The basics of shoulder anatomy: there's more to the equine shoulder than meets the eye. Here are the structures that help determine your horse's style of movement

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Author: Deb Bennett
Date: Mar. 2012
From: EQUUS(Issue 414)
Publisher: Active Interest Media
Document Type: Article
Length: 3,499 words

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While picking your ride for "stylishness" might seem as superficial as picking a horse by his color, in reality movement style otherwise termed "way of going"--counts for a lot. In large measure, it is the functionality of a horse's shoulders that dictates his style of movement.

Not only does a stylish way of going contribute to pride of ownership, in some circumstances the reach and flexibility of a horse's shoulders and arms may mean the difference between a safe, pleasurable trip and disastrous injury.

The external appearance and functioning of the upper part of the horse's forelimb is the product of a complex internal anatomy. To give you a good picture of how the shoulders work, in this article 1 focus on the muscles that mediate movement. We'll also look at the bones to which the muscles are attached and learn the unique functions of the shoulder joint. This article seeks to build a foundation for your understanding of shoulder structure and function so that you will be prepared to understand variations in shoulder conformation. Then you, as buyer, owner or trainer, will be better able to assess a horse's athletic aptitude.

COMPONENT PARTS AND A LITTLE PHYSICS

The horse's "shoulder" is the uppermost part of his forelimb. The shoulder proper and the arm lie against the anterior part of the thorax, which is structured by the vertebrae whose dorsal spines form the withers, the ribs and the sternum below. Bones of the shoulder--the shoulder blade (scapula) and the arm bone (humerus)--are anchored to the body by muscular attachments. Differences in the height of the withers, the shape and size of the anterior thorax, the length of the sternum, and the shape of the neck all exert an influence on "shoulder" conformation and function.

It is not useful to discuss the conformation of the horse's shoulder apart from the arm; both anatomically and functionally, the two limb segments must be considered together. The outline of the scapula bone, which structures the shoulder, is visible and easy to palpate. By contrast, the arm gets much less notice because its heavy covering of muscle and fat hide the humerus bone, which supports it. For this reason, traditionally the shoulder has received much attention and discussion by conformation judges, while the importance of the arm has been largely under-appreciated. Yet the arm is the much more moveable of the two components, and its length and resting angle are important determinants of your horse's style of movement.

The point of articulation between the scapula and humerus is the shoulder joint; its position dictates the surface prominence called the "point of shoulder." The angle formed between scapula and humerus at the shoulder joint is yet another factor influencing whether a given horse will naturally display "scopey" movement. In the lingo of con formation and biomechanics, "scope" is defined as the horse's ability to move the elbow away from his chest. The most important factor governing this is the length of the humerus, whether measured in absolute inches or centimeters or as a percentage of body length or withers height. The contrast is precisely that between the pendulum in a cuckoo clock vs. that in a grandfather clock; the longer the humerus, the wider the arc of movement at its lower end, where the elbow joint is. Likewise, for the same power input a longer pendulum takes more time to come up to speed and to complete its arc. In horsemanship, this is precisely the factor that creates the difference between speed and "elegance."

THE JOINT THAT ISN'T: SCAPULA AGAINST RIB CAGE

One of the most enigmatic and interesting parts of the horse's body is the surface of articulation between the scapula and rib cage. This junction--formed where the muscles that coat the inner surface of the scapula rub against the muscles that coat the outer surface of the rib cage is not a joint, for there is no bone to bone articulation as at the knee or hip. The equine shoulder "scrubs" against the rib cage with a complex circular motion like a hand cleaning the surface of a windshield, but unlike a windshield, the rib cage of the horse comprises a double curve. In top view, the double curve takes on the shape of a very shallow letter "S," somewhat like the shape of a broad-bodied rowboat with a canoe-like prow. At rest, the scapula seats fairly far forward; in movement, it slides back, traversing the concavity and then riding up onto the convexity. Movement over the double curve continually changes the orientation of the fiat surface of the scapula, surface that anchors one or more muscles. Finally, there is the lumbodorsal fascia, a tough, wide sheet that overlies the loin and croup area. It gives origin to the latissimus dorsi muscle, which inserts upon the humerus.

