Equine Anatomy and Biomechanics: A Primer of Equine Engineering Part II, Terminology

Introduction

Welcome back to this 17-part series exploring equine structure and mechanics. Months previous, we learned how to use anatomy charts more effectively. We've discussed how anatomy and conformation aren't synonymous. We also discovered the significance of viability and functionality. We also learned how to organize our creative process to best facilitate accuracy. We've also considered the spectrum of believability when it comes to realism,  and the Five Ps, or components that steer us towards more realism. We even learned about some common artistic errors in realistic equine sculpture. That was a lot to chew on, wasn't it?

Well, in this series we'll move beyond this rudimentary understanding and move into an intermediate-level of discussion. For that reason, this Part II will present some common anatomical terminology. Researching equine structure will inevitably present us with funky terms that refer to anatomical locations and orientations. We should be familiarized with such terminology so we can better decipher anatomical texts and visualize how anatomy is put together in our heads and in our sculptures. 

Now it should be mentioned that this terminology is often different between texts or resources. We have to remember that each dissection entails a new person trying to make logical sense out of chaotic, organic nature. That means that some terminology might be different, or anatomical structures may be organized in different ways. That's okay. If we have a firm grasp of the structures themselves, then transferring that understanding from text to text and diagram to diagram will be easier and more accurate. Everyone has their own way of doing it, and eventually we'll discover our own.

So to help us understand many of the structures presented in this series, let's discover some directional terminology and anatomical definitions...

Chart of Joint Locations

Directional Terms

These are common terms we'll encounter in anatomical text so it's a good idea to be familiar with them. They may also be combined to best describe the location or orientation of a structure.

Anatomical Planes

The main reference planes are the:

• Median Plane: Bisects the animal into symmetrical right and left halves. Any plane parallel to the median plane is called the...
• Sagittal plane: Any plane parallel to the median plane.
• Dorsal Plane: A parallel line that symmetrically divides the animal dorsally and ventrally, into a top and bottom half.
• Transverse Plane: Transects the animal perpendicular to its own length, creating a a front half and back half. With the horse, also called the Coronal, Cross Section, or Frontal Plane.
• Oblique Plane: A plane at an angle.

The main orientation terms are the:

• Medial: Towards the median plane.
• Lateral: Away the median plane.
• Ventral Plane: Towards the underside. 
• Dorsal: Towards the topside.
• Cranial: Towards the head.
• Caudal: Towards the tail. 
• Rostral: Towards the nose.
• Proximal: In reference to the limbs, those aspects closest to the body.
• Distal: In reference to the limbs, those aspects away from the body.
• Palmar or Volar: In reference to the lower limbs, the back or bottom of the forelimb. 
• Plantar: In reference to the lower limbs: the back or bottom of the hindlimb.
• Volar flexion: Flexion bent backwards.
• Dorsal flexion: Flexion bent forwards, often in relation to hyperextension from weight-bearing.
• Anterior: "In front of."
• Posterior: "In back of."
• Superior: An area up and towards the head.
• Inferior: An area towards the feet.
• External: All those aspects towards the surface of the body.
• Internal: All those aspects towards the inside of the body.
• Superficial: Those aspects towards the skin surface.
• Deep:Those aspects away from the skin surface.
• Adaxial: Those aspects towards the axis of the body.
• Abaxial: Those aspects away from the axis of the body.
• Ipsilateral: The same side of the body.
• Collateral: The other side of the body, referring to pairs.
• Adduct: In reference to movement, to draw towards the medial plane.
• Abduct: In reference to movement, to move away from the medial plane.
• Bilateral: Refers to paired body parts such as either side of the head, the legs, and the body, for instance.

Bones

Approximately two hundred and five bones make up the equine skeleton. The knee of the horse’s foreleg corresponds to the human wrist, and the hock corresponds to the human ankle. The cannons bones are equivalent to the bones in the palm of the human hand or instep of the foot. In both cases, the cannon bone corresponds to the middle, and longest, of the five bones in the human hand and foot. The first and second phalanges are a modification of the middle fingers or toes, and the coffin bone corresponds to the last finger or toe bone. The hoof is a modified version of the human fingernail or toenail.

