The Significance of a Horse’s Chest Sling Muscles Biomechanic Research by Hilary Clayton

Written by Betsy LaBelle - In Dressage - Friday, February 15, 2019

Seventeen years of collecting data on gait analysis for dressage horses in the equine laboratory at the McPhail Center at Michigan State University, biomechanics research veterinarian Hilary Clayton, BVMS, PhD, DACVSMR, MRCVS, conducted studies to evaluate a horse’s body during collection.

She ascertained how much weight each of the four limbs of a horse carries, how much propulsion are in each of the horse’s legs, the significance of a horse’s chest and trunk, and how crucial it is for a rider to have a well informed understanding on the sling muscles to aid a horse’s balance.

Unlike the human shoulder girdle where the collarbones (clavicles) attach the arms to the body, a horse has none. Without a collarbone, a horse has no bony connection between its front limbs and trunk. Instead, strong muscles connect the inside of its shoulder blades to its rib cage, which act like slings and suspend the chest between the horse’s two front limbs. The 'sling muscles' consist primarily of the serratus ventralis thoracis muscle assisted by the pectoral muscles. 

Contraction of these sling muscles lift the trunk and withers between the shoulder blades, raising the withers to the same height or higher than the croup. When a horse travels without proper contraction of its sling muscles, the horse's motion looks downhill and on the forehand.

Horses in Clayton’s studies were ridden in a working frame, a collected frame and a downhill frame in order to check the different populsion of each leg. Motion analysis markers were used to measure movement of the trunk, neck and croup as well all of the horse's legs during many data sessions on a horse's populsion and collectability. The average horse carries 58 percent of its weight on its front legs and 42 percent on its hind legs. She discovered the horse must learn to move in an uphill balance by pushing upwards with its forelimbs. The hind legs can then function as they should by sitting to carry more weight and by providing pushing power. In essence, the heavy chest needs to be up and out of the way for the hind legs to push.  

The Sling Muscles and Self-Carriage

The sling muscles are extremely important to the self-carriage of the dressage horse. The goal in dressage training is to teach the horse to use its sling muscles throughout the workout. With time, these muscles get stronger and the persistent elevation allows the horse to push and hold its hind legs under the center of gravity through its motion to be even more pronounced and uphill.

The toning of the sling muscles increases with a rider who balances the shoulders throughout training while also balancing with half-halts. This raising of the frame, if balanced correctly by the rider, will allow those muscles to become stronger and more elastic and aid in the horse learning to hold its own frame.

Riders tend to think crookedness comes from the back and hind legs of the horse. However, it is the horse’s serratus ventralis thoracis muscles and its shoulder blades that also play a role in the crookedness equation. Since a horse is stronger on one side than the other, it allows one shoulder to fall in on a turn or drift out on the other, depending on the stronger or weaker side. 

“These muscles,” Clayton explained, “fan out from the shoulder blade onto the ribs and on to the vertebrae at the base of the neck. When they connect they raise the withers so they emerge into a higher position between the scapulae and also raise the base of the neck.”

"In a young horse, the strength of its sling muscles are often asymmetrical on the left and right sides and that plays a significant role in its crookedness. Riders, therefore, must focus on teaching the horse to use and develop the muscles on its weaker side to make them more symmetrical for balance and self-carriage. In time, the horse will begin to balance in a more upright position without falling in or out of the turns." 

She also discovered that the horse's pectorals get bigger and grow stronger if the chest is balanced up during smaller circles, correct turns and going sideways (lateral work) because these muscles are important for holding the front legs in a vertical position during their stance phase and for crossing the forelimbs during their swing phase. 

A horse’s shoulders and trunk are heavy; therefore, in training and working toward collection with a horse, a rider must learn how to balance the chest and the trunk upwards so the hind legs can come underneath to provide propulsion and support. Clayton emphasized, “It’s the balance of the trunk that allows the push from the hind legs to go through the horse's body without pushing it onto the forehand.”

Posture of Both Rider and Horse

There is a distinct correlation between the rider’s posture and the horse’s posture as they train together. If the rider’s core muscles are not engaged, then the horse’s core muscles also will not be engaged. Even though the horse has a distinct advantage in having four legs, a rider must learn to hold his or her own posture in order for the horse to engage its own core strength, which is necessary for it to hold its frame up.Ashley Holzer and Sir Caramello at the Dutta Corp U.S. Equestrian Festival of Champions Photo: DH

A horse’s self-carriage is achieved through controlled tension of the muscle groups. There is a muscle ring that wraps around deep inside the horse through its back and abdominal muscles which allow it to maintain roundness of its back. The abdominal muscles encase the abdomen from the pelvis to the ribcage to the sternum. Contraction of these muscles and the back muscles allow the horse to be supple and loose to free its legs to push and carry all of its weight.

Equal Pushing Power

Most riders think that only the hind legs need development and push. More accurately, the push from the hind legs has to be supported by the upward push of the front legs. So pushing power of the hind legs must be harnessed by the elevation of the forehand so the horse can perform with controlled power in an uphill balance. 

About Hilary Clayton

“I grew up in England foxhunting and in pony clubs in Derbyshire, which is right in the middle of England. I am a veterinarian and I always wanted to study the horse’s biomechanics, but the technology did not exist when I came out of university. I've been studying equine biomechanics for almost 40 years and I am convinced that understanding the action of the horse's sling muscles and the role of the forelimbs are crucial to understanding the mechanics of self-carriage in the dressage horse."

Clayton said Mary Anne McPhail was instrumental in providing the facilities and equipment needed for this type of research. “Thanks to Mary Anne, I was able to make enormous strides in understanding the mechanics of the dressage horse and the rider's interaction with the horse at Michigan State University. I am so grateful that Mary Anne made that possible." 

Serving for almost seventeen years as the Mary Anne McPhail Dressage Chair in Equine Sports Medicine at the Michigan State University College of Veterinary Medicine, Clayton has since retired from her post. She continues to remain active in the research and presenting lectures that describe her amazing findings to the world. She is based at her home in Michigan during the summertime and migrates to Wellington, Florida for the wintertime. Currently, she is appying her knowledge to develop better tack and equipment for the dressage horse. 

Continuing to bring technology into riding, she said, “I love to do research and I'm currently working on several projects that have practical applications for dressage horses and trainers. That's what keeps me active and engaged." Always available to share her findings, Hilary Clayton gives great lectures and workshops all over the world.