

Dr. Clayton also described stimulated exercises that are used to further improve the horse’s core strength, balance and stability.

Clayton, in collaboration with Dr. Narelle Stubbs, has published a compilation of these exercises in a book and DVD entitled Activate Your Horse’s Core: Unmounted Exercises for Dynamic Mobility, Strength and Balance” and which details the best practices for the exercises. For example, at one time it was thought best to do these exercises after work, when the muscles were warm. Now experts recommend doing them immediately before work in order to activate the core muscles in preparation for the work that follows. They should be regarded as muscle activating exercise, rather than just a form of stretching.

Symmetry & Conditioning

The third segment of her talk was devoted to conditioning, straightness and symmetry. Among the causes of asymmetry in horses are:        
1.    Always being led and mounted from the same side
2.    Poorly fitted saddle
3.    The crooked rider who typically collapses the left hip and twists to the right
4.    Injury
5.    Asymmetrical anatomy of the body organs such as the cecum
6.    Acquired asymmetry of body structures such as the pelvis
7.    Heredity
8.    Sensory and motor lateralization within the brain are the major reason. Horses prefer the right eye to look at unfamiliar objects and are more likely to use the left eye to process a negative object. They are more reactive to a stimulus presented to the left eye. Horses’ very mobile ears move to a localized sound. They have an opposite ear preference for whinnies from a neighbor versus a stranger.  The right ear and eye project primarily to the left hemisphere of the brain and vice versa.
9.    We know less about what “leggedness” in horses than we know about human “sidedness.” Clayton uses the terms “stabilizing” and “mobilizing” to distinguish the functions of the two sides. For example, the leg that halts first is the stabilizing leg, and the leg that the horse moves up last and lifts off first is usually the mobilizing leg.

Consequences of sidedness are that horses prefer turning in one direction, usually fall on one shoulder, and take a heavier contact on one rein.

The hoof on the stabilizing front leg tends to be bigger. Foals reared in pasture may develop early signs of sidedness due to habitually graving with one forelimb forward (hoof becomes wider and flatter) and one forelimb back (heel becomes higher)

Equine sidedness is related to differences in strength between left and right limb. The difference in carrying and pushing power produces asymmetries in limb movements and/or body position. Asymmetries are easier to see in symmetrical gaits: i.e. walk, trot, passage piaffe, and at extremes of performance i.e., extensions and passage/piaffe.

Right and left sling muscles must be equally strong and able to hold up the ribcage centrally when there is weight bearing on one forelimb. However, with most horses one side is stronger resulting in asymmetrical positioning of the shoulders and ribcage on the left and right reins. This also carries through to differences in rein tension that is evident not only under the rider, but also when the horse works in side reins.

To illustrate her point, she gave a detailed explanation of how an asymmetrical piaffe reveals itself. Gaits with suspension use elastic rebound in the tendons and ligaments to bounce the horse off the ground. Piaffe has no suspension and relies on muscle strength to lift the body. Therefore, asymmetries in muscle strength and development are particularly evident in piaffe.

If the weaker mobilizing limb is not strong enough to raise the body, the horse uses its head and neck to increase the push against the ground, compensating for the weakness.  You see alternating short step and long step in the hind limbs because one limb pushes more forward and the other pushes more upward. This can be severe enough to simulate lameness.

Training will improve symmetry. “The rider should take care not to fall into the horse’s asymmetry pattern. Asymmetry might not be your fault, but you have the power to change it,” she said.

Some exercises to train the sling muscle to come into play so the horse can move in self-carriage are:
1.    Encourage engagement of the sling muscles on turns and circles by lightly using the whip on the shoulder.
2.    Be conscious of weighting the side of the mobilizing limb – sit heavy on that side to encourage the horse to carry more weight. Lateral work also helps to improve coordination.
3.    Halt by stepping up with either leg last
4.    Walk downhill with short, deliberate, equal steps. Halt on the downhill slope. On left and right limb, rock back and forward one other step at time. Include half steps on a slight downhill grade.
5.    Ride straight on uphill and downhill slopes.
6.    Do lateral work on uphill and downhill slopes.
7.    Walk across hillsides both directions.
8.    Strengthening left and right sling muscles and pelvic stabilizer also benefits agility and balance.   

Purity of Gaits

Over the past 20 years, the dressage horse has shown huge improvements in the quality of the gaits. Also improved has been the knowledge of biomechanics, in large part because of technology, especially slow motion videography. What we know now to be true about the biomechanics and movement of a horse cannot be seen by the human eye which doesn’t see fast enough. Therefore, in a sport such as dressage that is judged by humans, we need an objective description of what is correct.

“With slow motion video, we can see, for example, if the diagonal pair doesn’t hit the ground at the same time.

“When assessing whether contact of a pair of limbs is synchronous, remember that you can only judge this at the moment when the first hoof of the pair contacts the ground – not in mid-stance or mid-swing.”

Blossoming from our use of slow motion videography has been a heated discussion about the value of what is known as trot diagonal disassociation. A pure trot has a two beat stride with the diagonal pairs hitting the ground simultaneously. In a trot with a diagonal disassociation, the hind hoof hits the ground momentarily before the diagonal front hoof. According to Clayton, this happens because of the elevation of the forehand and uphill balance, all of which are positive attributes. “We need to consider the exceptional talents of the modern dressage horse and how these have affected the gaits and movements.

“These horses are being bred for self-carriage, and uphill balance is a feature of self-carriage.”

Currently Dr. Clayton is researching the fact that there is less concussion on the limbs with hind first dissociations compared to synchronous contacts. Her goal is further understanding of self-carriage and collection.

As the day drew to an end, several of the attendees were quick to request from Debranne Pattillo, Equinology’s founder and owner, to set up a return engagement soon with Dr. Clayton. Dates are in the works.