Monday, October 31, 2016

Equine Anatomy and Biomechanics: A Primer of Equine Engineering Part XI, The Forelimb


Whoops! We got a little out of synch here, didn't we? This installment was supposed to come after the torso, but I hosed it up. But we're back on track now.

So hello again and welcome to this 17-part series exploring equine anatomy in a bit more depth than we did in Anatomy 101. So far we've discussed evolution, the neck, head, and torso as well as some common anatomical terminology and orientation terms along with the curiosities of equine evolution. Now it's time to get to the limbs, starting with the forelimb.

The forelimb is very complicated to sculpt, perhaps more so than the hind limb. That's because the play of line, angle, curve, and plane have to be exact for the forelimb to be accurate. Plus the knee structure is a bit more complex than the hock. Truly, much can be learned about a sculptor's prowess simply by studying the forelegs they sculpt. 

Yet one of the things that can make sculpting the forelimb far easier is to understand its anatomy and biomechanics. When we understand the skeletal and fleshy underpinnings of it, we gain clarity and that guides our hands much better. That said, we should also expect we need to sculpt many forelimbs until we approach accuracy. Yes, they really are that tricky. So enough talk! 

Let's go!...

Basic Structure of the Forelimb

The forelimb doesn't just entail the the radius down to the fore hoof. Nope! It encompasses everything from the shoulder to the toe! So there will be a lot of muscles listed. 

Because of all this, the shoulder, arm, and forelimb need to be considered as a whole unit because it's all bound together, interdependently and synergistically. (Refer back to Part IV: Systems regarding the Stay Apparatus of the Forelimb.) There are also a lot of "bridging" muscles linking the forelimb to the torso, so keep in mind there will be some overlap.

Overall, however, it's important to understand that muscles cease at the knee, therefore, all motion from the knee down is accomplished by tendons acted upon by their muscle counterparts above the knee in an organic pulley-system acting on every bone, down to the coffin bone. In other words, there's no muscle contraction below the knee.

Note how the entire forelimb section works as a system, compressing and expanding like a drafting lamp? Look at the scapulae and the radius, and the humerus and the metacarpal. The same goes for the hindlimb. Look at the femurs and metacarpals.

Skeletal Structure

The forelimb is comprised of the scapula, humerus, radius, ulna, carpus (knee), metacarpal (front cannon), sesamoids, first phalanx (long pastern bone or os suffraginis), second phalanx (short pastern bone or os coronae), third phalanx (coffin bone or os pedis) and the navicular bone. 

There's a total of eleven moveable joints in the forelimb. The degree of their motility is dependent on their individual joint structures. Specifically, the joints are comprised of the intersection between the:
  • Shoulder Sling (scapula-torso; not actually a bony “joint" but a muscular sling)
  • Shoulder joint (scapula-humerus)
  • Elbow joint (humerus-radius)
  • First knee joint (radius-first layer of carpals)
  • Second knee joint (first layer of carpals-second layer of carpals)
  • Pisiform joint (the carpals with the pisiform bone)
  • Fetlock joint (metacarpal-first phalanx)
  • Sesamoidean joint (sesamoids and the metacarpal and first phalanx)
  • Pastern joint (first phalanx-second phalanx)
  • Coffin joint (second phalanx-third phalanx) 
  • Navicular joint (navicular bone-third phalanx-second phalanx) 
The scapula (shoulder) is a large, flat triangularish bone, the wide end at the wither and the point at the joint with the humerus. The part by the wither is cartilage (dorsal cartilage) which never completely ossifies and is the part of the scapula that's in closest proximity to the torso. Externally, the scapula is divided lengthwise by its spine. At the bottom of the scapula towards the humerus is the corocoid process, marking the end of the scapula itself. 

