Equine Anatomy and Biomechanics: A Primer of Equine Engineering Part XIII, The Hindlimb

Introduction

Here we are again, back on track with the proper sequence in this 17-part series discussing equine anatomy and biomechanics in a bit more detail than we did in Anatomy 101. It's recommended to read that first, then dive into this series as that beginner level post provides a foundation for understanding this more advanced series.

So in this Part XIII we'll be exploring the hindlimb, which in this case is the femur down to the toe. In Part XII we discussed the pelvis, treating it separately since it's a component of both the spine (Part X) and the hindlimb and warranted its own post because of this interdependence. In previous posts, we've also touched on the head, neck, torso, forelimb plus evolution and some terminology. There's a lot to digest, but understanding our subject from the inside out is important not only for creating accurate work, but responsible work. The more we know about his biology, the more informed are our creative decisions. 

While the the hindlimb is a bit more complicated in function than the forelimb, it's also a snidge easier to sculpt from a sculptural point of view. The planing and angles are a tad simpler than the nuanced aspects of the forelimb, with the hock having simpler articulation. That being said, however, the hindlimb is nothing to sneeze at—it's tricky. Indeed both the forelimb and hindlimb are complex and fascinating, being finely-tuned, highly-developed, incredibly sophisticated running mechanisms. In particular, the stifle and hock flummox many artists in both structure and mechanics which is why we see so many problems in these areas. 

Nonetheless, wholly unique in the animal kingdom, his legs are a distinctive characteristic of Equus. So we have to get them as correct as possible which means we have to pay close attention to their structure, function, and topography when we sculpt since general approximations can lead to errors. Truly, we can tell a lot about an artist's simply skill by how they sculpt the legs.

So let's learn more...

Basic Structure of the Hindlimb

The hindlimb is constructed of alternating angles, starting at the LS-joint then at the femoral joint, then at the stifle joints, then at the hock, then at the fetlock joint, and on down into the foot. This causes the hindlimb to compress in flexion and lengthen in extension not because the bones themselves are shortening or lengthening, but because the angles of these joints are closing or opening. This structure also causes the hindlimb to flex and extend like a drafting lamp as a tensionally balanced system. 

It's also important to understand that muscles cease at the hock therefore all motion from the hock down is accomplished by a pulley system of ligaments acted upon by their muscle counterparts above the hock (like with the knee). Therefore, the femur and hindlimb should be regarded as a whole unit because of the mechanisms binding the structures together (as we learned in our discussion about the Stay Apparatus). 

We should also be aware that the shape of the hind hoof is different from that of the fore hoof due to the mechanics of propulsion and thrust, being more upright and pointed at the toe. (For more detailed information about the hind hoof, and hooves in general, please refer to my blog post, Steppin' Out: Hooves From An Artistic Perspective.) At birth, the hooves are the similar shape, but within three days, they begin to develop their diverging shapes due to these forces. This is an important detail for sculptures of newborns.

Skeletal Structure

The hindlimb consists of the femur, patella, tibia, tarsus (hock), metatarsal (hind cannon), sesamoids, first phalanx, second phalanx, third phalanx, and navicular bone. 

Joints are comprised of the femural joint (femur-pelvis), femoropatellar joint (patella-femur), femorotibial joint (femur-tibia), tarsal joint (hock), fetlock joint (cannon-first phalanx), pastern joint (first phalanx-second phalanx), coffin joint (second phalanx-third phalanx), both sesmoidean joints (with the cannon and first phalanx), and the navicular joint (navicular bone-third phalanx-second phalanx). This makes a total of ten joints in the hindlimb (eleven if we count the LS-joint). The degree of their motility is dependent on their individual structures and mechanisms. 

The femur is the largest long bone in the horse. It articulates on top with the pelvis deep within the hindquarter and below with the tibia and patella. The top part of the femur has a head, the middle is a shaft and the bottom part ends in a trochlea. The head articulates with the pelvis and has the trochanter major, which serves as the attachment of the Deep gluteus and the Middle gluteus muscles. The shaft has both the lesser trochanter on the inside, the attachment of the Psoa muscles, and the tertiary trochanter on the outside, the attachment of the Gluteus superficialis. The distal end has a front trochlea, comprised of two ridges forming an articular surface with the patella and the internal and lateral condyles that articulate with the condyles of the tibia. These two joints form the stifle joint. The whole thing is lashed together with ligaments, tendons, and muscle.