Bone is the internal support system for muscles and fleshy tissues. There are five basic types:

• Long bones: Act as levers to support and move the body. Example: femur.

• Flat bones: Protect organs and provide attachments for flesh. Example: cranium. 

• Short bones: Absorb shock. Example: knee and hock.

• Irregular bones: Protect the spinal cord. Example: spine. 

• Sesamoid bones: Increase torque and keep tendon insertions at a constant angle. Examples: sesamoids and navicular bone.

There are terms that describe the different parts of bone. Sometimes these parts, such as protrusions and ridges, are subcutaneous boney landmarks on the surface of the animal. The bony parts are described as:

• Angle: A corner in a bone. The inferior (lower) and superior angle (upper).
• Body: The main portion of a bone.
• Crest: Prominent border or ridge.
• Line: Similar to a crest, but not raised as much; can be relatively faint.
• Diaphysis: Main section or shaft of a long bone.
• Epiphysis: The end part of a long bone, usually with a larger diameter.
• Facet: A smooth, flat articular surface.
• Lacunae: A cavity or depression in bone.
• Canaliculi: A small channel or duct in bone.
• Fissure: A long, crack-like hole for nerves and blood vessels.
• Foramen (pl. foramina): A round hole through which nerves, blood vessels, or ligaments pass.
• Fossa (pl. fossae): A depression or hollow in a bone. 
• Malleolus: A bony projection with a hammer-head shape, often at the end of a bone.
• Tuberosity: Large round nodule or oblong projection that looks like a raised bump that are attachment for muscles, tendons, or ligaments; larger than a tubercle.
• Tubercle: Small tuberosity that's a round nodule or warty outgrowth to act as an attachment for connective tissue such as muscles, ligaments, or tendons.
• Condyle: A rounded, smooth prominence at the end of a bone where if forms a joint with another bone.
• Intercondylar notch: The channel between two condyles.
• Epicondyle: A "bump on a bump," a rounded projection located on or above a condyle and typically serving as an attachment location for ligaments and tendons.
• Trochanter: A large, blunt protrusion at the proximal and lateral part of the shaft of the femur; larger than a tuberosity.
• Greater: Larger, as in Greater Trochanter.
• Lesser: Smaller, as in Lesser Trochanter.
• Intertrochanteric line: A line on the anterior side of the proximal end of the femur.
• Intertrochanteric crest: A bony ridge on the posterior side of the femur's head, stretching down and medially from the top of the great trochanter to the lesser trochanter. With the intertrochanteric line on the anterior side of the bone's head, they mark the transition between the femur's neck and the femur's shaft.
• Head: The ball of a ball and socket joint.
• Margin: Edge of a flat bone.
• Meatus (pl. meati): A tube-like opening or channel extending within a bone.
• Neck: The narrowed section of long bone between the head and the shaft, usually at the base of the head.
• Septum (pl. septa): The dividing wall that separates tissues or cavities.
• Notch: A V-like depression in the margin or edge of a flat area.
• Process: A raised area or projection.
• Ramus (pl. rami): Curved portion of bone.
• Fovea (pl. foveae): A cup-like depression or pit in a bone.
• Sinus: Cavity within a bone.
• Sulcus (pl. sulci): A groove, crevice, or furrow to accommodate a nerve, tendon, or blood vessel.
• Linea aspera: A prominent ridge, crest, or roughened surface on the middle third of the posterior side of the femur to which muscles and the inter-muscular septa attach.

Some bones, like those of the neck and back have what are called "processes." These serve as important connection points for muscles, tendons, and ligaments and, therefore, are important levers for soft tissues to articulate. There are two different basic types of processes:

• Spinal processes: Branches or slender projections of bone that divert upwards, slanting either upright, forwards or backwards. Seen on the top of the cervical, thoracic, lumbar, and sacral vertebrae.
• Transverse processes: Branches or slender projections of bone that divert out and laterally, slanting either straight outwards, forwards and/or backwards. Seen on the sides of the cervical, thoracic, lumbar, and sacral vertebrae.

