Sport Horse

Strengthen the Sling

Want your horse to achieve better collection? Help him strengthen the muscles of his thoracic sling. Second of two parts.

By Hilary M. Clayton, BVMS, PhD, Dipl. ACVSMR, FRCVS

In my last “Sport Horse” column (“The Thoracic Sling,” January/ February), I looked at the structure and function of the muscles— known as the thoracic sling—that suspend the horse’s trunk from the weight-bearing forelimbs. In a standing horse, well-toned sling muscles improve the horse’s posture by raising the forehand, which

causes the withers to become more prominent between the scapulae. During locomotion, the sling muscles contribute to an uphill posture and to centering the horse’s chest between its forelimbs. Poorly toned or inactive sling muscles allow the forehand to slump between the scapulae, making the withers less prominent.

The main component of the thoracic sling is a muscle called serratus ventralis, which runs downward from the inner surface of the shoulder blade to the ribs and the lower cervical vertebrae on each side. Besides raising the withers, serratus ventralis resists forward and downward motion of the trunk when the horse’s forelimb(s) are on the ground. In other words, it helps to prevent the horse from falling onto the forehand.

Another characteristic of serratus ventralis is that it is encased in a strong elastic sheath that stretches as the limb bears weight, then recoils as the limb pushes off. The elasticity cushions force transmission to the horse’s trunk (and ultimately to the rider’s back) when the limbs hit the ground during such movements as the extended trot, which involve rapid deceleration just after limb contact.

In summary, we can think of the functions of the serratus muscle as: • Providing attachment of the forelimbs to the trunk • Determining how the forehand is suspended relative to the position of the forelimbs, both from front to back and from side to side • Lifting the forehand to put the horse in an “uphill” balance • Centering the forehand between the forelimbs after the horse has been straightened • Restraining the forehand in the face of decelerating forces that tend to tip the horse onto the forehand.

Visualize the sling muscles acting in harmony on the left and right sides to support the forehand in an uphill balance, with the ribcage centered so the rider can sit in the middle of the horse with both seat bones supported equally.

As you can see, a strong thoracic sling is necessary for good dressage performance. Fortunately, there are many ways of strengthening a horse’s sling muscles. Some exercises are done unmounted while others are performed under saddle. Read on to learn how.

TERRAIN TRAINING: Walking on a downhill gradient is a “magic bullet” for strengthening the muscles of the horse’s thoracic sling

Exercises from the Ground

Many readers are already familiar with the use of core-training exercises to stabilize the horse’s core muscles. Owing to their role in positioning the horse’s chest, the sling muscles are considered part of the core musculature. I will describe how to perform the following unmounted exercises from the horse’s left side, but they should be done with an equal number of repetitions on the left and right sides. Even

THE SLING IN ACTION: When a horse engages its sling muscles, the withers lift

SLING ANATOMY: Side view of the muscles that attach the horse’s scapula to its chest. Serratus ventralis is the main component of the thoracic sling.

though you may not be aware of it, horses often transfer their weight imperceptibly toward the handler, which affects how the muscles are used to control their balance.

Baited stretches. The first level of core-training exercises is baited stretches or “carrot stretches.” These exercises require the horse to stabilize his center of mass so that it remains within the base of support provided by the grounded limbs. The horse follows the bait downward (rounding exercises) or around to the side (bending exercises). Start by rounding your horse’s neck and back: •Stand on the left side facing your horse’s shoulder. •Holding the bait in your right hand, pass your hand between his forelimbs from behind. •Use the bait to entice him to move his muzzle down and back between his forelimbs. •Encourage him to hold the rounded position for several seconds before releasing the bait. •Vary the height of the bait from knee to fetlock level. •Do three repetitions from each side.

As the horse reaches down, the sling muscles stabilize his chest and avoid the tendency to tip forward. Try to keep his chest centered between his forelimbs as he stretches. If he twists his neck, have a helper hold the halter to guide his head and neck and keep them straight. In most horses, the sling muscles are naturally stronger on one side, and by being a stickler for straightness, even in the simplest of exercises, you can work toward improving symmetry.

The next exercise is to perform baited stretches with the horse bending to the side through his full range of motion. • Stand on the left side of the neck with the bait in your right hand. • Entice your horse to move his muzzle around to the side. • Holding the treat at the height of the stifle, take his muzzle as far backward as possible. • Hold the position for a few seconds before releasing the bait. • Do three repetitions on each side. • Repeat the exercise, taking the muzzle as far as possible toward the hind fetlock. • Hold the position for a few seconds before releasing the bait. • Do three repetitions on each side.

