Wednesday, October 26, 2016

Equine Anatomy and Biomechanics: A Primer of Equine Engineering Part XII, The Pelvis


Hi gang! This is Part XII of a 17-part series discussing equine anatomy and biomechanics in more depth than we did in Anatomy 101. It's recommended to read that first, then read this series because the beginner level has some introductory ideas that help our understanding of this series.

Anyway, in this part we'll be exploring the pelvis. We're treating it separately since it's a component of both the spine and the hindlimb and so warranted its own post because of this interdependence. Now for previous posts, we've touched on the head, neck, torso, forelimb plus evolution and some terminology. Understanding our subject's structure is necessary for creating authentic work because we have a better grasp of what goes where and how it works. 

While the pelvis may seem simple at first, it's actually a rather complicated structure. Being the interface between the spine and the hindlimb gives it a distinct shape and as well as shared musculature. That's to say the pelvis functions as an integrated system and not independently, just like everything else in the equine. This can introduce some peculiarities we need to know before we dive into sculpture. But once its properties are grasped, it's not so hard!

So let's go!...

Basic Structure of the Pelvis

The pelvis is a fused girdle of bone consisting of the ilium, ischium, and the pubis. There are no joints in the pelvis, forming a solid "box" of bone. Between each wing of the ilium lies the LS-joint and through which the spine passes. The lumbar lie in front of the pelvis and the sacrum lies on top, creating a "roof" over the pubis. On each side is a large ball and socket joint to which the femur attaches to form the femoral joint.

Skeletal Structure

The gluteal plane of the ilium faces upward beginning with the tuber sacrale (point of the croup) then flows forward to the tuber coxae (point of the hip) and, finally, extends down and back to the femoral joint. This is the largest portion of the pelvis. Regardless, it's not uncommon for one tuber sacrale to be slightly higher than the other simply because the wings of the ilium don't grow in perfect symmetry. 

The ischium projects horizontally backward from the femoral joint and determines both the point of the buttock (tuber ischii) and, in conjunction with the point of hip (tuber coxae), the length and angle of the hip.

The pubis, the lowest section, establishes the floor of the pelvis and joins its mate under the tail and between the thighs, bridging the two halves. As such there's a large hole created through which a foal passes, which is why equine pelvises are strongly sexually dimorphic; the mare has a larger pelvic cavity than a stallion. The tuber ischii should also be wider apart in mares than in stallions for the same reason. Nonetheless, the equine pelvis is wider in front across the tuber coxae (points of the hip) than the posterior when viewed from above, which tapers inward at the ischium bones. The tuber coxae should be straight across from each other, perpendicular to the spine when the spine is straight. What's more, each point of the tuber coxae should be equidistant from the center of the spine, as should the tuber sacrales and tuber ischii.

There's a floating joint that attaches the pelvis to the spine, the Sacroillicac Joints (SI joints), of which there are two on each ilium between each sacral wing on either side, which are important connections for functionality. However, there's also two Sacro-lumbar joints (SL-joints) between S1 and L7 on either side. There's the LS-joint between the wings of the ilium, too. That means there are five joints within a span of two inches which connect the hindend to the spine.

Basic Musculature of the Pelvis

The lateral sacroiliac and sacrosciatic ligament help to attach the sacrum to the pelvis and are part of the dorsal ligament which blends with the nuchal ligament. In particular, the sacrosciatic ligament creates a roof over the pelvic cavity and is a strong brace for the pelvis with the spine.

The basic muscles of the pelvis are:
Psoas minor: Flexes the lumbo-sacral joint. Important muscle for bascule.
Psoas major: Helps to flex the lumbo-sacral joint, draws the femur forward and rotates it outward. Important muscle for bascule.
Iliacus: Draws the femur forward and rotates it outward.
Quadratus lumborum: Stabilizes the last two ribs and lumbar vertebrae and creates lateral flexion in this area.
Quadratus femoris: Folds the hindleg by extending the femur and adducting the thigh.
Obturator externus: Adduct the femur and rotates it outward.
Obturator internus: Adduct the femur and rotates it outward.
Gemellus: Rotates the femur outward.
Superficial gluteal (or gluteus maximus) : Abducts the limb and flexes the hip joint. The top portion of this muscle is visible as a letter “v”, enveloping the bottom portion of the medial gluteal. 
Medial gluteal (or gluteus medius) : Abducts the hindlimb and extends the hip joint. Primarily responsible for the convexity of the croup.
Gluteus profundus (or deep gluteal) : Abducts the femur and can also rotate the hindlimb inward.
Quadriceps femoris (or crural triceps) : This is a large muscle that covers the front upper parts of the femur and is referred to by its four heads: rectus femoris, vastus lateralis, vastus medialis and the vastus intermedius. This group of muscles extends the stifle joint and adducts the femur. The rectus femoris also helps to flex the hip joint while the vastus intermedius helps to raise the stifle joint during its extension.
Semitendinosus: Extends the hip and hock joints, also creates flexion of the stifle and inward rotation of the hindleg. Part of the hamstring group. Largely creates the posterior outline of the hindquarter. somewhere during evolution, the Semitendinosus also developed a thick tendon in the middle of its muscle belly to amplify its forces and to become part of the Reciprocal Apparatus. So the dock on the modern horse really begins at the third tailbone and that bump on the tail head is really the Semitendinosus muscle.
Semimembranosus: Extends the hip joint and abducts the hindlimb. Part of the hamstring group. Largely creates the fleshy mass of the inner quarters, along with the gracilis, when seen from behind.
Biceps femoris: A muscle with a very complex function, it basically extends the hip, stifle and hock and abducts the hindlimb. Its longest branch can sometimes be called the long vastus muscle, that large crescent shaped muscle that dominates the appearance of the hindquarter. Part of the hamstring group.
Gluteal fascia: Functions as connective tissue.
Tensor fascia latae: Flexes the hip joint and extends the stifle. Also acts as connective tissue and a brace in the standing position. Also steadies the pelvis and helps to resist the motion of the muscles on the other side, thereby helping to keep the croup somewhat steady during movement. When at rest with a cocked hindleg, this fascia helps to support the animal with minimal muscle strain. The ilio-tibial band lies within it and stretches from the external patellar ligament to the point of hip. It is very apparent on a fit horse, especially during movement, as a groove or cord from the patella to the point of hip. Also often seen thusly on the supporting hindlimb on resting horses with a cocked hindfoot.
Gracilis: Aids in the adduction of the hindlimb and is nearly completely superficial by forming that inner bulk of the inner thigh.
Sartiorus: Abducts and flexes the femur.
Pectineus: Adducts the hindlimb and flexes the hip joint.
Adductor: Rotates the femur medially, adducts the hindlimb and also extends the hip joint.
Iliocapsularis: Raises the stifle joint during flexion of the joint.
Panniculus Carnosus: Serving as a skin muscle, it is the “fly shaker” and the most superficial muscle in the horse. It is located mostly on the neck and trunk, but also lies over the pelvis and around the forearm and gaskin. On the forearm, it creates the basis of the elbow flap and on the trunk, creates the basis of the stifle flap.