The humerus (or arm) is a short thick bone, about the length of the scapula minus the cartilage. It descends down from the scapula at an approximate 90˚angle. The openness or acuteness of this angle, however, is dependent on the individual's conformation. Sometimes the angle is more open, more like 94˚, for example. Regardless, it articulates with the scapula, and its external tuberosity forms the point of shoulder. A ridge lays in front, between the external and internal tuberosities, also forming a visible aspect near the point of shoulder at times. Below the external tuberosity is a flat area ending in the deltoid tuberosity. At the bottom of the humerus, the internal and external condyles articulate with the radius and ulna.

In the horse, the radius and ulna (forearm) are fused, making one bone, keeping the leg permanently oriented forwards. The radius is the main bone of the forearm and the ulna constitutes the elbow (or olecranon process) which is partly subcutaneous. The radius has an external tuberosity at its upper end, near the ulna, that is visible below the external condyle of the humerus. At the bottom of the radius, the external and internal malleolus are subcutaneous and distinctly visible, especially the internal malleolus. The internal shaft of the radius is also subcutaneous about halfway down the bone. When seen from the front, the radius doesn't descend perpendicular to the ground to the cannons, but on a slight medial slant, like the human femur, to join with the knee at an angle. However, the forecannons are perpendicular to the ground.

The image depicts the left foreleg.

The carpus (knee) is comprised of seven irregular bones stacked into two layers between the end of the radius and the top of the metacarpal (cannon). It articulates between the radius and the first carpal layer (radial-carpal articulation) and between the two carpal layers (inter-carpal articulation). It doesn't articulate between the second layer of metacarpals and the cannon because this layer is tightly bound to the top of the metacarpal by ligaments. The pisiform (or accessory carpal bone) is part of the first layer and is located on the back of the carpus, towards the outside, which articulates with both layers in flexion. A complex network of ligaments binds the carpus together. 

The metacarpus (cannon bone) consists of three bones, the cannon and the two splint bones which become tightly conjoined in maturity as to form one structure. The middle bone is the largest, forming what is known as the cannon bone. The internal splint bone is a bit longer than the lateral splint bone.

The first phalanx (long pastern bone) is the largest and longest of the phalangeal bones. The sesamoids are two small bones behind the fetlock joint that are tightly bound by ligaments. The second phalanx (short pastern bone) is a short, wide bone, and the third phalanx (coffin bone) is shaped like a little hoofie, and with the crescent-shaped navicular bone, is buried inside the hoof. On a healthy foot, the top of the hoof capsule is located at or below the front point of the coffin bone. If the front point of the coffin bone lies below the top of the hoof capsule, enclosing a portion of the second phalanx, that indicates a "mechanical sinker," a pathological foot. (Refer to Steppin' Out: Hooves From An Artistic Perspective for more in-depth information of the foot.)

Basic Musculature of the Shoulder

The foreleg isn't attached to the torso by a bony connection, but by a sling of flesh composed of ligaments, muscles, muscle tendons, and muscle aponeurosis. The primary ligament of this system is the dorsoscapular ligament which originates on the thoracic portion of the Rhomboideus, Splenius, and Complexus muscles. This ligament gives attachment of the limb to the truck by thickened fascia of the third, fourth, and fifth thoracic spines. After giving origin to these muscles, it thins to form many lamellae which network the scapular portion of the Serratus ventralis and attach to the scapula.

The basic ligaments of the forelimb, in general, are the capsular ligament of the shoulder joint, the collateral ligaments of the elbow joint, the collateral ligaments of the carpal joint, the collateral sesamoidan ligaments, the collateral ligaments of the coffin joint, superficial digital flexor (or perforantus ligament), the suspensory ligament, deep digital flexor (or perforans tendon), common digital extensor (or extensor pedis tendon), the check (or subcarpal) ligaments and the Suffraginis ligament. From the knee down, nearly all the bones and ligamentous and tendinous structures are subcutaneous and are readily apparent on a clean-legged horse.