The patella is a solid little bone equivalent to the human kneecap. It's housed in a complex tight network of ligaments, tendon, and muscle. However, when standing, it forms the base of the depression of the overhanging muscles and when the hindlimb is flexed, becomes readily apparent as it protrudes outward.

The stifle joint corresponds to the human knee and consists of two joints, the femoropatellar joint and the femorotibial joint. A network of five important ligaments bind the femoropatellar joint and transfer communication to the femorotibial joint. Likewise, a network of four important ligaments bind and stabilize the femorotibial joint and transfer communication to the femoropatellar joint. 

The fibula of the horse is rudimentary, therefore the tibia (gaskin) bears the weight of the animal and is the most developed bone between the two. It's a long bone extending from the stifle to the hock. At the top, it articulates with the femur and at the bottom with the tarsus. The top part is large, three sided, and dominated by its medial and lateral condyles. The shaft is large and widens distally. The bottom part has an articular surface with two grooves angled about 12º-15º, slanting medially upwards to the dorsal plane, with the hock joint having a corresponding slant to mesh with this articular surface. The bottom of the tibia has a medial and lateral malleolus; the medial is more prominent. The fibula is a slender shaft with a top part that articulates with the lateral condyle of the tibia and a bottom part that's fused to the tibia, acting as an attachment for muscle and ligaments.

The spiral joint of the hock, left hock seen from the front.

Movement of the spiral joint, left hock seen from the front.

The tarsus (hock) is comprised of six bones formed into three layers, creating the distinct shape of the hock. The calcaneus is the largest hock bone, its proximal tip being the tuber calais or calcaneum (point of hock). The hock is composed of a number of articulations that can be generalized into three joints. The tarsocrural, intertarsal, and the tarsometatarsal. However, these joints are lashed together with ligaments, making the only working joint between the bottom of the tibia and the top of the metatarsals.

The three metatarsal bones (hind cannon), the cannon and its two splint bones, have the same basic structure as their metacarpal counterparts. However, the metatarsal is characteristically longer than the metacarpal. The external splint bone is also longer than the internal on the hindlimb, the reverse of the forelimb. 

The sesamoids on the hindlimb are a hair smaller than those on the forelimb. The first phalanx is also a bit shorter. The third phalanx, or coffin bone, is more narrow and pointed as well, mirroring the difference in hoof shape of the hindfoot. 

Basic Musculature of the Hindlimb

The principle stabilizing ligaments of the hindleg are the network of stifle ligaments, capsular ligament of the hip joint, lateral and medial patella ligaments, lateral collateral ligaments of the tarsal joint, plantar ligament, the suspensory (or interosseous) ligament, navicular ligament, and the sesamoidean ligaments. From the hock itself and down, nearly all the bones and ligamentous and tendinous structures are subcutaneous and are readily apparent on a clean-legged horse, morphing in and out of distinction dependent on motion and "dryness." 

Speaking of which, newborn and young foals typically have very "dry" legs with distinct ligamentary and tendinous definition on their knees, hocks, and lower legs. A great way to learn about such structures then is to study foals, especially those references depicting them in motion. Indeed, studying foals is a great way to learn about bony and fleshy landmarks, given we understand that their muscles and bones aren't fully developed yet.

Anyway, the muscles of the pelvis and hindleg are so interdependent because of its bridging muscles that there's some overlap in this inventory, so please reference back to the pelvis installment (Part XII) of this series. 

The basic muscles of the femur are:

(See Basic Musculature of the Pelvis in Part XII)

The basic muscles of the hindlimb are:

• Long digital extensor (or extensor pedis): Extends the phalanges, flexes the hock and fixes the stifle during motion.
• Lateral digital extensor (or peroneus tertius): Assists the long extensor in hock flexion and extension of the digits.
• Deep digital flexor (or perforans): Flexes the digits and extends the hock.
• Long digital flexor (or accesorius): Also referred to as the medial head of the deep digital flexor. Assists the action of the perforans muscle.
• Superficial digital flexor (or perforatus): Flexes the digits and extends the hock joint. Mostly tendinous.
• Tibialis cranialis (or deep flexor metatarsi): Flexes the hock.
• Popliteus:. Flexes the stifle and helps to rotate the leg inward.
• Soleus: Assists the gastrocnemius.
• Gastrocnemius: Extends the hock joint or flexes the stifle joint; these two motions cannot occur simultaneously.