Biomechanics of the Pelvis

During evolution, the equine pelvis changed by lengthening both ichium bones to increase leverage for the developing hamstring muscles that propelled him forwards, the Semitendinosus and Semimembranosus muscles. This increased the power and stride of his hind legs which helped him escape predation more effectively. We can see this in effect whenever the horse squats and coils his pelvis in preparation of rapid take-off into a gallop.

Yet the pelvis itself isn't articulated. Being a solid girdle of bone, it moves as a single unit with the spine at the LS-joint. As a result, the pelvis “box” is always constant regardless of motion. In other words, the points of sacrum, hip, and buttock must always be perpendicularly centered on the spine and always be level to their pair and parallel to each other despite motion. It also means that the tuber ischii don't move with the rear legs. And because the pelvis is lashed onto the spine, primarily to the sacrum, it follows the motions of the spine, tilting and rocking in various ways as the spine allows, but always as a single fused "box." This also means that when the spine hollows out, the pelvis levels out, too, such as in a show stretch. And when the spine twists, so does the pelvis such as with cutting. Then when the spine laterally bends, the pelvis follows, such as with bending work. In short, the pelvis is a clear expression of what the spine is doing since it cannot articulate independently.

Furthermore, mass is expressed into the pelvis up through the hind legs, easily observed when the pelvis tilts with the impact of each hind leg. The more relaxed and supple the horse, the more pronounced this undulation, sometimes referred to as "dancing." This is because his spine is relaxed so it can function freely without braced muscles that would cause the back to stiffen. This tilting effect can also occur in the standing posture as one hind leg is drawn either forwards to backwards to a marked degree against a perpendicular hind leg. So predictably, the more stiff or hollow-backed the horse, the less pronounced this undulation is because the animal is bracing, most likely attempting to protect himself from careless riding. Similarly, the classic stance of a horse's pelvis in motion is when he's resting with a cocked hind leg, which causes the pelvis to tilt in sympathy and downwards with the hind leg that’s bent, clearly displaying its union with the spine and its structure as a box. This posture occurs when the horse has initiated his Stay Apparatus on the straight hind leg allowing the other leg to be cocked. 

As for the LS-joint, it lies between the last thoracic vertebrae and the sacrum, making it the primary mechanism by which the pelvis is curled under the body (a prerequisite for bascule, or self-carriage). It's a hinge joint, capable only of extension and flexion. When this joint is flexed, it leverages the sacrum to curl the pelvis under the body, rounding the back (primarily in the lumbar span) and lifting the base of the neck which, in turn, causes the neck to arch and open the throat, dropping the head at the poll. This curling of the hindquarter under the body is often referred to as “tucking." Such hindquarter activation is naturally initiated to canter or gallop as in collection. Being so, a stiffened lumbar section hampers proper dorso-ventral rotation of the pelvis and therefore sound kinematics of the hind legs. And because the hindlimb is part of this system, too, the coiling of the loins dictates the reach and length of stride, which is the reason why bascule results in pronounced hindend engagement. In doing so this increases the stride length which results in stronger, more athletic and fluid motion.

The tuber sacrales are two small bumps visible on the top of the croup. Each one is the tip of each ilium wing, or the wings of the pelvis. Each wing of the pelvis is attached to a nodule on the sacrum, or the SI-joints. However, acute SI-joint strain can cause pelvic misalignment that lifts one tuber sacrale higher than the other, causing severe lameness. This is because the SI-joints aren't closed, fused joints, but instead are designed to float with a minuscule range of motion of less than 1°. Indeed, all SI-joint damage is due to excessive movement of these joints that destabilize them. This is because the thrust of the hindlimb is continually trying to tear the pelvis off the sacrum and spine because they're attached from below and not from above. As a result, these joints form another suspensorium which can be over-stretched, stressed, or injured resulting in Hunter’s Bump, Jumper's Bump, or Racker’s Bump. A typical cause is found in steeple chasing (or jumping at speed) when the horse is fatigued and gets lazy and jumps low and hooks a leg to recover balance, jamming the landing oddly to one side of the spine and knocking asunder the SI-joint, either sliding it off the pelvis or breaking the pelvic suture at the bottom on the pelvic floor. This causes an asymmetrical slide off the sacrum. This injury is extremely painful and almost assures permanent lameness. On the other hand, sacro-lumbar subluxation is more serious because it involves the spinal cord and typically destroys natural hind leg coordination and movement.