I defer a complete discussion of the "reciprocating" or forelimb locking system until we come, later in this series, to discussion of the lower part of the forelimb. In brief, however, when an anatomically normal horse places weight upon either forelimb, he engages an inbuilt, tensionally co adjusted system of ligaments that lock the knee and shoulder joints. Weighting a forelimb locks the limb in such a manner that the scapula cannot rotate downward toward the rear because the shoulder joint cannot close. The automatic forelimb locking system is helpful under a normal workload, because it functions to prevent collapse of the weighted limb. However, when the horse moves abnormally by carrying himself upon the forehand, locking of the forelimb acts to delay breakover until the body has advanced far past the contacting forehoof. Delay in breakover is made worse when hooves are out of antero-posterior balance, with long toes and run-under heels. Delayed breakover is a major cause of strains and tears to the muscles that bridge from the neck to the shoulder.

SHOULDER MUSCLE PROFILES

A budding conformation judge needs to become familiar with all the "bridging" muscles of the shoulder--those that simultaneously function to attach the shoulder and arm to the body, while also being major players in creating forelimb movement. Ideal functioning of these muscles makes a plain horse workmanlike and a beautiful horse breathtaking in movement. When through lack of knowledge or wrong philosophy riders force the horse to move abnormally, the horse will gradually become more restive and "resistant," not because he is stub born, but because the rider is making it impossible for him to perform his work with ease and is thereby pushing him toward soreness and lameness.

Trapezius: Lying atop other muscles and just beneath the skin, the trapezius is divided into an anterior part that arises from the nuchal ligament, and a posterior part that arises from the dorsal ligament. It primarily acts to lift the shoulder and/or to raise and hollow the neck and back. When a horse habitually moves in good balance, his cervical trapezius will be no thicker than a few sheets of paper and its triangular contour will not be visible through the skin. The thoracic part of the trapezius is somewhat thicker, but in a horse who moves normally it does not develop into a knotlike bulge. There are several ways, however, in which the development of the trapezius may reveal lack of good horsemanship or training skills:

* In horses who move on the forehand especially those who have been made to wear weighted shoes or boots in quest of artificially changing the way of going--the cervical trapezius becomes tremendously hypertrophied, just as a weightlifter's "traps" become thickened when he repeatedly performs "shrugs" while holding heavy dumbbells. The cervical trapezius muscle can become so overdeveloped in a horse that its edge stands above the general contour of the neck like a sirloin steak. If the knowledgeable and humane owner finds such overdevelopment, a long period of rehabilitation can be anticipated that will incorporate normal shoeing plus various types of neck-stretching exercises to facilitate the horse's ability to lower his head while moving.

* The thoracic trapezius muscle commonly becomes hypertrophied through the wrong execution of dressage style "extensions" of the trot. In this case the horse, pushed forward by the rider so that he must carry him self upon the forehand, tightens both trapeziuses. The hollowing of the neck and back that this creates produces the flashy hut highly incorrect "flicking" of the toe of the protracted forelimb that unfortunately has become an icon of competitive dressage.

* The forepart of all saddles--those that have trees as well as those advertised as "treeless"--are meant to rest upon the thoracic trapezius. Whenever the saddle tree is too narrow, or when overtightening of the girth causes the saddle to dig in, the trapezius can become bruised or even abraded. Pressure from an ill-fitting saddle causes muscular cramping, which stiffens and greatly restricts the scope of forelimb movement.

Rhomboideus: Forming a muscular layer just beneath the trapeziuses, the rhomboideus too has both cervical and thoracic parts. The cervical rhomboideus originates upon the cordlike part of the nuchal ligament, while the thoracic rhomboideus originates at the top and along the rear slope of the withers from the most superficial leaf of the dorsoscapular ligament. The rhomboids insert upon the upper corners of the scapula and the inner surface of the scapular cartilage, acting when the forelimb is unweighted to pull it forward and upward. If instead the horse places weight upon his forelimb--engaging the limb's automatic locking system--contraction of the cervical rhomboideus acts to raise the head.

When the horse habitually moves with a high head, the cervical rhomboideus muscle commonly becomes hypertrophied. Occasionally, we find a horse in which this is so ingrained as to manifest even when the animal is at liberty, but generally as with other bad habits high-headedness is taught to the horse by humans, often through the use of devices that prevent the horse from lowering his head or that tie the head down and thus act to isometrically develop the cervical rhomboid muscles. Numerous examples of this have been presented in previous installments of this series. In a normal horse, the lower, more substantial part of the cervical rhomboideus is about as thick as the heel of a man's hand, but overdevelopment can enlarge the muscle so much that it casts a shadow in noon sunlight and raises the topline of the neck into a lumpy and unattractive false crest.