The muscles of the neck, torso, and shoulder are so interdependent that some overlap in the inventory occurs, in which case, reference is made back to the respective inventories. Therefore, the basic muscles of the shoulder are:
  • Trapezius: See Part IX: The Neck. Helps toe flex the shoulder and elevate it. Part of the Shoulder Sling. 
  • Rhomboideus: See Part IX, The Neck. Helps to rotate the scapula backwards or partially elevate it. When at rest, the cervical portion helps to lift the neck. Part of the Shoulder Sling.
  • Latissiumus dorsi: See Part X: The Torso. Helps to pull the humerus up and back, flexing the shoulder joint. Part of the Shoulder Sling.
  • Brachiocephalicus: See Part IX: The Neck. Helps to elevate the shoulder and pull it forward. Part of the Shoulder Sling.
  • Omotransversarius: See Part IX: The Neck. Helps to elevate the shoulder and pull it forward. Part of the Shoulder Sling.
  • Anterior superficial pectoral: See Part X: The Torso. Helps to adduct the forelimb or draw it forwards. Part of the Shoulder Sling. 
  • Posterior superficial pectoral: See Part X: The Torso. Helps to adduct the forelimb. Part of the Shoulder Sling. 
  • Anterior deep pectoral: See Part X: The Torso. Helps to adduct the forelimb and draw it forwards. Part of the Shoulder Sling. 
  • Posterior deep pectoral: See Part X: The Torso. Helps to adduct the forelimb. Part of the Shoulder Sling. 
  • Serratus ventralis (cervical and thoracic portions): Each portion works antagonistically to respectively pull the shoulder forwards or backwards, or together, pull the shoulder upwards. Part of the Shoulder Sling.
  • Deltoid: Flexes the shoulder joint, abducts the arm and tenses the scapular aponeurosis.
  • Supraspinatus: Aids in shoulder extension and with its two tendons, aids in stabilization of the shoulder joint.
  • Infraspinatus: Acts as a lateral ligament of the shoulder joint, abducts the forelimb, and permits outward rotation and extension of the shoulder joint.
  • Teres major: Flexes the shoulder joint and adducts the humerus. 
  • Teres minor: Helps flex the shoulder joint, abduct the arm, and rotate the forelimb outward.
  • Subscapularis: Functions to stabilize the shoulder joint and adduct the arm.
  • Coracobrachialis: Provides some stabilization to the joint and mildly aids the adduction of the humerus and flexion of the shoulder.
  • Capsularis: Helps to stabilize the joint and aid in adduction of the limb.
  • Scapular Aponeurosis: Tensed by the Deltoids and Anterior deep pectoral. Its external aspect provides attachment for muscles that connect the forelimb to the torso.
Basic Musculature of the Humerus

The humerus or arm is often ignored both in anatomical and conformational references, with the shoulder getting most of the limelight. However, the arm and its angles with the shoulder and radius are of pivotal importance to motion. An open angle with a long, sloping shoulder produces flashy motion whereas a more closed angle produces the "grass cutting" long and low foreleg motion. This is why the foreleg-humerus angles of Saddlebreds and hunter hacks are so different, for example. The humerus is also a massive bone, with a twist in the middle. This is because the fore fins were the first to flip over to form limbs during evolution, which is where the twist comes from and why the forelimb is much more stable. (The hindlegs were the last, which is why the stifle joint, or knee, is so jury-rigged.)

That in mind, the basic musculature of the humerus is:
  • Biceps brachii: This important muscle flexes the elbow joint and fixes the shoulder and elbow when standing. Also it has a significant influence on forelimb articulation because it directly connects the scapula with the radius, being unattached to the humerus. Therefore, if the scapula is raised, so must the radius follow automatically; the shoulder and radius are directly linked by this powerful muscle, i.e. the shoulder joint and the elbow joint are directly linked as well.  
  • Brachialis: Flexes the elbow by directly flexing the humerus with the forearm.
  • Triceps brachii: This large and important muscle is a powerful extensor of the foreleg.
  • Tensor fasciae antebrachii: Helps to control the elbow joint and the flexion of the forearm.
  • Anaconeus: Helps to extend the foreleg and also raises the capsule of the joint, preventing the capsular ligament from being pinched during extension.
Basic Musculature of the Foreleg