Basic ligaments of the hindlimb are:

• Straight patellar ligaments: Stabilize the stifle joints.
• Internal and External ligaments of the stifle: Help to stabilize the stifle joints.
• Superfical digital flexor (or peroratus) : The tendon of the peroratus. Helps to flex the foreleg and support the forelimb.
• Deep digital flexor (perforans): The tendon of the perforans. 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 is mostly inelastic, but does retain some elasticity. Its branches travel over the first phalanx, clearly visible on a clean-legged horse.
• Metacarpal ligaments: Helps to stabilize the hock. 
• Check ligament: Helps to relieve the strain of standing and force generated by motion; sometimes absent in the hindlimb.
• Long digital extensor tendon (or extensor pedis tendon): Helps to extend the foreleg and stabilize the forelimb.

Biomechanics of the Hindlimb

The hindquarter and, therefore, the hindlimb, is responsible for propulsion and impulsion; it's the engine of the whole system. This is why the hind foot is shaped differently from the fore foot, and why the hindlimb is built as a series of angles to add "spring" for forward motion.

Notice how the system articulates like a drafting lamp?

Hindleg motion begins with the femur (though technically it begins in the spine) in a ball-and-socket joint. The femural joint is very stable due to the depth of the "cup" of the pelvic aspect. It's the femur that initiates the drafting lamp-like motion, the pulley system of the tensionally-adjusted arrangement of tendons and ligaments that automatically flex or extend the entire hindlimb. This means the femoral joint, stifle, hock, fetlock, and foot joints are mechanically moved by this tensionally-balanced, interdependent system and cannot move independently. So when the femoral joint flexes, so must the stifle and the hock, and when it extends, so must the stifle and hock, all at similar angles. If there's any out-of-synch articulation in this system it means there's a catastrophic rupture, a serious injury. 

So any sculpture depicting motion exhibiting such a rupture is fundamentally nonviable since no horse would be able to adequately move with such an injury. It also means that if there's slack in the system, which can naturally happen during motion, the foot can wobble or flip upwards or backwards as we sometimes see during performance or fatigue. Sometimes when the hind limb is extended backwards and the hoof flexed, we'll again see slack in the system as a moderate bump caused the by the crumbling of the ligaments and tendons on the back of the cannon.

Sometimes we'll see the ligaments and tendons "bunch up" as a function of the slack created by a flipped and flexed foot.

The stifle joint corresponds to the human knee. It's a bicondylar joint, consisting of two joints, the femoropatellar joint and the femorotibial joint. Being the largest and most complex in the horse, it's primarily only capable of extension and flexion (like a hinge joint) with an articular angle of about 150º. However, there's some minor rotational play when the hind leg is strongly extended, such as the extension phase of the gallop, slightly spinning the entire limb down to the hind toe towards the median; seen from the front, the spin is slightly clockwise in the left and slightly counterclockwise in the right. The patella slides in synch with the tibia due to the thick casing of ligaments and tendons; down when flexed and up when extended, creating the distinct profiles of this area with the different hindleg articulations. This also means that the distance between the patella and the upper rim of the tibia remains about the same despite extension or flexion. There can be no lateral or medial play at the stifle. All lateral and medial motion can only come from the femoral joint, so the entire hindquarter (and therefore the spine) is engaged during such movements.

Abduction and adduction of the hindlimb. Note how it happens at the femoral joint and not the stifle?

The tarsocrural joint is composed of astragalus and tibial articulation. The astragalus (or talus) is the upper portion of the tarsus, right next to the calcaneus, and is distinctive since its trochlea consist of two oblique ridges that articulate and correspond to those of the bottom part of the tibia. This creates a spiral hinge-joint directed at an angle of about 12º-15º, slanting medially upwards to the dorsal plane. Therefore, the hock cannot articulate straight forwards and backwards like a classic hinge joint (like the elbow), but on an oblique inward angle that spins the metatarsal (counterclockwise for the left hock and clockwise for the right hock when seen from the front), especially when articulation is extreme. This allows the hindcannon to be positioned relatively straight forwards, readying the hind hoof for straight-forward impact and take-off despite the inwards angulation of the gaskin produced by the popped-out stifle. This mechanism increases efficiency and speed while reducing injury and interference. In short, hindlegs cannot move on a straight vertical plane from stifle to toe.

The intertarsal (between the bones of the hock) and the tarsometatarsal (between the tarsal and the metacarpal, or hind cannon) articulations are planar joints, but a tight binding of ligaments and tendons bind the bones of the tarsus into one unit and also lash the tarsals to the metatarsal, forming a long lever activated by muscle action on the calcaneus and the front of the hindcannon. It's the astragalus that articulates with the distal end of the tibia then to create hock flexion, with the remaining tarsus bones and the metatarsal acting as a single structure. In other words, the calcaneum is an extension of the hind cannon and cannot articulate independently. Therefore, the hock is distilled it into a strict hinge joint, capable only of extension and flexion on its unique oblique angle.