Joints

When two or more bones join together, they form a joint. Joints are the mechanisms of movement; without them motion would be impossible. As such, joints may be immovable, slightly moveable, or very moveable depending on their structure and fleshy connections. Joints also absorb shock and serve as connectors and linkages. The equine has lots of flexible joints, varying in degrees of motility.

Each joint's location and parameters of motion are unique, and collectively define equine motion. Yet no joint in the equine operates independently, or in other words, all joints involve the movement of other joints or affect another portion of the body. The jaw joint is the only one with the least amount of influence in this way. Therefore, it's important to understand the location and nature of the joints to understand equine biomechanics because they act as a series of symbiotic systems that also work as a symbiotic whole.  

Basic terms related to joints:

• Male surface: Convex surface of a joint.
• Female surface: Concave surface of a joint.
• Simple joint: Having two articulating surfaces; male and female.
• Compound joint: Having more than two articulating surfaces.
• Degrees of freedom: The number of axes in which the bone in a joint can move.

Joints are categorized in two basic ways, by their design and by their motion. Basic design of joints:

• Ovoid: When one oval, or egg-shaped male surface fits into another oval female surface, permitting movements in two planes allowing flexion, extension, abduction, adduction, and circumduction. Also called a condyloid joint, or ellipsoidal joint, or bicondylar joint. Example: Joint between the Scapula and Humerus.
• Saddle-shaped: Convex in one plane and concave in the perpendicular plane, with a saddle-like articular surface. Allows motion on the sagittal and frontal planes. Example: Pastern and coffin joints.
• Ball and Socket: When a bony ball fits into a cup-like socket of another bone. Although motion is restricted by the nature of the ball and cup-like receptacle, these tend to provide the greatest range of movement. Also called a circumduction joint or spheroidal joint. Example: Joint between the pelvis and the femur.
• Gliding: When an even surface of two bones glide along each other, allowing for mobility in one or two directions without any circular motion. Example: patella.
• Hinge: The most simple form of joint, it produces only flexion and extension. Also called a planar joint. Example: elbow.
• Pivot: When a round or conical part of a bone fits into a ring formed by a bone or tendon, permitting rotation only. Example: The joint between the Atlas and Axis vertebrae. 
• Bicondylar: Biaxial joint with a degree off motion that differs within each axis because of asymmetrical formations, but primarily moves in one plane. Example: stifle.
• Synarcosis: A joint that’s formed by muscle attachment and multiaxial. Example: shoulder sling.

Basic types of joint motion:

• Uniaxial joints: Motion in one plane. 
• Biaxial joints: Motion in two planes. 
• Multiaxial joints: Motion in three planes. 
• Chord: The shortest path between two points within a joint.
• Arc: The longest path between to points within a joint.
• Angular motion: Makes bones closer or farther apart from each other. There are four types: "Flexion" which is closing the angles of a joint, "extension" which is opening the angles of a joint, "adduction" which is movement towards the median of the body, and "abduction" which is movement away from the median of the body.
• Rotation: Spinning of the bone in the joint on a longitudinal axis. The revolving of a bone around an axis. Two types of rotational motion in the horse are "internal rotation" and "external rotation." Internal rotation is towards the axis of the body whereas external rotation is away from the axis of the body. When rotation is independent of other movements it is called "adjunct rotation" or when rotation is dependent on other movements it is called "conjunct rotation." Rotation most often concerns the limbs, spine, and neck of the horse.
• Co-spin: When the effect of adjunct rotation adds to the normal spin.
• Anti-spin: When the effect of adjunct rotation nullifies the normal spin.
• Translational motion: A gliding motion as one articular surface slides over another. It often serves as a shock absorber such as with the shoulder or hock joint.
• Circumduction: Making conical circles with a long bone in the joint.
• Pure swing: Having no spin, traces the path of a chord; also called a Cardinal swing.
• Impure swing: Having a spin, traces the path of an arc; also called an Arcuate swing.
• Flexion: The bending of a joint or series of joints.
• Extension: The straightening of a joint or series of joints. 

[Note: Flexion and extension dovetail into the terms flexor and extensor; flexor refers to flesh that creates flexion of a joint (decrease the angle of the joint) and extensor refers to flesh that straightens the joint (increase the angle of the joint).]