The musculature of the foreleg is based on a system of pulleys and leverages. Contraction or relaxation high on the limb results in extension, flexion, adduction, abduction, or rotation lower on the limb.
  • Flexor pedis perforans: Flexes the knee and digits, and supports the foreleg.
  • Flexor pedis perforatus: Flexes the knee and digits and supports the foreleg.
  • Common digital extensor (or extensor pedis): Extends the knee, digital joints, and flexes the elbow.
  • Lateral digital extensor (or surraginis): Extends the digits and the knee, and helps to stabilize the foreleg.
  • Extensor metacarpi obliquus : Helps to extend the knee and can rotate the cannon bone slightly outward.
  • Extensor carpi radialis (or extensor metacarpi magnus): This powerful muscle is straightens the knee. 
  • Ulnaris lateralis (or flexor metacarpi externus): Flexes the knee and extends the elbow. 
  • Flexor carpi ulnaris (or flexor metacarpi medius): Flexes the knee.
  • Flexor carpi ulnaris-humeral head (or flexor metacarpi internus): Flexes the knee. This muscle is readily visible because the large subcutaneous vein of the forearm runs between it and the posterior portion of the radius.
Now those are the activators (above) that act on these following tendons and ligaments, i.e. some of the following are the actual extensions of the muscles above:
  • Superfical digital flexor (or perforatus tendon) : Helps to flex the foreleg and support the forelimb.
  • Deep digital flexor (perforans tendon): Helps to flex the foreleg and support the forelimb.
  • Suspensory ligament (or interosseous ligament): A strong brace for the fetlock joint that relieves the strain and effort required for the horse to stand and move. It's mostly inelastic, but does retain some elasticity. Its branches over the first phalanx and is clearly visible on a clean-legged horse.
  • Superior check carpal ligament: Helps to support the perforatus tendon. 
  • Carpal ligaments: Helps to stabilize the knee.
  • Sub-carpal check ligament: Helps to relieve the strain of standing and force generated by motion.
  • Common digital extensor tendon (or extensor pedis tendon): Helps to extend the foreleg and stabilize the forelimb.
  • Sesamoid ligaments: The network of ligaments of the fetlock, sesamoids, and pastern joints that form strong bonds to stabilize these areas and also modify their outward appearance. 
  • Navicular ligament: Stabilizes the navicular bone.
Biomechanics of the Forelimb

We can refer to the forelimb system as an Appendicular System while the torso, neck, and head are the Axial System. And one of the rules of equine motion and posture is that Axial motions govern Appendicular motions. That is to say, what the torso (i.e. the spine) does, so must the limbs. Therefore, it's the oscillations of the spine that initiate and determine motion, not the legs.

The scapula-thoracic “joint” is an synsarcosis joint, sliding over the torso, independent of its paired counterpart and consistent to the soft tissues that bind it to the torso. Because the shoulder is connected to the torso only by flesh, it's motion very fluid, supple, and dynamic. Besides motion, however, the shoulder (i.e. the Shoulder Sling) also absorbs shock (as we seen when the foreleg is jammed into the torso when landing a jump, for example), increases stride length (by stretching forwards or backwards), influences the function of the neck and torso (since they share musculature), affects coordination and balance (as the forelegs shift up and down in relation to the torso), and dictates the motion of the foreleg (through the pulley systems). During evolution, the equine lost the clavicle, too, because such a large herbivore dependent on agility, endurance, and speed would break them in short order otherwise. Also as a reminder, the delicate hyoids inside the head (the swallowing mechanism) is also directly attached to the forelimb (as we learned in Part IV: Systems). 

The joint between the scapula and humerus (the shoulder joint), it's a shallow ball and socket joint and saddle joint combined, yet a network of muscles, tendons, and some ligaments inhibit the full potential of articulation at this point. It's rather curious that muscles provide more stabilization to this joint than ligamentary connections, or even the joint construction itself. Nonetheless, its large head (on the humerus) guards against dislocation. Its primary motion is extension and flexion, like a hinge joint, that can be extended to about 145˚ and flexed to approximately 80˚, give or take. It does, however, have a goodly degree of rotation, adduction and abduction, helping to rotate the forelimb, or move it outward or inward such as with half-passes or in cutting. 