Indeed, it's not the sacrum that supports the pelvis but it's the pelvis that supports the sacrum. In turn, the pelvis is supported by the hind legs, making the athletic, coordinated motion of the hind legs critical to a sound SI-joints. For this reason, the SI-joint isn't built for compressive force but for a tiny amount of sliding motion and suspension. This joint can also fuse or ossify with age, plus poor riding technique can stretch the ligaments of this joint, causing it to reseat crookedly and fuse to make the pelvis permanently crooked on the spine, making the horse unable to move straight which is a prerequisite for bascule. The SI-joint is also susceptible to trauma such as a a fall, bad slip or landing, or unnatural, uncoordinated motion that can reseat or fuse the pelvis to the sacrum. Likewise, if the horse is made to move crookedly, the ligaments of the SI-joint can adapt to the torsion and stretch to orient the pelvis crookedly on the spine, too. 

What's more, hind leg motion doesn't start at the stifle but in the spine through the LS-joint and then at the femoral joint. For this reason, the pelvis is also engaged with all hind leg motion as well as spinal motion; it's the bony conduit through which the spine communicates to the hind toe. Furthermore, the head tries to stay in front of the pelvis, perpendicular to the ground, under natural conditions of motion when leaning. However, if under saddle, this natural coordination can be interrupted as riding tends to force the neck to continue the curve and the head to deviate from the perpendicular orientation rather than be used as a counterbalance in a lean. This is okay though given the horse is supple and balanced enough to accommodate, which again is why collection is so important to responsible riding.

Lastly, the Accessory ligament, which is unique to equines, binds the femur to the pelvis in such a way as to prevent extensive lateral motion of the femur. This is why horses can’t kick sideways like a cow. As for curious muscles, the lliopsoas run from the lumbar to underneath the pelvis while the Psoas minor runs from the lumbar to the femur (its tendon can be felt between the hind leg and the groin). This is why a relaxed back or "schwung" helps to coil the loins by activating this muscle to contract and release with each stride to maintain collection.

Landmarks and Reference Points

Boney Points of Reference

While most of the pelvis is buried deep in muscle, some points are palpable. The tuber sacrale (point of the croup) and the tuber coxae (point of hip) can be felt under the flesh. With practice, the tuber ischii (point of the buttock) can be located despite being covered in muscle. Likewise, some points around the femoral joint can be felt as well.

Fleshy Points of Reference

The pelvis is understandably covered in ample musculature, but the groupings converge on key areas of the pelvis nonetheless. The Gluteus maximus flows over the femoral joint to converge with the Tensor fasciae latae about mid-femur. And the Semitendinosus runs down the back of the hindquarter, forming that typical grove with the branches of the Biceps femoris. On the inside of the leg, the Gracilis forms the bulk of the musculature in the groin while the Semitendinosus often forms another groove with it. 

Artistic Aspects to Consider about the Pelvis

Because the pelvis is a fused girdle of bone that doesn’t articulate internally, its only points of articulation are with the femur at the femoral joint and (marginally) with the sacrum at the SI-joints. This has important consequences for artists. First, it means that the points of buttock, the points of hip and the points of croup must always be aligned as a perfect box in all phases of motion. And, second, the pelvis should be centered on the spine and move in concert during all phases of motion. The pelvis also "follows" each hind leg to the ground, causing it to tilt in response to the motion of each hind leg. Horses have fluid, flowing motion, and factoring in the motions of the pelvis with the spine and hind legs goes far in capturing this quality.

The muscle planes of the hindquarter are important as well. The area surrounding the femoral joint and flowing down the femur tend to be the most protruding plane with the other planes flowing away from these markers, forming a kind of "T" running down to the stifle. This means that the hindquarter musculature isn't flat, when seen from the rear or from a three-quarter front view. We should see the "horizon" of this "T." On that note, the Gracilis is a hefty, robust muscle that creates the rounded cleavage going up the groin to line up to the tail. In addition, remember that the points of hip are broader than the points of buttock.

Different breeds have differently planed hindquarters. For example  certain drafters and Friesians tend to a more "boxy" type of hindquarter while Arabians have more of a squared-oval type, and stock horses are more rounded and robust. And as with overall musculature, different breeds have different degrees of definition of hindquarter musculature. For example, Quarter Horses and race-fit Thoroughbreds tend to have highly defined musculature whereas "smooth-bodied" breeds tend to have less definition.

Nonetheless, the most typical areas of definition are between the Semitendinosus and the Biceps femoris muscles, and between the branches of the Biceps femoris. Also the grove between the Gracilis and the Semitendinosus can often be traced on many individuals despite breed.

The muscular aspects of the hindquarter also change dramatically between standing and motion postures; it morphs quite a bit depending on posture or movement. When there is a lot of effort or strain visited on the hindquarter, musculature tends to become more defined and pronounced whereas at rest, these things tend to soften back to a resting state.

