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

Introduction

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