As the horse turns, many of the core muscles are recruited to stabilize the forehand. Initially, most horses find it easier to turn in one direction, so one of our goals is to make them more symmetrical in their ability to shorten the muscles on one side while lengthening the muscles on the opposite side. As you know, the ability to coordinate muscle contractions on the inside and outside of the bend is an important skill in dressage horses!

Lifting the withers. From the ground, we can teach our horses to lift their withers in response to a stimulus applied under the chest. [

LIFT THE WITHERS: I’m applying pressure with a blunt tool under the horse’s chest to encourage him to engage his sling muscles and lift his withers

• Stand on your horse’s left side facing his shoulder. • Hold a blunted tool, such as a hoof pick, in your right hand. • Reach forward between his forelimbs with your right hand and apply upward pressure over the sternum. • Maintain steady pressure as you slide your hand back slowly between the forelimbs and across the girth line, taking about five seconds to complete the maneuver. • The desired response is for the horse to raise sequentially through the base of the neck and the withers as the pressure moves back. • Start with light pressure, as some horses are very sensitive. If your

horse does not respond to light pressure, apply gradually increasing pressure or make a scratching motion with your fingernails or a curry comb. Some horses respond better when pressure is applied in the opposite direction, from back to front. Bear in mind that horses with back issues may find it painful to raise the back, so be sensitive to your horse’s responses. • Do three repetitions on each side.

Horses that are resistant to lifting their withers in the beginning usually become more responsive as they understand what’s required. After practicing this movement for a week or two, you should see the withers and back rise up by several inches. Most horses seem to enjoy the feeling of stretching their backs after they become accustomed to it.

Lifting the withers and rotating

the ribcage. This exercise combines the lifting of the withers with bending the back just behind the withers and rotating the ribcage (see photos below). It exaggerates the rounding and bending motion we seek under saddle. • Stand on your horse’s left side facing his shoulder. • With your right hand, reach under the girth line toward the opposite side of his ribcage. • Using your fingers or a blunt tool, such as a hoof pick, apply pressure, “pulling” the lower ribcage toward you. • Watch the top of your horse’s back to see his ribcage rise on the left side. • Maintain pressure and hold the position for five to 10 seconds. • Allow your horse’s back to relax completely before repeating. • Do three repetitions on each side, comparing the amount of lifting and rotation on the two sides.

Backward weight shift. The value of this exercise lies in the fact that the sling muscles play a role in preventing the chest from pushing forward— or, to put it another way, the sling muscles help to position the trunk over the hind limbs. To introduce your horse to how this feels, use the backward weight-shift exercise.

1 2 3

•Stand facing your horse’s left shoulder. •Place your left hand on the front and middle of his chest with your thumb pointing upward over the highest part of the sternum between the pectoral muscles. •Put your right hand in the hollow at the side of the withers. •Gently push backward with your left hand just enough that your horse shifts his weight backward, but not so much that he takes a step back. •Try to feel the contraction of the sling muscles with your right hand as he pulls his trunk back. •Hold the position for several seconds, then release the pressure with your left hand. Watch as your horse’s trunk moves forward; this is often more obvious than the subtle backward shift. •Start by performing the exercise once on each side, and gradually increase to three repetitions on each side daily.

Weight shifts with a forelimb

lifted. The sling muscles not only move and stabilize the forehand; they also play a role in resisting loss of balance. In this exercise, a destabilizing force is applied while one forelimb is lifted so that you stimulate the sling muscles on the side of the grounded forelimb to resist being pushed off balance. This exercise is especially recommended for horses that carry their shoulders asymmetrically and that tend to fall in or out over one shoulder when turning. •Lift your horse’s left forelimb and hold it up with your right hand while you stand facing him. •Make sure he is comfortable (limb not too high), and use good ergonomics yourself, keeping your back flat and knees slightly bent. •Place your left hand over the point of his left shoulder. •Gently rock your horse in different directions: toward the opposite forelimb, the diagonal hind limb, or the hind limb on the same side.

Use a slow, steady rhythm, taking

about four seconds for each rocking motion. •You can also sustain the displaced position for five seconds.

In these exercises, the sling muscles contract to resist pushing the horse off balance. The slow, rhythmic rocking turns the muscles on and off repeatedly, whereas the five-second hold requires sustained contraction.