Common Artistic Faults with the Pelvis

Sculptures have a strong tendency towards broken pelvic girdles. This is seen when the points of hip, buttock, and croup don't form a perfect box, but more of a trapezoid. Sculpted pelvic girdles also have a tendency to articulate incorrectly with the spine, often being laterally bent at the LS-joint and SI-joints. Many times, too, a sculpted pelvis doesn't follow the motion of the spine, being treated almost as an afterthought. This suggests a very stiff spine, something unnatural for the horse in motion or indicative of poor riding. On that note, many pelvic girdles don't tilt and undulate, or "follow" the hind legs to the ground, making the spine appear rigid and stiff as well. To compensate, the artist will have to arbitrarily lengthen or shorten each respective hind leg to get the sculpture to stand perpendicularly to the surface, when seen from behind, causing a marked asymmetry in the hind limbs. In addition, many pelvic girdles aren't seated on the spine correctly, with each side not being equidistant from the spine and so not centered on it.

On the other hand, many sculptures have disengaged hindquarters, a significant fault in biomechanical terms and particularly so in performance depictions. The horse must tuck his hindquarter to gallop, canter, push off, cavort, or achieve collection and if our sculptures don't reflect this, we aren't only creating stiff motion, but motion inconsistent to his natural coordination. Along with this, many sculpted pieces depicting a show stretch don't have a pelvis moving in kind with the spine, to level out, tipping the points of buttock up. Instead, the hind legs are simply stretched backwards at the stifle rather than accounting for the mechanics higher up on the hind quarter and spine that originate the posture.

The qualities and nature of the muscles also morph with movement. That means the expression of the muscles will change between gaits and postures. For instance, when the hind leg is brought forward, the Iliacus and the Quadriceps femoris muscles will bunch up and pooch out. On the other hand, they'll stretch when the leg is extended backwards along with the Tensor fasciae latae. For that reason, we cannot sculpt standing hindquarter musculature onto a moving hindquarter, or visa versa. We also can't forget that the stifles must pop around the bulk of the barrel, which also changes the planes and nature of the musculature of the hindquarter. Remember the horse doesn't move like an articulated anatomy chart!

Musculature planes in sculpture are often incorrect as well, lacking that "T" so typical of the planing. Many times, too, the Semintendinosus is misshapen or not connected properly, giving the hindquarter a "chicken leg" appearance. Yet sometimes the points of buttock are moving with the hind legs as though they articulated with the pelvis, making them asymmetrical when seen from above and behind. Other times, the Gracilis is atrophied, giving the sculpting a "split up the rear" flaw (something not applicable to newborns).

Biological Aspects to Consider about the Pelvis

The hindquarter is the seat of all motion; power is thrust from it through the "transmission" of the spine and onto the forequarter with forelegs that act as pole-vaulter poles. Yet like the forelegs, all hind leg motion is caused by the oscillations of the spine, meaning that the spine automatically engages the pelvis. When we neglect to infuse this into our sculptures, the depicted motion will appear "off" and artificial.

A broken pelvis renders a horse painfully lame. Therefore if we create such a pelvis in our sculpture, we've rendered our horse functionally nonviable. It's very important to get the pelvis as perfect as possible despite it being buried in muscle.

Conclusion to Part XIII

Phew! Well that's it for Part XII. While it's a solid girdle of bone, the pelvis certainly isn't as simple as one would think! Incredible forces are visited on and produced by this feature, making it a critical component to an authentic sculpture.

So in the next part, we'll take a look at the hind legs!

"Success is when I'm out on location and can pull off a decent painting. It's also when I can convince someone who is afraid, to put brush on canvas and feel the joy."
~ Linda Blondheim


Wednesday, October 19, 2016

Equine Anatomy and Biomechanics: A Primer of Equine Engineering Part X, The Torso


Hello and we're back to this 17-part series exploring equine anatomy and biomechanics from an Intermediate perspective. We've already had a starter introduction, so now it's time to dig deeper. There's always more to learn with equine anatomy—it's a living subject. Literally!

In previous parts, we've discussed the head, neck, and topics ranging from terminology to evolution So in this Part X, we'll be discussing the equine torso. It's the biggest portion of the animal and full of its own special properties that make it a dynamic, integral aspect of equine art. Actually, in a very real sense, the torso is the most important element for depicting motion! Let's learn why...

Basic Structure of the Torso

The torso is like the support bridge and transmission of the horse, all in one. It's also what supports the saddle and cradles the rider. The spine consists of the neck (previously discussed) and the thorax, lumbar, sacrum, and tail vertebrae. The pelvis can also be thought of as part of the spine (but it'll get its own treatment in this series for the same reason the neck did). In terms of conformational references, the torso itself entails the loins, croup, dock, withers, chest, barrel (the ribs), sternum, groin, and tail.

The equine body is wider and flatter at the rear and narrower at the front. In essence, the equine torso isn't shaped like a tube or like a 2x4, but like a canoe with a narrow "keel" at the front and a wide, sprung shape towards the flank with a flat underside towards the groin. However, different breeds can have a different spring of rib, and therefore slightly different barrel shapes. The Arabian, for example, has a rounder, well-strung barrel, the Saddlebred has more of a heart-shaped barrel, and the Friesian can have more of a tube-like barrel. Moreover, the hindquarter, from point of hip to point of hip, is usually wider than the forequarter, from point of shoulder to point of shoulder. There are exceptions, however, such as chunky draft or pony breeds which can have a more equal width between the two at times. 