Movement

There are some basic concepts to understand regarding movement such as...

General

• Center of Gravity: The horse’s center of gravity is located approximately near the center of the ribcage, behind the girth area. It's estimated that the horse carries approximately 60% of his weight on the forehand.
• Logic of Design: Relates to the biological "rationale" behind the evolution of the animal's body to adapt to his lifestyle and habitat. For this reason, the equine is considered a prey animal so his logic of design is constructed for the flight response.
• Base of gait: Refers to how far apart the horse’s feet are, as determined by front or rear inspection. The base of gait is dependent on an animal’s individual conformation; a wide-chested horse will have a wide base of gait and a narrow-chested horse will have a narrow base of gait. Base of gait is also dependent on motion; a rearing horse will have a wide base of gait in the hindlegs for stability and speed tends to narrow the base of gait, bringing the legs towards the median plane. 
• Angle of gait: Refers to the angle of the legs in relation to the medial plane. Depending on speed, the equine should have an angle of gait in the forelegs and the hindleg, from the stifle down, towards the median at impact. The faster the gait, the more towards the median the leg tends to impact as a function of natural coordination.
• Action: The style of motion, characteristically an expression of his build or mood.
• Cursorial: Another term for "running," as in "cursorial lifestyle."

Phase of Gait 

• Landing or Impact Phase: The support phase for the body as the foot lands and the leg begins to accept weight. This creates the sound characteristically heard as a “beat"...
• Loading Phase: When the body moves over the foot, pressing weight, and force of motion down through the limb. This is usually the moment of maximum flexion of the foot joints. Each loading phase is a "beat."
• Stance Phase: The foot bones spring up to comparable angulations when at rest as the body moves in front of the foot; usually the most stressful point for the structures in the foot.
• Heel Lift Phase: The moment when the heel lifts as the limb is about to be flexed and the toe begins its pivot for the swing phase.
• Breakover Phase: When the toe is in full breakover and the limb is being actively flexed.
• Suspension or Swing Phase: When the leg is flexed and is lifted off the ground to swing forward and extend in preparation for the next Landing Phase. Suspension should count as a beat and it’s also the only time when the horse can change the placement of his foot, i.e. change his direction or gait. Also, suspension isn’t a rest period, as often misinterpreted. 
• Stride: The cycle of motion of one leg from the Landing through to the Swing Phase to the Landing Phase.
• Marching: A gait with a measured footfall with at least one foot always on the ground.
• Suspension: The moment in a stride when all four feet are off the ground.
• Diagonal: The synchronized stride of opposing pairs of feet (such as the right fore and left hind then the left fore and right hind at the trot).

Gaits (counting the suspension period)

• Walk: A four-beat symmetrical marching gait with no suspension. The spine’s rotary nature is showcased at the walk in a “rocking boat” type of motion in the saddle. The walk can become pacey if the horse is stiff.
• Trot: A four-beat symmetrical suspension gait with alternating diagonals; two beats and two suspension periods. The spinal motion of side to side is showcased but also having an up and down motion along with rotation motion.
• Gaited: The same footfall pattern as a walk, gaitedness is literally walking with more energy. Gaited footfalls can become pacey if the horse becomes stiff. 
• Pace: A lateral four-beat symmetrical suspension gait where the legs on each side move in unison, two beats and two suspension periods.
• Canter: A four-beat assymetrical suspension gait with one set of diagonals; three beats and one suspension period. The most energy efficient gait for the horse. The spine’s up and down flexion is characteristic of the canter along with coiling of the loins; the horse must coil his loins to canter.
• Gallop: A five-beat assymetrical suspension gait when the horse is at full speed and using all four feet to gain maximum speed; four beats and one suspension period. Horses that are able to deeply coil their loins and recapture collection with each stride make the fastest runners. 