 The calcaneum functions as an extension of the metatarsal. Notice the rounded tip of the calcaneum? Flexed hocks aren't pointy.

The articulations of the hind foot are similar to those of the fore leg, so please refer back to that part of the series in Part XI: The Forelimb.

When the horse swings his hindleg forward, the stifle must clear the widely sprung posterior portion of the barrel and, therefore, the stifle pops out around it, spinning and forcing the hindlimb inwards at a slant with an outward rotation. Then the spiral joint of the hock "straightens" and spins the metatarsal to orient it more forwards for effective planting and push-off. Now when bearing or especially when pulling great weight, the hocks tend to come together, increasing leverage and tork. Furthermore, in motion, the horse naturally angles his entire hindlimb inwards at the toe, towards the median, as a function of coordination, speed, and physics. Indeed, the faster the gait, the more towards the median the foot is placed.

All of these stifles are "popping out" to get around the wide barrel. Note how it angles and rotates the hindlimb.

Landmarks and Reference Points

Boney Points of Reference

The great trochanter and third trochanter of the femur can be found under the flesh as well as the bottom ridges of the medial and lateral femoral condyles. The patella can be palpated along with the upper rim of the tibia. In the tibia, the external malleolus, nearly the entire internal aspect of the bone and the internal malleolus are all superficial and important landmarks.

The hock is devoid of muscle tissue and is therefore easily palpable under the skin, tendons, and ligamentous bindings. These bindings and bones are prominent landmarks on the surface of the hock, especially the point of hock. The ligaments of the hock and hindcannon are sometimes readily visible, especially during movement and on clean, "dry" legs.

The three bones of the hindcannon are all subcutaneous and easily palpable, as are the first phalanx and the sesamoids. While many of the ligaments of the pastern can be pin-pointed, the suspensory ligament, is particularly noticeable. The upper portion of the second phalanx is also palpable.

A healthy hock, cannon, fetlock, pastern and pastern joint will not be smooth and uniform, but possess the crisp definitions and distinct geography of the internal surfaces that lay beneath the skin. In other words, they have specific lumps n' bumps characteristic of its design.

Like the knee, the hock is also constructed in a trapezoidal design, when seen from the front:

Left hind leg, seen from the front.

Fleshy Points of Reference

The Biceps femoris group is often readily apparent, especially in extreme motion as is the Semitendinosis, which is subcutaneous. The Tensor fascia latea can often be seen in certain movements along with the Gracilis on the inside of the leg. On the gaskin, the Extensor digitorum longus and the Extensor digitorum pedis lateralis are superficial and their definition can sometimes be seen during articulation or under forces. 

On the inside, the Tibialis anterior, Flexor digitorum pedis longus and its tendon can often be seen as can the Tibialis posterior and the Flexor hallucis longus. The tendons on the hock also contribute to its shape and are often clearly visible, especially in motion. The hamstring is subcutaneously composed of Achilles tendon and the Superficial flexor tendon. The hind cannon has definition similar to the forelimb's cannon.

Artistic Aspects to Consider about the Hindlimb

The hock's only joint exists on the end of the tibia and the top of the hock grouping so the calcaneum (point of hock) moves in synch with the metatarsal (cannon). When the cannon is bend, the point of hock lowers, away from the tibia; when the cannon is straightened, the point of hock rises, towards the tibia. The point of hock also has a distinct shape so when it becomes apparent in articulation, it's peculiar softly-rounded, squared off tip becomes more pronounced. From the back, the point of hock is also broader than the Achilles tendon and the Superficial flexor tendon, producing a "bulb" at its point. This means that the point of hock is neither pointy, as we so often see in sculpture, nor is it narrow to match the back of the hind cannon.

The correct plumb-line from the point of buttock down the back of the metatarsal. Notice the flexion of the fetlock between the two stances.

Hindlimb muscles markedly morph, stretch, goosh, and pooch in motion such as during flexion and extension because the femur is so thickly encased in flesh. So we need to not only know the skeletal structure underneath all that flesh, but also to pay attention to fleshy changes and planes at the different phases of a gait or movement. This is why the hindlimb may appear to "shorten" in extreme flexion (such as with Hackney ponies or Saddlebreds) and "lengthen" with extension (such as with a halter stretch) as the tibia is alternately squished into the hindlimb musculature or stretched out from it. 