The torso has bones (such as the ribs and sternum) in the front half, but a series of thick layers of aponeurosis fascia as supportive structures towards the rear (thoracic tunic); there are no ribs emanating from the lumbar vertebrae. The spinal column itself is slightly arched upwards to support the heavy viscera (despite what many erroneous diagrams and mounts  depict, having straight columns), and has a downhill slant towards the front. It's the spinous processes of the thoracic vertebrae that form the familiar concave curve of the back between the peak of the withers and the point of croup.

However, different breeds or types of horses require a different tilt to the spinal column in order to go about their function properly or have proper type. For example, riding breeds such as the Arabian, Saddlebred, Morgan, sport horses, etc. should have an uphill tilt to their torso. In other words, the base of the neck should be level or higher than the LS-joint. In contrast, in speed, cow horse, and many draft breeds such as the Thoroughbred, Quarter Horse or Shire should have downhill balance in which the base of the neck is below the LS-joint. However, many drafters should alternately be of level balance, too, depending on the breed or type. Breeds that are of general use should have level balance such as the Akhal-Teke or Morab. 

Skeletal Structure

The horse has approximately eighteen thoracic vertebrae (back), each bearing a pair of ribs. The spinal process of the fifth thoracic vertebrae generally is the longest, forming the peak of the withers. The fourth thoracic vertebra typically forms the visual junction between the neck and the torso. All the thoracic vertebrae articulate with each other and with their ribs, but within the limitations of their structures and connections.

There are approximately five to six lumbar vertebrae (loins) whose straight transverse processes establish the width of the loins. These transverse processes are long, wide, close together, and tightly bound to each other by ligamentary attachments. All the lumbar vertebrae articulate with each other, but within the limitations of their structures and connections.

The sacrum (croup) consists of five to six fused vertebrae, making a single inflexible bone triangular in shape (when seen from above). There are no joints in the sacrum, it being a solid block of fused bone. The limbo-sacral joint (LS-joint), between the last lumbar vertebra and the sacrum, is the point where the hindend is tucked and curled in bascule, cantering, galloping, pivoting, stopping, spinning, jumping, etc.

And bringing up the rear, the horse has generally eighteen coccygeal vertebrae (tail) that decrease in size to a pointed tip. The tail is the most flexible part of the spine even if the first coccygeal vertebra becomes fused to the sacrum in maturity. But because evolution caused the "semis" to pirate part of the coccygeal vertebrae, the actual, flexible part of the tail doesn't start until the third coccygeal vertebra.

The sternum consists of seven “sternebrea” that never completely ossify, though always stationary. It has a keel (the keel of the canoe-shaped barrel), and is deepest between the forelegs ("depth of girth") and most shallow and flat at the eighth rib, where it ends. It's an anchor for the pectoral muscles of the chest, as well as others.

The horse has eighteen ribs which descend at a slightly backward slant. From the first to the sixth rib, they're flatter and wider to become more arched and narrow towards the hindquarter. Only the first eight ribs attach directly, with cartilage, to the sternum and so are termed “true ribs." In contrast, the remaining ten ribs are called “false ribs" because they "float" with no connection to the sternum despite the thick cartilaginous "rod" that connects them together at the bottom, and to the last true rib. The ribs help to form, with the abdominal muscles and thoracic tunic, the external shape of the barrel. 

Basic Musculature of the Torso

Though there are many ligaments that lash the vertebrae together, the major one in the torso is the dorsal ligament, which blends with the nuchal ligament of the neck. 

In turn, the basic muscles of the torso are:

  • Trapezius (thoracic portion): Moves the neck and scapula, or helps create a shrugging motion.
  • Rhomboideus (thoracic portion): Flexes the shoulder.
  • Latissimus dorsi: Helps to pull the humerus up and back, flexing the shoulder. 
  • Longissimus dorsi: Helps to flex the back and neck laterally, round the back, and lift the base of the neck in bascule. It is the single largest muscle in the horse. On a well conditioned horse, the groove of the spine lies between the two parallel masses of this muscle (when seen from the top).
  • Longissimus costarum: Aids the longissimus dorsi.
  • Semispinalis dorsi: Aids the longissimus dorsi. Often regarded as part of the longissimus dorsi. It continues as the semispinalis colli, up the neck to the axis bone.
  • Multifidus dorsi: Aids the longissimus dorsi.
  • Serratus dorsalis anterior: Fleshes out the back and can cause hollowing of the back when contracted.
  • Serratus dorsalis posterior: Fleshes out the back and can cause hollowing of the back when contracted.
  • Serratus ventralis (thoracic part): Supports and rotates the shoulder backward and also helps to articulate the ribs in breathing. Works with or antagonistically with its cervical counterpart  (See Neck-Basic Musculature of the Neck in Part IX). Part of the shoulder sling.
  • Intercostal muscles: These fill the spaces between the ribs and costal cartilages and have an external and internal layer. These muscles help with breathing.
  • Internal abdominal oblique (or rectus abdominis): Aids in breathing, provides support for the contents of the torso and helps to bascule the back, flex the lumbo-sacral joint and aids in lateral bends of the back. Horses that work at the gallop have strong abdominal muscles and a subsequent tucked up belly, or "lady-waist."
  • External abdominal oblique (or great external oblique): Aids in breathing, flexion of the lumbo-sacral joint and support for the contents of the torso. It joins its mate in a tendinous connection called the “white line” or linea alba down the underside of the belly.
  • Transversus abdominis: Aids the external abdominal oblique.
  • Abdominal tunic (or aponeurosis of the obliquus abdominis externus): Acts to support the contents of the belly.
  • Anterior superficial pectoral: Helps to pull the forelimb to the median.
  • Posterior superficial pectoral: Helps to adduct the forelimb and tenses the fascia of the forearm.
  • Anterior deep pectoral: Helps to adduct the forelimb.
  • Posterior deep pectoral: Helps to pull the forelimb backward.
  • Biceps brachii: Helps to flex the elbow. 
  • Coccygeal muscles: Move the tail.
  • Panniculus Carnosus: Serving as a skin muscle, it's the “fly shaker” and the most superficial muscle in the horse. Located mostly on the neck and trunk, it also lays over the pelvis and around the forearm and gaskin. On the forearm, it creates the basis of the elbow flap, and on the trunk, creates the basis of the stifle flap.
Biomechanics of the Torso