Gait Terms

• Beat: Beat refers to the moment when a hoof hits the ground; it’s often heard and can be counted. The pattern of beats, or footfall, distinguishes a gait from others while the speed of the footfall distinguishes the speed of the gait. 
• Rhythm: The regularity of the beats, another way of describing the pattern of footfall or beats.
• Tempo: This term describes either the speed of the rhythm, whether being fast, medium or slow, relative to each individual horse or the speed of the gait. Also referred to as “rapidity."
• Cadence: The number of beats per minute or the clarity of rhythm in terms of the horse’s ability to produce a footfall pattern that’s definite, clear, and “confident." Also referred to as “regularity."
• Travel or Path: The path of the hoof flight as compared to the median and other limbs, as viewed from the front or rear; related to Angle of Gait.
• Directness: The straightness with which the limb travels forward, as viewed from the front; related to Angle of Gait.
• Height or Arc: The height of the arc of the stride when viewed from the side; how high the foot is lifted in motion.
• Placement: How the horse places his food during the Landing Phase.
• Overtrack or Tracking Up: How far the prints of the hindhoof over-shoot the prints of the forehoof.

Athleticism of Motion

• Impulsion: Impulsion derives its meaning from the German word relating to rocketry. Unlike the misconceptions of modern understanding of the word, because modern riding has largely been pirated by “push-pull” riding (or "frame" riding), the term doesn’t mean to go forward or even to go forwards faster; it doesn’t mean forwards thrust. Instead, it means upwards thrust, to go up. Therefore, impulsion relates to the horse’s ability to bounce or be springy upwards, to be light on the forehand, a direct result of bascule or roundness. 
• Engagement: Refers to how deeply and completely the horse is using his hindquarter via the coiling of his loins, sometimes gauged by observing the flexion of the hindlegs. This is closely related to impulsion and can sometimes be regarded as the same thing.
• Power: The forces that are propelling, thrusting, or balancing the horse.
• Suppleness: The controlled and athletic flexibility of the horse, a positive influence on motion; a stiff horse has an inferior quality of motion.
• Lightness: Spring and “float” of the forehand in motion; directly caused by bascule so that base of his neck is higher or of equal distance from the ground as his coiled loins.
• Freedom of Movement: How far the horse can reach with his forelegs and hindlegs, in all directions, during motion. This permits fluid athletic motion and is a product of impulsion, suppleness, and bascule.
• Energy of Gait: The “oompf” or energy level of the strides. In the trot, this can sometimes lead to a slight disconnect of the timing in the landing phase of the fore and hind hooves, which is a natural outcome; it’s not a fault or indication of inferior gaits or fitness. Some biomechanical or dressage text will argue otherwise, but this is because they’re taking a “robotic” regional approach rather than a “living” systems approach, losing sight of the living nature of equine motion. In essence, they’re a bit misguided on how the horse really works as a whole, especially in bascule. Energy of Gait also relates to gaitedness, which by sharing the same footfall pattern as walking, can simply be considered as walking with more “brio."