The hindlimb is also capable of a great range of motion, thanks to the femoral joint, making it a powerful and dynamic component of movement. This also means that when the hindlimb is bearing weight or pushing off with impulsion, many of these muscles become more readily apparent under the thrust but relax and soften when not weight bearing. Sometimes muscle striations can be seen, often on the Vastus (long muscle of the biceps femoris group) under exertion. Sometimes in extension, wrinkles can be seen between the tail and the femoral joint, in the biceps area, or on the "semis." 

Different breeds or lineages within a breed sometimes have different hindquarter builds, some being rather angular (such as the Teke), square-ish like the Friesian, or round and bulbous (such as the drafter). Certain breeds even have extra development around the dock such as often seen on the Iberian, or an "apple butt." This is noticeably so with asses and sometimes mules.

We also need to know the proper angulation of the hindlimb; that would be a plumb line up the back of the metatarsal to the point of buttock. When standing, this plumb line holds true no matter how the hindlimb is positioned, whether postured behind the body or underneath.

Different breeds have different degrees of limb "dryness." For example, thin-skinned breeds such as the Arabian, Teke and other "hot" breeds will have sharp, clear definition of their leg details. However on heavier breeds such as drafters and draft ponies, or "cold" breeds, there will be less definition.

Common Artistic Faults with the Hindlimb

The hindlimb is commonly flawed in the mechanisms dictated by the Stay Apparatus and Reciprocal Apparatus (both discussed in Part IV: Systems) with joints articulating independently rather than together as a system. Typical faults are a stifle and hock out of synch, or the fetlock and hock out of synch. More subtle, because it's housed in flesh, is a femur out of synch with the hock. Subtler still is a patella out of synch with hindlimb articulation, as it's often forgotten or misplaced in sculpture. This problem typically leads to the oddly shaped formations between the stifle and the tip of the tibia especially in flexion, manifested as too many or too few profile "bumps" along the front of the area, or misplaced bumps. But it's also seen in extension and standing as well. We have to remember that the patella slides in synch with the flexion or extension of the tibia which means that the orientation of and distance between each "point" in relation to the other stays about the same, and is important to correctly render in sculpture. We also often see a femur not long enough and sometimes with a missing patella, producing a roundish profile to the front of the stifle area in flexion. This is usually paired with a standing femur with a flexed hock and a fetlock joint not flexed enough, indicating a ruptured Stay Apparatus and Reciprocal System.

Like the foreleg, the hindlimb even more so moves like a drafting lamp. Note the orientations of the femurs and metatarsals along with the scapulae and the radii, and the humeri and the metacarpals.

Another common error is a straight plane during hindlimb flexion that misinterprets the anatomical structure and biomechanical function of the stifle and hock. Seen from the front, the equine hindlimb cannot flex on a straight plane due to the "popping out" of the stifle and the spiral construction of the hock. Only in extension does the hindlimb straighten out, more or less. Likewise, when the sculpture depicts a standing horse, the hindlimb is usually facing forward on a straight plane (as so often erroneously illustrated in conformation texts) when, in fact, it should be slightly oriented outward as a whole unit, from stifle to toe, away from the median. This is the correct, natural angulation of the hind leg and shouldn't be confused with sickle-hocks. When the hind leg is oriented straight forwards from stifle to toe, the hind leg is actually bow-legged, a conformational fault.

Other common faults are found in the misinterpretation of the stifle, hock, fetlock and foot joints in function or structure, creating mistaken biomechanics, misshapen forms, or incorrect topography. A lack of symmetry from the hock down can also be found, most notably in the fetlock joint and the structure and articulation of the foot structures and joints. The hock is also typically flawed in topography, having "lumps and bumps" in the wrong locations, either seen from the side, or from the front and back. More often, the hock is indistinct and puffy, lacking the necessary lumps n' bumps altogether. Additionally, the hocks's calcaneum is also often too pointy or not moving in synch with the metatarsal, being curved either upwards or downwards away from its proper alignment. Sometimes a flexed hock will be compressed in the front aspect, as through the bones themselves were squished. However, the hock bends by leveraging, not by compression. 

These photos demonstrate the natural and normal outward angulation of the hindlimb. The hindlimb should never be oriented on a straight forwards plane when standing.