With all the leg motion, we might come to think that the horse moves on his legs to create forward motion. And he does. But the fact is all motion begins in the spine and not the legs. His legs are simply initiated and moved by the oscillations of the spine. The neck should also be included in "the spine" even with biomechanics, but it got its own post because of how involved it is.

Its structure and ligamentary network make the equine spine relatively stiff and unyielding to lateral motion. The conventional concept of "straightness" is misleading, when, in fact, the issue is far more fascinating and complex than simply "the fore quarter in front of the hindquarter." The most potential for lateral bend is before the sixteenth thoracic vertebra (specifically most of it is between T9 and T16), but lateral bend is most pronounced and facilitated by bascule. A hollow back cannot laterally bend very well which is why horses that move thusly lack suppleness and agility. Most transversal rotation occurs T9 and T14, about where the rider's thighs are. Overall, the horse's spine is more flexible than a camel's back, but less flexible than a dog's or cat's spine. Also, in order to bend, the horse pushes his ribcage outside of the turn, to "ride the rim" so to speak. He may also hold his tailbone in line with the curve.

The horse's relatively rigid back evolved to best facilitate structural stability for the body of a large herbivore who escapes danger at sustained speed. It speaks directly to the animal's evolutionary lifestyle. Indeed, a fast canter is the horse's most energy efficient stride! That said, however, the thoracic vertebrae can create an undulating up and down oscillation of about 2.25 inches (5.7cm) to hollow or round. These oscillations can also be induced by the rising or lowering of the neck. For example, hollowing occurs in a halter stretch starting in the spine, not the legs. Rounding occurs during bascule. The spine is also capable of a some rotational motion to allow the forehand to pivot more or less independently of the hindquarter, often exhibited on cutting horses. Most of this rotation occurs in the thoracic vertebrae since the lumbar are relatively more inhibited in this due to its tightly lashed lateral processes.

Note how the back hollows, the hindquarter sinks below the withers, and the pelvis levels out. The more extreme the stretch and the higher the head is held, the more pronounced these effects. However, this shouldn't be confused with lordosis.

All the ribs articulate with their vertebrae and adjacent ribs in both an up and down and rotational front and back motion simultaneously. This makes the torso expand and contract in synch with breathing. However, one side of the ribcage cannot move independently of the other side; they work together.

The lumbar vertebrae, because of their tight ligamentary network between their lateral processes, are resistant to lateral flexion, though capable to a marginal degree. They're also capable of the same up and down motion of the thoracic portion, but it's designed specifically to "coil," or tuck the hindquarter under the body to launch the horse forwards efficiently. 

Note how the lumbar are coiling in response to the flexion of the LS-Joint.

This wee foal perfectly demonstrates what the lumbar vertebrae are designed to do: coil the loin and tuck the hindquarter. That LS-Joint is also flexing, amplifying the effect.
Photo courtesy of Maria Hjerppe

The sacrum functions rather like a lever by articulating with the seventh lumbar vertebra at the lumbo-sacral joint (LS-joint). This flexion tucks the hindquarter under the body, and arches the spine upward to "round the back" and lift the base of the neck during engagement and bascule. It cannot laterally bend or rotate. Similarly, the LS-joint cannot laterally bend or rotate but can only operate as a "yes" motion up and down motion to alternately coil and uncoil the hindquarter under the body.

Depending on the specific situation the barrel will also swing like a pendulum over the supporting hind leg, referred to as "schwung" in German. However, it does so unless it's being pushed left or right in response to a turn or pivot, more or less. The ribs will move in an up and down rotational motion, too, when breathing as the rib articulates with its own thoracic vertebra and those adjacent. 

Despite its relative rigidity, the equine back is flexible enough to accommodate a wide range of movement. All motion of the spine is amplified by bascule and reduced by a hollowback.

Here we see abduction of the hindlimbs and lateral bend in the front of the thoracic column; the spine isn't bending at the lumbar. That's an illusion created by the bent thoracic column around the wither. Note how the forelimbs are oriented in the opposite direction of the hindlimbs. Paying attention to what the spine is doing helps us capture that graceful, supple motion so characteristic of horses. Can you imagine how clumsy this sculpture would appear if it lacked this movement of the spine? Yet it's an error often found in equine sculpture.

Furthermore, the torso "see-saws" in relation to the orientation of the legs, when seen from the side. In other words, with the distance of the torso to the ground being "shortened" by hind legs drawn alternative forwards or backwards, or both forwards or both backwards, the hind quarter sinks below the withers rather than staying at standing balance. Think of the fore legs as a pivot for the torso when moving the hind legs.