That means outward or inward rotation, or abduction or adduction of the foreleg does not occur at the  elbow, it being a strict hinge joint. Rather, these motions are created by the rotation, adduction, or abduction of the scapulo-humeral joint which moves the whole forelimb laterally outward or inward or also rotated outward or inward. This is why the whole chest expands and compresses during abduction or adduction, or why the entire leg spins when that joint is rotated. The physical appearance of the pectorals is also affected and altered by shoulder motion, being morphed, compressed, pooched, or stretched in synchronicity. The scapula can also "pop out" in a minor rotational lengthwise motion in extreme flexion, such as when the horse lays down, due to the "push up" of flesh from the ground.

Adduction and abduction of the forelimb can only happen at the scapula-humeral joint, not the elbow. The example here depicts abduction, or movement away from the median.

 We can see here that abduction and adduction (below) can only happen at the shoulder joint.

Abduction and adduction cannot happen at the elbow, which is a strict hinge joint.

More still, because of the Stay Apparatus (discussed in Part IV) , the shoulder, humerus, and foreleg operate together as an interdependent, symbiotic system. But remember, shoulder articulation (i.e. spinal oscillation) creates foreleg motion; foreleg motion does not create shoulder motion. This means that nothing in the forelimb can articulate independently of the other parts; the forelimb operates like a drafting lamp, compressing, stretching, extending, and flexing together as one system. That's to say, what the scapula does, the radius wants to do. Or put another way, what the humerus does, the metacarpal really wants to do as well. In short, all the joints of the forelimb are synchronized in a tensionally-balanced system.

The joint between the humerus and the top of the radius and ulna (the elbow joint) is a classic hinge joint, capable only of extension and flexion with no lateral play or rotation. Being so, it's a highly stable joint. Unlike us, the ulna of the horse is fused to the radius, permanently and automatically orienting the forelimb forwards for efficient, sustained flight (as we learned in Part VI and VII: Evolution Parts 1 and 2). The ulna works in leverage with the radius; it moves an one unit with the radius. That means if the radius is flexed the ulna will drop, or if the radius is extended, the ulna will straighten in relation to the shaft of the radius.

There are two functional joints in the knee. The first one is between the bottom of the radius and the first layer of carpals, and the second is between the first and second layer of carpals. The intersection between the second layer of carpals and the top of the metacarpal is tightly lashed with ligaments, and so doesn't articulate. This is why the top part of a flexed knee is more pointy while the second bend in the knee is more rounded. The radio-carpal articulation is capable of almost a 90˚ opening and the inter-carpal articulation is capable of about a 70˚ opening. The pisiform (or accessory carpal bone) is part of the first layer of carpals, and is located on the back of the carpus, towards the outside. It forms a lever, a connection for and a stabilizer of the ligaments that run down the back of the forelimb. It also moves in synchronicity with the first and a bit with the second layer of carpals, altering the appearance of the knee joint between flexion and extension. Both of these knee joints are hinge joints that have some rotational and circumventing play, rotating or moving the cannon slightly outward or inward such as we see when the horse lays down, for instance.

The fetlock joint, pastern joint, and coffin joint are all hinge joints that have a limited amount of both rotational and lateral play seen when a foot pops out laterally in energetic motion, or when the horse lays down, or makes a sharp turn or balance correction. Depending on the horse's natural coordination of motion (each horse can be different in this), the foot bones can curl to different degrees of flexion when the forelimb is flexed, when seen from the side. Some horses tightly curl the foot bones and delicately land the foot while some let their foot bones dangle and "plop" the foot onto the ground with a thud. These differences (and the spectrum in between) can be a function of his mood or level of exhaustion, too. Also if the hoof capsule is properly at or below the front top point of the coffin bone, the foot will be able to curl in flexion even more, since it's not "trapped" inside the hoof capsule. The sesamoids move in synch with the first phalanx and the second phalanx, sliding along the round posterior surface of the metatarsal in flexion or extension. They stabilize the tendons and ligaments that run down the back of the forelimb into the foot, maintaining a constant angle of insertion, and increase the torque exerted on the hoof. Curiously, the navicular bone lacks periosteum, meaning that it's really an ossified ancient bursa. Biomechanically, it stabilizes the insertion of the tendons by maintaining a constant angle of insertion onto the coffin bone.