WEIGHT SHIFT WITH FORELIMB LIFTED: With one forelimb lifted, the handler gently rocks the horse in different directions. Because he’s destabilized, his sling muscles must work harder to maintain the horse’s balance.

Stepping up with the forelimbs

onto a raised surface. Stepping up onto a raised surface requires the extensor musculature (including the sling muscles) of the limb that takes the first upward step to contract strongly to lift the forehand. It’s the same action the horse uses to step up into a trailer. In designing a platform to step onto, it’s important that the surface is not slippery or unstable and that it can support the horse’s weight. A used tractor tire filled with sand can work well.

The height of the step can start in the range of six to 12 inches, increasing until it becomes difficult for the horse to reach the step up. •Lead your horse close to the step and allow him to investigate it. •Encourage him to step up, and reward him when successful. •Maintain the elevated forelimb position for 10 to 30 seconds. •Back him off the raised surface slowly and carefully. •After he becomes comfortable with stepping up, increase the number of repetitions up to three per forelimb.

Just as people have a preferred leg for initiating a step up, so do horses. After your horse learns the basics of the step-up exercise, encourage him to become equally comfortable and confident stepping up with either forelimb on command—for example, by using a tap with a whip to cue the desired forelimb.

Under-Saddle Exercises In dressage, our goal is to strengthen the sling muscles equally on the left and right sides—which is a component of making the horse straight. As we’ve discussed, muscles attach the horse’s forelimbs to its chest. The attachment usually is not symmetrical, meaning that the shoulder is less well stabilized on one side.

The rider’s first challenge is to align the horse’s shoulders ahead of his haunches and to maintain this alignment throughout the ride. Unless the horse’s shoulders are correctly aligned, the impulsion generated by the hind limbs cannot go through his body. Furthermore, when one shoulder tends to collapse, the horse cannot raise his forehand to develop self-carriage. Activating and strengthening the sling muscles on both sides is a crucial step in the horse’s training progression.

Under-saddle exercises that strengthen the sling muscles are

based on their role in supporting and stabilizing the forehand when a horse turns, performs a downward transition, or descends a gradient. Jumping also activates the sling muscles, especially during landing. The common denominator in all these exercises is that the sling muscles control the position of the chest between the forelimbs and resist the tendency to fall on the forehand.

Turns, circles, spirals, and lateral

work. Because turns and circles use the muscles on the left and right sides of the horse’s body asymmetrically, they provide the rider with an opportunity to focus on the functions of the inside and outside limbs. Horses naturally lean to the inside of a turn and have to be taught how to maintain a vertical orientation while bending their bodies through the turn. When the sling muscles and the pectoral muscles are correctly coordinated, they help to support the forehand with the limbs vertical during turning. Both forelimbs must push up through the shoulders in order for the horse to develop uphill balance.

Start by working on largediameter circles, decreasing the size as your horse learns how to turn in balance and develops sufficient strength to support a vertical position through the turn. Spiraling in and out is particularly beneficial in this regard because the muscular demands change constantly as the circle gets larger or smaller.

Frequent changes of direction improve the coordination between the muscles on the left and right sides and offer an opportunity to compare the feeling on the two reins, which should become more symmetrical over time. If one forelimb persistently falls in or bulges out on the circles, it may be helpful to flick your whip lightly against that shoulder to activate the lazy sling muscles.

Lateral work further challenges the horse’s ability to coordinate the supporting action of the sling muscles with the body moving forward and sideways at the same time. Each of the lateral movements calls for a different pattern of muscular coordination, and it is important to strive to perform them equally well in each direction.

Transitions. Transitions within and between gaits are excellent strengthening exercises for the sling muscles, especially downward transitions that are associated with a marked decrease in speed. In a transition from trot to halt or canter to walk, for instance, the hooves press against the ground to slow the forward movement, but the trunk has momentum that continues to push it forward. To avoid falling onto the forehand, the horse must learn to use his core muscles, including the sling muscles. Every transition should recruit the core muscles to maintain correct posture throughout the change in gait and/or speed.

When you prepare for a downward transition, visualize the sling muscles restraining your horse’s forehand and lifting his withers. Perform a collecting half-halt before and after the transition, and use your core muscles to keep your own weight back; if the rider tips forward, it makes the horse’s job more difficult. Note that the half-halt should be given when the horse’s body is at its highest point in the stride, which in trot and canter is during the suspension phases. Doing so cues the horse to activate his sling muscles at the correct time to maintain his uphill posture.