Seen from the back, the calcaneum and associated tendons and ligaments down the back of the leg are also often mistakenly the same width instead of the subtle curve and width of the calcaneum in relation to its surrounding structures. In other words, there's no "bulb" to indicate the tip of the calcaneum. Hindlimb's can also be pathological, having puffiness, swellings, depressions, or asymmetries where none should be found. Also found is a misunderstanding of the structure of the tibia, cannon, and pastern bones as seen in a curvature of these bones, often referred to as "spaghetti legs," which we see in the forelimb as well, at times.

The hindleg is set towards the outside of the hindquarter, not in the middle.

Sometimes we also see hind legs oriented in the middle of it's haunch, as though the bulk of the Gracilis didn't exist. This causes a narrow hind end and stance. Instead, the hind legs are oriented towards to outside of the haunch, away form the median.

Furthermore, a lack of adequate physics in motion is typical, with the hindlimb not sufficiently demonstrating the power of thrust, weight-bearing, or stopping necessary to best mimic the effort we see in life. We can sometimes see this in the fetlock joint not being depressed downward, but instead the foot bones are "perched" like a bird on a branch in relation to the ground.

We also find a confusion in the topographical planes of the hindlimb, most notably those planes falling away from the femur and stifle as well as those of the gaskin and especially the hock. Musculature can be confused, especially in how the hindquarter musculature merges with that of the gaskin. Gaskin muscles are also often too bulbous or engaged when they should be relaxed, which can be seen in some extended hindlimbs. Muscles can be carved in for delineation rather than indicated by soft curves and the 3D quality of muscle masses. In other words, the muscles of the hindquarter are defined by lines, or gouged tracing rather than fleshy indications; the musculature expresses more as soft troughs and bulges rather than harsh lines. Think of bulk rather than defined lines indicating muscle. The muscles of the hindquarter also undergo a great deal of morphing in motion, so we shouldn't see "standing" musculature on a piece depiction motion, and visa versa. Likewise, standing muscle definition and delineation is too often seen on hindlimbs that depict motion. Remember, the horse's muscles don't manifest like an articulated anatomy chart...they morph and contort, merge, goo, and pooch in response to articulation and physics. 

Biological Aspects to Consider about the Hindlimb

The equine hindlimb was designed by nature for thrust and propulsion while the forelimbs are designed more as rods, pole-vaulting the forequarter forwards in motion, especially in the gallop. This gives the animal a springy, powerful stride and contributes to that look of hovering flight. And the faster the animal goes, the more important that coordination of the pole-vault motion of the forelimbs becomes. This is why horses trip in the forelimbs whereas they fall out from under themselves in the hindlimbs. It's also why horses forced to move too slowly or out of synch with the coordination of a normal gait can plod, trip, or kick up dirt with their fore hooves because the pole-vaulting effect of the forelimbs cannot be adequately exploited by the hindlimbs. We can sometimes see this with "peanut-rollers" or Western Pleasure horses made to move with impure gaits.

Furthermore, motion that's consistent to natural equine coordination and movement will preserve the integrity of the hind leg joints, even under extreme performance demands. Injuries and pathologies of the hindlimb occur when the animal is forced into unnatural postures that alter natural coordination and function, which can lead to puffiness, bumps, and swellings. We often see this in dressage as so many horses are forced into false collection due to rollkur and "push-pull" or "frame" riding. Indeed, false collection can lead to many pathologies in the hindlimb from String Halt to being "strung out" behind, to even ruptured stifle ligaments.

The hindlimb also moves fast. Those legs positions and push off occur in the blink of an eye and the forelimbs "catch" the forward motion to support the forequarter. Capturing this kind of nimble movement is important for an equine sculpture since a "clunky" feel can really "stop" the motion visually.

Conclusion to Part XIII

Phew! That was a lot to chew on, wasn't it? The hindlimb maybe be a bit easier than the forelimb, but it's complicated nonetheless. So many details to factor into our equation! But how we sculpt the hindlimb can mean the difference between a convincing sculpture and one that's not since the physics of impulsion influence them so greatly. If done right, they can really capture the idea of mass, thrust, and force, as well as fleeting agility so characteristic of this animal's motion.

So now that we have this last aspect under our belt, let's delve into details in the next installment. The equine has lots of them that deserve our careful attention, plus they're fun to infuse into our work because they really bring a piece alive.

So until next time...propel forwards in your understanding equine biology!

"The better an artist can mimic how the eye sees, the more effective and natural the paintings will be."

~ Kenn Backhaus

Equine Anatomy and Biomechanics: A Primer of Equine Engineering Part XIII, The Hindlimb

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

Introduction

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

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