The horse usually and naturally angles his entire foreleg inwards when he moves, slightly towards the median, as a function of natural coordination. In other words, the horse doesn't tend to plant his forelegs perpendicular to the ground during movement, especially when moving at speed. This is referred to as "tracking down the middle," or "tracking down the median."

Landmarks and Reference Points

Boney Points of Reference

The most obvious parts of the scapula are the dorsal cartilage, the spine, the top posterior portion and in many horses, the coronoid process (as a "button" around the point of shoulder). We often can see the "shoulder bed" created by the muscles attaching to the neck and surrounding the scapula.

The "shoulder bed," or ledge of the shoulder where it connects to the neck.

On the humerus, the external tuberosity is easily found and palpated around the point of shoulder as well as the bony ridge which forms a visible aspect of the point of shoulder. The deltoid tuberosity is also a discernible landmark. These three points are distinguishable both by their surface effect and also by the muscles that converge on them. 

Many aspects of the radius are subcutaneous such as the external tuberosity of the radius below the external condyle of the humerus, at the top of the radius. Also subcutaneous and obvious is the external malleolus and especially the internal malleolus of the radius. In fact, much of the internal shaft of the radius is subcutaneous and forms a definite surface landmark with the muscles laying around it. The olecranon process of the ulna 
(elbow) is subcutaneous, too, and very obvious on the horse, particularly when the foreleg is flexed. 

The knee is devoid of muscle tissue and is therefore essentially subcutaneous and easily palpated. The pisiform on the back, in particular, is especially prominent when the leg is standing. A rounded bump on the lower rim of the radius and the top of the metacarpal are clearly visible, from the front and in profile, as a break in the profile between the radius and the knee and metacarpal. The lower "break" in the profile is more common than a top one. From the front, the internal "point" at the bottom of the radius is prominent while its external point is slightly less so, but clearly obvious and palpable. The lower aspects of the knee are more rounded, both internally and externally.

The front of the metacarpal and the spint bones on the side are easily palpated and subcutaneous. The first and second phalanx, and the tip of the coffin bone (if the hoof capsule is properly located at or below it) are all palpable and subcutaneous, as are the sesamoids. While many of the ligaments of the foot can be pin-pointed, the suspensory ligament is particularly noticeable. 

A healthy knee, cannon, and fetlock will not be smooth, but possess the crisp definitions and distinct topography of the internal surfaces that lay beneath the hide. In other words, they'll have all the bumps and grooves indicative of a "clean leg." 
The foot bones will be "clean" as well. 

Fleshy Points of Reference

The forelimb has many fleshy points of reference, simplifying the sculpting process quite a bit. This is because much of the forelimb from the forearm down is subcutaneous or partially so.

Starting at the top, the Trapezius can be clearly seen on fit horses. The Supraspinatus is often visible as are the Deltoids and the Infraspinatus. The Biceps and Superficial pectorals are distinct as are the Triceps. In fact, on a fit, muscular horse, the groove between the two portions of the Triceps can be visible as well.

On the forearm, nearly all the muscles and their tendons are subcutaneous. In particular, the Extensor carpi radialis is clearly visible on the front while the Flexor carpi ulnaris is visible on the back. Sometimes the Extensor metacarpi obliquus can be seen, too. The Extensor Pedis and the Ulnaris lateralis form a "W" on the lateral side and their tendons are often visible over the knee. The Extensor Pedis has a tendon down the middle, which can be sometimes seen, especially when the foreleg is flexed or extended. Often the Suffraginis is visible, too. Then over the knee in the front, sometimes the Annular ligament is seen as can the tendon of the Extensor carpi radialis. 