Latissimus dorsi: Formed as a sheet that covers the upper half of the rib cage, this large muscle spans the entire distance forward from the last rib to the arm. It originates upon the lumbodorsal fascia and the supraspinous ligament all the way from the last lumbar spine forward to the top of the withers, and inserts upon the medial aspect of the humerus. A powerful retractor of the forelimb that averages about a half inch thick in a normal horse, it is thinner along the topline but becomes about one inch thick as its fibers converge toward the tendon by which it is inserted onto the humerus.

The anterior third of the latissimus dorsi, including its insertion, is hidden because it is overlain by the very thick triceps muscle. Due to the pattern of circulation within muscles, when soreness develops it is often greatest near the tendon of origin or insertion. There is thus very commonly a sore spot at the particular point in the latissimus dorsi--marked by a red dot in the drawing on page 62--which lies just at the rear margin of the triceps, where the latissimus dives under the triceps and becomes tendinous. A hard lump or "knot" may also be palpable.

Soreness in this area may be caused by the saddle, since it is crossed by the girth. The cinch ring on a Western saddle or the buckles on an English saddle may press into the flesh and cause bruising. However, the saddle is not the most common cause of soreness in the latissimus dorsi. Especially where photo or video analysis demonstrates that the horse is ridden upon the forehand, soreness will be accompanied by a general thickening or hypertrophy of the muscle. This is because when a horse travels upon the forehand, effort that ought to come from muscles of the hindquarter is shifted to the latissimus dorsi, which then becomes the major muscle of propulsion. Instead of propel ling himself forward by thrust from the hindquarters, a horse who travels on the forehand first plants a forefoot and then contracts the latissimus, pulling his body forward. This is analogous to the effort made when a person climbs a rope. Anytime you see a photograph of a horse in a "trot" with the animal standing upon one forefoot while at the same time having both hind feet off the ground, he is by definition travelling upon the forehand and you may expect "sore shoulders" to he a chronic complaint.

Brachiocephalicus: This muscle of the neck balances anti opposes the latissimus dorsi of the thorax; the brachiocephalicus is the major protractor of the forelimb while the latissimus dorsi is its major retractor. The brachiocephalicus originates upon the rear part of the skull and the first four neck vertebrae and inserts upon the lateral aspect of the humerus. In horses who move normally, the thick, powerful brachiocephalicus contracts when the forelimb is unweighted and easy to move, thus contributing to the "fine and airy gait" which poet Emily Dickinson famously limned. If, however, the animal goes upon the forehand, the brachiocephalicus will try to fire during the prolonged period during which the limb is weight ed and will find itself in a tug of war with the latissimus dorsi. To increase the leverage of the brachiocephalicus, the horse will raise his head, stiffen his neck anti tilt it to the side. In movement this creates the appearance of the neck "tick tocking" woodenly from side to side a ridiculous spectacle, a strain to the muscle, and a complete waste of the true talents of good horses. Not surprisingly, the brachiocephalicus is another very common site of muscle soreness, sometimes manifesting as a lump or "knot" where the muscle overpasses the fifth or sixth cervical vertebrae.

Pectorals: There are tour pectoral muscles, falling into two groups--one that attaches to the scapula and three that attach to the humerus. A good way to picture the layout of the pectoral "complex" is to turn your hand palm-up; the palm represents the horse's sternum, and your upwardly pointing fingers represent the rib cage. Up from the sternum extend four pectoral muscles:

* The anterior deep pectoral muscle. I encourage students to refer to this muscle as the "scapular pectoral," because it is the only one of the four that extends from the sternum to the scapula. It is relatively small and thin and can weakly pull the top of the scapula forward. In reality the major function of the scapular pectoral is to act as part of the "sling of muscles" that cradles the thorax between the forelimbs. Uneven development and function of left and right scapular pectorals, often accompanied by soreness, occurs when the rider permits the horse to continually place more weight upon one forelimb than the other, which is to say to permit the animal to lean to one side as he travels or to go crooked.