The spine also bows from side to side in relation to the gaits, most notably at the walk. When the hind leg is drawn forwards, the spine has a tendency to bow away from this leg, orienting the pelvis curved around it in the back. Likewise, when the other hind leg is drawn forwards, the oscillation reverses.

Landmarks and Reference Points

Boney Points of Reference

Most of the sternum is palpable and marks the muscular junction of the neck to the chest at the front, about level with the points of shoulder on a standing horse. The top part can be felt between the two fleshy “bumps” crowning the Anterior superficial pectorals. The keel is subcutaneous and exhibits itself as the median grove between the pectorals, and the terminal end, the xiphoid process, can be felt as well. The tail dock is another useful landmark.

The ribs are also palpable under the flesh (the last one is a particularly helpful landmark) as are many parts of the spinal and transverse processes of the thoracic and lumbar spine, though these areas are quite sensitive so proceed gently. The dip of the LS-joint can be gently felt as can the point of sacrum and its spinal processes. The vertebrae of the tail are all palpable under the skin.

Fleshy Points of Reference

The thoracic portion of the Trapezius can be clearly seen as can the Latissimus doors and thoracic portion of the Serratus anterior on a fit horse. The groove between the posterior portion of the Pectoralis minor and the bottom of the Serratus is a common fleshy landmark. The "white line," or groove down the belly from sternum to groin is another landmark as is the sometimes seen groove between the two sides of the horse, down the back from wither to tail.

Artistic Aspects to Consider about the Torso

We need to keep the canoe-shape in mind when we sculpt the torso. But while the torso sticks to this shape, there are variations on it with each individual, too. So try not to sculpt the exact same barrel on every piece.

The torso can also appear to compress or elongate depending on the effort of the gait or posture. For example, a strong trot can make the torso appear to compress, or shorten, while a show stretch can make the torso appear to stretch, or lengthen.

We also have to careful not to invert the superficial muscles with the deep muscles as well. We should also see that the chest (when seen from the front) has a nice oval (or roundish for a stout drafter), rounded aspect to it, running lengthwise perpendicular to the ground.

It should also be mentioned that the chest can widen or become more narrow based on how developed the Shoulder Sing is. A wide chest shows good development whereas a narrow chest implies a lack of development. Along these lines, the chests of Big Lick Tennessee Walkers are often bull-doggy and wide due to the weighted "packages" on the front feet build up the muscles of the Shoulder Sling. 

As for the musculature, some breeds are typified by "smooth muscle," meaning that their torsos are more subtly muscled with less definition with "flatter" musculature, their bodies being more a reflection of planes rather than delineations. On that note, drafters aren't really examples of muscle definition either, being more an expression of bulk and mass rather than delineation. Again, planes define them more than actual definition. In contrast, muscled breeds such as the Quarter Horse, have much more definition and more rounded, bulkier musculature. 

The tail itself is a reflection of the size of the vertebral column at its root. We have to keep this in mind when we sculpt the dock area.

Common Artistic Faults with the Torso

Remember the canoe-shape in mind when we sculpt the torso, with its narrow front and gradual wideness towards the back. However, many sculptures are too wide in the front to create a rectangular-like barrel (when seen from above). Or sometimes the barrel will be bulbous in the back (when seen from above), making it appear the sculpture swallowed a beach ball! When it comes to pregnant mares, the entire rear portion of the ribcage is pushed outwards, creating a softer line to the expanded barrel than the beach-ball effect too often seen in sculpture. Sometimes, too, the portion towards the groin will be rounder rather than flatter. Other times, the sides of the barrel are asymmetrical (when seen from above), indicating a serious injury or deformity. Less often, the sculpture is slab-sided, meaning that the desired spring of the ribs is missing.

Musculature on the torso can be mistaken as well, with the deep muscles being confused for the subcutaneous ones. This is especially so with the muscles that "bridge" the appendicular structures with the axial body. On the other hand, "smooth muscled" breeds can be sculpted with too much muscle definition, compromising a believable depiction.

As for the shape of the toppling, it times, though rarely, lordosis, or excessive downwards curve of the spine behind the wither, is present. Lordosis is common with Saddlebreds, for example, and an artist simply copying what's seen will make the mistake of creating a sculpture with this genetic defect.

What's more, the structure and function of the back is often flawed in both structure and mechanics. Generally speaking then, the motions and influences of the back are largely ignored in realistic equine structure. We see many standing backs with moving legs, or they lack "schwung," which is why so much sculpted motion appears stiff and awkward. Often times, the spine is laterally bending or rotating in the wrong location, as well, indicating a broken back. Even more, many performance-depicting pieces have hollow, stiff, or plank (flat) backs when bascule and collection, or other torso motion, are needed.

Likewise, too many stretched halter sculptures have straight backs and level balance or, in other words, they have standing square backs and standing square pelvic girdles and scapulae with cranked-back hind legs or cranked forward forelegs. Often times the hind legs are arbitrarily lengthened to contrive level balance as well. So if such a horse were standing square, his hindquarter would tower over the forequarter. Yet what actually happens in a halter stretch is the back hollows which tips up the pelvis at the point of the buttock to set the hind legs back, and increases the visual length of the underline. The hindend subsequently drops downward, below the wither, creating an uphill balance with the forehand (the LS-joint dips below the withers). In short, the show stretch begins in the spine and not the legs. An incorrectly represented halter stretch is often tied into another common error: docks that are placed too high on the hindquarter, sometimes almost in the middle of the croup, which indicates an incredibly short sacrum.