Flowing down the metacarpal, the tendons of the muscles are often seen on a clean-legged horse, often forming distinct landmarks, especially when the leg is extended or flexed. Behind the metacarpal, the Suspensory ligament, the Perforans tendon, and the Perforatus tendon form those clear tendon grooves typical of the cannons. 

We can clearly see the ligaments on the front of the cannon bone.

On the inside, the Extensor carpi radialis, a terminal branch of the Biceps and the Flexor metacarpal internus sandwich the internal aspect of the radius.

Moving further down, the tendon of the Extensor Pedis and the Suspensory ligament are clearly seen across the foot bones on a clean-legged horse.

Artistic Aspects to Consider about the Forelimb

We can think of the scapula as a long, skinny triangle, with the wide portion at the wither coming to a point to form a joint with the humerus. In turn, we can think of the humerus as a twisted tube, to form a joint at its end with the top of the radius and ulna. The radius is a shaft of bone, slightly curved forwards, sitting atop a semi-circular wall of carpal layers. Similarly, the metacarpal is a shaft of bone slightly skinnier than the radius when seen from the front. The form made between the first and second phalanx, from the front, with all their ligaments and tendons, is bell-shaped and inserts into the lateral cartilages.

The knee shouldn't be smooth and indistinct, with smooth, rounded lines. It should be crisp and bumpy according to the underlying topograph; it should appear to be made of bone and ligaments, in clean, precise definition. Likewise, the metacarpal should be crisp and defined and not puffy, bulbous, or smooth. The same can be said of the fetlock and pastern, too. From the front, the fetlock is smaller than the knee; the two shouldn't be the same width. And when it comes to the foot, please refer to my blog series on the feet.

The planes of the foreleg are rather complicated. Therefore, it's a good idea to work from lots of reference photos from many angles in the desired position. And how the muscles and tendons manifest can be slightly different between horses, and different between standing and motion. So again, reference photos are our surest bet to getting things right aside from field study. The equine forelimb is best learned through comparison, so exploit many reference photos and opportunities to do field study.

Common Artistic Faults with the Forelimb

Flaws with the forelimb are common in realistic equine sculpture owning to its difficulty. But perhaps the biggest error is a lack of crisp topography. Many sculpted forelegs, especially the joints, are puffy, indistinct, and smooth when, in reality, they should have specific lumps n' bumps 'n grooves indicative of their underlying anatomy so typical of a clean-legged horse. Often times, too, forelegs and joints have bumps n' lumps that don't belong there, as though the artist simply made them up. More still, sometimes these bumps n' lumps aren't scaled properly, usually being too big.

Joints are often misshapen as well, especially the knee, and especially from the front. The equine knee has a rather specific shape from the front, that of a trapezoid, with three very distinct points owing to little tuberosities. From the side, it forms a another trapezoid with the pisiform, with lots of ligamentary and bony detail, both inside and out. 
Many sculptures are missing their pisiforms as well, lacking that "bump" at the back of the knee.

Some Pisiforms are more prominent than others, this being about the medium manifestation. Many sculptures lack this essential bump, creating a smooth profile to the back of the leg, which is an error.

A very common conformational flaw in sculpture are calf-knees. The carpals should meet the top of the metacarpal at a 90˚ angle; otherwise the leg is calf-kneed. Even a slight calf-knee is a serious conformation fault. Other distortions on legs indicate injury or pathology such as bowed tendon, bucked shins, splints, ringbone, etc. Ringbone is particularly common since the artist makes an overly rounded, pronounced coronet band around the hoof. "Spaghetti legs," once common in the past, have waned a bit more in current years, but still occur, especially when seen from the front. We have to be absolutely precise and detailed in order to create correct forelegs.

This mare demonstrates a properly "straight" forelimb; no calf-knees!

Another common flaw are forearms 
perpendicular to the ground, when see from the front. In life, the forearms angle inward to the median at the knee, with the metacarpals being perpendicular. In other words, horses are slightly knock-kneed, seen from the front, as we discussed previously.

 Image is depicting the left fore leg.