Rein back. The rein back can be a powerful training tool when performed correctly, in a controlled manner. One of the challenges in performing a correct rein back is to keep the horse’s body balanced over his limbs so that the entire horse moves in one piece. The limbs should be raised and set down without giving the appearance that the horse is leaning backward.

Typically, we practice four or five steps of rein back, according to the requirements of the dressage test. A larger number of steps can be used when using rein back as a conditioning tool, but each step must be deliberate and not rushed. It is also useful to alternate rein back with walking or trotting forward, reversing directions without a sustained halt and varying the number of steps of each gait.

Gradients. Working on a downhill gradient is a magic bullet for training the sling muscles. When a horse descends a slope, the effect of gravity increases the forelimb loading. Due to the increased load and risk of concussive injury when working on a decline, it is recommended that downhill postural training be performed only at a walk. A prerequisite to using a downhill slope for strengthening the sling muscles is that the horse must be able to negotiate the slope in self-carriage without leaning on the reins.

Start by walking down a gradual slope and, over a period of weeks, progress to steeper gradients as your horse becomes comfortable with the exercise. Take slow, short steps in good balance with a light rein contact. In order to achieve a controlled descent, the horse must push up and back with its forelimbs, using the same muscles that are used to increase collection.

After you and your horse are able to walk down a hill in a slow, controlled manner, the next step is to introduce some halts. Many horses have a preferred limb to bring up last into the halt and to step off first out of the halt. The sling muscles and other muscles involved in balancing tend to be stronger in the limb that halts first and moves off last. Therefore, work on getting your horse to halt and to move off equally easily on either forelimb. The goal is for him to take well-balanced steps into and out of the halt. The time for which the halt is sustained can be varied.

A more advanced version of the downhill exercise is to combine the effects of the slope with the rein back. Use a gradual gradient for this exercise. Establish a balanced halt, then rein back—just a couple

of steps at first, but increasing the number over time. Cue your horse to step back with the left or right diagonal first. Progress to alternating between walk and rein back, with or without a pause between.

When the rein back is well established, reduce the number of steps between the forward and backward transitions. The ultimate goal is for the horse to alternate taking a single step backward and forward with either forelimb, which requires balancing on the grounded limbs.

Jumping grids. Jumping is not for everyone, but if you enjoy it, grid work can be a useful form of crosstraining that strengthens many muscles, including the sling muscles.

For this purpose, I like to work with small fences set at bounce distances so that the horse lands and immediately takes off again. The forelimbs control the landing and reverse the direction of movement from downward to upward, which involves engaging the muscles that play a role in postural control and self-carriage. The fences do not need to be big: 18 to 24 inches is adequate. Start with three fences and, as your horse’s technique improves, increase the number of jumps to increase the endurance of the sling muscles. This type of exercise provides a powerful stimulus to activating and strengthening the sling muscles that can be likened to plyometric training. Strengthen to Straighten and Balance We’ve looked at a number of exercises that engage and strengthen the muscles of the thoracic sling. It’s certainly not necessary to perform all the exercises every day. Choose the ones that work best for you and your horse. Core training from the ground is recommended for all horses, regardless of the stage of training, so choose a couple of the unmounted exercises to perform before you tack up each day. It’s fine to mix and match these exercises on different days.

Many of the movements that engage the sling muscles during locomotion are part of your normal dressage training routine. By focusing on correct posture and performance, adding a few extra repetitions of these exercises will target the development of straightness and uphill balance. Cross-training using hills and jumping grids are valuable conditioning and strengthening exercises that add variety to the workouts for those who enjoy getting out of the arena.

Meet the Columnist

Dr. Hilary Clayton is the professor and Mary Anne McPhail Dressage Chair emerita. She was the original holder of the Mary Anne McPhail Dressage Chair in Equine Sports Medicine at Michigan State University’s College of Veterinary Medicine, East Lansing, from 1997 to 2014.

A world-renowned expert on equine biomechanics and conditioning, Dr. Clayton is president of Sport Horse Science, LC, which is dedicated to translating research data into practical advice for riders, trainers, and veterinarians through lectures, articles, and private consultations. A USDF gold, silver, and bronze medalist, she is a longtime USDF Connection contributing editor and a past member of US Equestrian’s Dressage Committee. In 2020 she was inducted into the Roemer Foundation/USDF Hall of Fame.