On the other hand, we sometimes see a crooked spine, when seen from the top. Unless our sculpture depicts a lateral bend to the torso (up to T16), the line of the spinal column from the withers to the dock should be straight. Deviations indicate a break or a pathology, especially if the bend or deviation occurs at the loins, LS-joint, or sacrum. On the other hand, "push-pull" riding, or "frame" riding, can cause a horse to brace his spine and therefore causes kinks along its length. It takes quite a bit of rehabilitation to straighten out such a horse, so we should avoid simply parroting what we see in modern riding.

What's more, the little "buttons" above the pectorals don't articulate or shift: they represent the attachment of the Panniculus to the top of the sternum and are therefore fixed since the sternum is fixed. As the sternum is angled, so are these "buttons." They cannot "slide" up and down in relation to each other.

Speaking of the sternum and body angling, many sculptures have a sternum that's broken away from the ribcage since it's not directly underneath the spinal column. To elaborate, when the spinal column is leaning, the sternum should be angled in the opposite direction. Think of a straight stick with the top being the spinal column and the bottom tip being the sternum. Those two ends should always have a straight line connecting them. When the spine rotates, it takes the ribcage and therefore the sternum with it, re-angling the sternum between the forelegs.

Furthermore, if we aren't careful about the shape of the chest (when seen from the front) we can create a boxy, square chest that looks odd. When seen from the front, the chest should be on a spectrum of roundish ovals. We can also confuse the configuration and shape of the pectorals as well to omit some sections of pectorals, or add in new ones. 

Along those lines, the pectorals morph quite a bit in motion, so we need to be quite clear on their construction. It's a mistake then is to sculpt the same kind of pectorals onto every piece we sculpt since their characteristics are dependent on the individual and the specific situation. In other words, the pectorals on each of our pieces should reflect their mercurial nature in response to individuality, posture, or movement; they shouldn't be the same between sculptures.

Getting back proportion right is imperative, too. But the fact is, backs are typically far longer in life than they're interpreted in art. Indeed, there seems to be a communal blindspot when it comes to torso length. On many sculptures, in fact, we're left wondering where the saddle is supposed to fit! On a normal back then, there should be a head length between the top of the withers to the point of the sacrum, with stallions sometimes being a snidge shorter and mares being a snidge longer. Back length is also dependent on breed or type. For example, a bit of length to the back in the Connemara is deemed acceptable whereas with an Arabian, it's not preferred.

The number, placement, nature, and slenderness of the ribs is often in error, too. They're often too few and too big, or they aren't parallel to each other. Sometimes they've been sculpted in too harshly, being hard grooves rather than subtle indications. Often times, they're the wrong structure as well, being long all the way to the flank when they should diminish in length and leave a gap between the back of the ribcage and the point of hip.

When it comes to the tail, two primary errors occur. First is making the tailbone too thin or too fat. Remember that it reflects the size of the spine as it erupts from the dock. Second, it's often too long. It takes practice and experience to estimate its length, so keep at it and use references.

Refer to Common Artistic Errors in Realistic Equine Sculpture for more torso insights.

Biological Aspects to Consider about the Torso

When we sculpt we should keep the integral nature of the spine in mind as we design posture or motion. It cannot be overstated enough that motion begins in the spine, not the legs. That means the torsos we sculpt need to be synchronized and moving in accordance to the gait, posture, or movement depicted. If not, our sculpture will appear stiff, artificial, and rigid, lacking that natural coordination and fluidity of motion so characteristic of the horse.

Mares also tend to have slightly longer torsos, or be "lower to the ground," more rectangular, than geldings or stallions simply because they have to bear foals. Their ribcages should be well-sprung as well for bearing foals.

A horse that moves in bascule will have a deeper waist, especially when compared to a racehorse, which tend to have "lady waists" from all the LS-Joint flexion and extension. So if our sculpted dressage horse doesn't have a deep waist, for example, we made an error that indicates poor horsemanship.

Achieving bascule is a mechanical process that starts with the flexion of the LS-joint and continuing throughout the spine and neck. It creates a distinct posture called "self-carriage" that results in very definite biomechanics results. In contrast, false collection, which is typical of modern riding, results in the opposite effects that are equally obvious. Learning about bascule and false collection is perhaps one of the best things an artist can learn because it helps us promote positive visuals rather than harmful ones.

Breeds also have different types of torsos from the massive chunk of the European drafter to the svelte desert-dryness of the Egyptian Arabian to the stretchy elegance of the Saddlebred to the round Baroqueness of the Morgan to the squat pudginess of the Shetland. That means we can't just sculpt the same torso for each breed we do, but have to pay attention to it just like we would the head.

Conclusion to Part X

So those are some of the biological and artistic aspects of the equine torso. It's not simply a way to connect the legs, artistically speaking, like some passive expanse to simply fill up. It's an integral part of motion, type, and conditioning. In the truest sense, when we design a new sculpture, it should be the torso we think of first. So mull all this over, and next time we'll get to Part XI, the forelimb!

Until next time then, let's get back to basics! (ha ha ha ha. Get it? Ok.. I'll let myself out...)

"The arts celebrate multiple perspectives. One of their large lessons is that there are many ways to see and interpret the world."

~ Elliot W. Eisner

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