The forelimb is also typically faulted by incorrectly articulating joints, most commonly the elbow, shoulder, and pastern. Either the joint itself is misrepresented such as a rotating or laterally bending elbow, or the forelimb isn't functioning like a drafting lamp, so we have a standing shoulder with an extended foreleg or a rotated shoulder with a standing foreleg. Often times the elbow joint is incorrect in its articulation, with the point of elbow often nonexistent or not pronounced enough. Many times the knee is misshapen in articulation since the artist didn't understand how it changes in flexion.

Another common problem is a lack of physics expressed in the forelimb. Mass, inertia, momentum, impact, and centrifrugal force are all relevant energies we need to imbue in our sculpted forelimbs and since the horse carries about 60% of his weight on his forehand, these energies find ample expression here.

Biological Aspects to Consider about the Forelimb

Again, the forelimb has a slight knock-kneed stance. This cannot be overstated enough since it's such a common omission. In other words, the limb doesn't descend from the torso straight down, perpendicular to the ground. The knees angle inwards (because the radius angles slightly inward) towards the median a snidge, with the metacarpals being perpendicular to the ground. They're oriented very much like our femurs and tibias, when seen from the front.

Errors in the articulation of the forelimb indicates an grave injury. So if we have a trotting sculpture exhibiting a standing shoulder but with a flexed foreleg, we've actually sculpted a horse with a terrible injury, one who certainly wouldn't be trotting around. This is how these types of flaws aren't just logic flaws, they're flaws in realism, even if the rest of the piece is absolutely spot on. We have to train ourselves what to look for, to see how those joints articulate together, if we hope to infuse that into our work.

As for motion, when the horse moves, he "rolls over" his forelegs like a pole-vaulter. That's to say, his hindend is the impulsive drive; it's what moves him forward. As his hindlegs drive him forward, he uses his forelegs like pole vaults to keep his forehand upright and continue the stride. That means the horse is a "rear-wheel drive" animal; he doesn't pull himself along, he pushes himself along. And since he carries an estimated 60% of his weight on his forehand, that's a lot of weight to coordinate and move. These ideas are some of the reasons why the fore hoof is round as compared to the more pointed toe of the hind leg hoof. So if we make the fore hooves and hind hooves the same, we've created an error in realism.

In regards to the foot, the forefoot shouldn't impact the ground toe-first, but heel-first. It's important to know that many feet on the domestic horse are pathological (and science is continuing to discover just how much), ones that often land toe-first (so we can't simply sculpt what we see). This causes a cascade effect in the foot that's destructive and typically leads to injury, so avoiding that stance in our sculpture may be a good idea. However, often a toe-first stance helps along a more graceful composition or allows it to free-stand, so it's up to artistic license to decide how to orient the toe. For more in-depth information on the foot and hoof, please refer to my previous blog series.

One of the goals of bascule, or collection, is to shift more weight onto the hindquarter for carrying capacity of the rider and a "lighter," more agile forehand. To be "heavy on the forehand" is to be lacking in this quality, often meaning that the horse lacks self-carriage altogether. So if we design a composition of a horse with his weight shifted backwards, we need to be careful not to flex his fore fetlocks so much, right? As to indicate excessive weight bearing? They should be more passive, in comparison to the more flexed hind fetlocks, which are presumably carrying more weight.

Conclusion to Part XI

Well, that's a good primer for the foreleg, one that will set the stage for further proactive learning. The foreleg may be based on simple principles, but there's hardly anything simple about it! It takes a lot of study, practice, and diligence to get them right, so keep at it. And it's worth it to get it as right as possible. So many flaws abound with it in sculpture, that to get it even partway right is to give ourselves a big comparative advantage.

In Part XIII then, we'll then look at the hind legs, those complicated features that move the horse forwards. (We already discussed the pelvis in Part XII because we got a little bit ahead of ourselves.) 
So until next time...get a leg up and master those forelegs!

"Without craftsmanship, inspiration is a mere reed shaken in the wind."

~ Johannes Brahms


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

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