* The posterior deep pectoral muscle. I encourage students to refer to this muscle as the "V pectoral" after its outline, which is palpable and visible on the chest of the horse. By far the largest of the four pectoral muscles, this one extends from the rear part of the sternum forward to insert upon the lower and middle parts of the humerus. The normal function of this muscle is to assist the latissimus dorsi in retracting the forelimb. However, it also functions to adduct the elbow, drawing it inward toward the midline of the chest. We will say much more about this aspect of pectoral function in upcoming installments, because there are literally thou sands of horses whose movement is made less pretty and less free because they move with their humeral pectoral muscles held stiff and tight.

* The anterior superficial pectoral muscle. I encourage students to refer to this muscle as the "breast pectoral"--for obvious reasons. The contour of this muscle forms the shape of a heart in front view. It connects the upper lateral aspect of the humerus to the sternum and acts to draw the humerus forward in cooperation with the brachiocephalicus. It also acts to adduct the humerus.

* The posterior superficial pectoral muscle. I encourage students to refer to this muscle as the "girth pectoral," for it is over its fibers that the saddle's girth goes. It connects the midsection of the humerus to the mid-section of the sternum, and its action is almost wholly to adduct the elbow, drawing it directly inward.

Serratus: As with the trapeziuses and rhomboids, there are two serratus muscles of importance, cervical and thoracic. Originating from the transverse processes of the last several cervical vertebrae and from the lower parts of the first nine ribs, the two muscles form a single continuous fan whose fibers converge to the inner upper surface of the scapula. The size of the serratus fan is impressive, extending almost half the length of the body and covering, in a full size horse, about four square feet of area.

The cervical and thoracic parts of this muscle can act separately; when they do, they oppose one another. The cervical part acts to draw the top of the scapula forward (i.e., to retract the lower part of the limb), while the thoracic part has the opposite effect. If both parts contract simultaneously, the shoulders are drawn down tight against the rib cage anti become "frozen" in place. Teaching the horse how to move without tensing the serrati is therefore one of the most important skills that any rider can possess.

Important though its movement functions are, of still greater importance is the role of the serrati in supporting the forequarter. It is primarily by means of the serrati that the shoulder is attached to the thorax. The tendons of insertion of the cervical and thoracic parts are impressively thick and strong. In front view, the configuration and function of the serrati as a sling are evident. The horse's thorax is suspended between its forelimbs primarily by the elastic fibers of the serrati.

The bottom line that all riders should derive from study of the shoulder musculature is that the ability of the horse to move his shoulders freely is not primarily the product of conformation, but rather of the state of tension or relaxation in the bridging musculature, especially the serrati and pectorals.

THE SHOULDER JOINT: UNIQUE IN MANY WAYS

The joint between scapula and humerus is potentially the most freely movable one in the horse's body. Smoothly curved like the top of a grapefruit, the head of the humerus presents a wide surface against which the cupped lower end of the scapula can glide. Any type or combination of opening (extension), closing (flexion), adduction, abduction or rotation are possible at this joint. It is important to notice that no other joint in the forelimb can rotate, abduct or adduct to any significant extent, so if the horse crosses his forelimbs during a reining type spin or while executing a half-pass, the in-and-out motion is produced entirely by the shoulder joint.

Such flexibility requires that the joint be held together by elastic tissues that will not easily tear under lateral or twisting motion. The shoulder joint is the only joint in the body that is not supported by collateral ligaments--short, stiff bands that hold such joints as the ankle, stifle and knee together. Collateral ligaments work great when fore-aft motion is the main action anticipated, but they will tear if adduction, abduction or rotation is required. Instead of collateral ligaments, the shoulder joint has a thick, elastic joint capsule that contains lubricating synovial fluid. Directly overlying this is the deep layer of intrinsic shoulder muscles--those, you recall, being muscles that originate on the scapula and insert upon the humerus. The deep layer of shoulder intrinsics comprises the subscapularis, supraspinatus, infraspinatus and teres major. The main function of all four of these muscles is simply to hold the shoulder joint together. Lying above them are the biceps brachii and triceps muscles--respectively the major extensors and flexors of the shoulder joint--which also act to stabilize the shoulder joint.

Once again, there's an important bottom line to this lesson in anatomy: the very same muscles that can potentially flex or extend the horse's shoulder joint are also major muscles of support. As with the serrati and pectorals, if we hope to see the horse perform at his best, we must teach him that he can move without continuously tensing these muscles. Only the horse who moves without tension can freely open and close the shoulder joint, thus fully displaying his beauty.

By Deb Bennett, PhD

Source Citation

Source Citation   

Gale Document Number: GALE|A282214929