Collection

• Straightness: When the sternum is centered between the horse’s elbows and is key for achieving bascule. Roundness is a by-product of straightness, not visa versa.
• Crookedness: When the horse isn’t centered on his sternum and habitually leans to one side in motion. Leaners often have a flared hoof on the favored, leaned-to side with a clubfoot on the other. A leaner's head and neck will also veer towards their favored side, particularly during a turn. A crooked horse cannot achieve bascule or quality gaits because he doesn't move “plumb” to his build.
• Symmetrical Motion: Motion that is evenly balanced, placed, timed, and weighted; a characteristic of straightness.
• Assymetrical Motion: Motion that is unevenly placed, timed, and weighted; a characteristic of crookedness. Also referred to as “being off” without clinical signs of injury. Lameness is a clear example of asymmetry of motion (one step long and one step short).  
• Bascule, Self-Carriage, Carriage, Collection, or Roundness: A particular posture of the vertebral chain that reinstates its original anatomical arch, letting the horse carry a rider with agility, lightness, and athleticism without longterm pathology. To achieve bascule, the horse must attain certain things. First, he must coil his loins by flexing the LS-joint. This is also called “flexion of the loins," “engagement of the the hindquarter," “engaging the hocks," or to “break down in the back.” Second, bascule is continued when loin coiling lifts the center of his back, a nearly simultaneous reaction. And third, bascule follows through when he raises the base of his neck, arching his entire cervical vertebrae and dropping his head at the poll. His tail also becomes arched in a passive response to the use of his spine, called a "rainbow tail." All quality ridden motion originates in bascule.
• Hollow, Stiff, Above the Bit, or Inverted Frame: The opposite of bascule, indicative of a tight, stiff back with a bracing stiff neck and poll. He often carries his head high because the base of his neck is very low due to uncoiled loins. He’s also usually hurried, anxious, in pain and therefore difficult to ride both physically and emotionally. This type of frame is often pathological and leads to permanent physical damage. All poor quality ridden motion originates with an inverted frame. "False collection" is created by an inverted frame.
• On the Forehand: A horse that lacks self-carriage, leaning onto his forehand to pull himself along rather than carrying himself with his hindquarter to push himself along. His head is often lowered with a low base of neck and his hindlegs seem to trail behind. This can lead to pathologies, even those of the forelimb.
• Rollkur: When the horse's head is pulled beyond the vertical, often touching his chest. Erroneously believed to create roundness, this misguided practice causes physical and psychological harm to the horse. Unfortunately, however, it's ubiquitous in modern dressage training.
• Behind the Bit or False Collection: Typical with rollkur, this is a posture that looks like collection to the unawares, but it's really a bad posture for the horse that hurts him physically and psychologically. It's basically an Inverted Frame, only the head is cranked in beyond the vertical, nose to chest.
• “Broken Neck Syndrome”: Flexion at C2-C3 rather that a lifting of the base of the neck and flexion at the poll. Also referred to as “Double Hinge Neck." Typical of rollkur or "push-pull" or "frame" riding.
• False Extension: An extended gait in the Inverted Frame. At the trot, for example, the horse will typically flick up his front hooves at the end of protraction and bend at the knee before breakover at retraction, trailing with his hindlegs. He may also be pacey at the walk.

Extension vs Collection

• Collected: When bascule is recovered at every stride, allowing the up and down occilations of the spine to be neutral, with a high roundness, and slower and higher steps.
• Working Gait: A mild version of deep collection; a more “working” everyday version of high school degrees of collection. In short, having a bit more up and down occilations of the spine and recovering bascule at every stride.
• Medium Gait: A more extended version of collection, but not in full extension; having a back that occiliates somewhat more in the entire range of motion and recovering bascule at every stride.
• Extended Gait: A fully extended mode of motion in which the back oscillates through the entire range of motion from low to high with each stride. The risk is that bascule may not be recovered with each stride, commonly leading to false extension. 

Modes of Balance:

• Lateral Balance: Side-to-side balance or agility manifested at motion. Bascule facilitates maximum lateral balance by creating lightness which frees the forehand to reach out to change direction or maintain balance. Also, the more engaged the horse’s inside hindleg, the more agile he is in a turn.
• Longitudinal Flexion: Front-to-back balance, another term for collection, see below. This gives the horse coiling power and stored energy, enhancing athleticism and the quality of his gaits. 
• Lateral Flexion or Bending: Despite what many believe, the horse cannot bend in a smooth, even curve, but in a series of kinks. In reality, the horse’s spine isn’t built for lateral flexion, but for coiling. Nonetheless, with the small degree of lateral flexion it can produce, it can greatly enhance the quality of motion. Specifically, while the joint between the skull and the 1st cervical vertebra may only be able to do a "yes" motion, there's a minor degree of lateral slippage when the head is tucked. This is referred to as "twirling of the head." Then the rest of the neck from C4-C7 has a great deal of lateral flexion. In turn, the thoracic vertebrae has the greatest degree of lateral bend before the T13 while the lumbar have barely any and the sacrum has none. The tail has a great deal of lateral bend. The horse also pushes his ribcage outward, "banking into the turn" or "riding the rim" of a lateral bend, stepping deeply under himself with his inside hindleg and “following through” with his poll. Maximum lateral flexion can only be achieved with bascule which allows the spine to assume more flexible postures. Bascule also allows him to bring his inside hindleg under himself more, enhancing his ability to turn.

Colloquialisms:

• Headset: How the horse carries his head and neck in motion. Often misinterpreted to be the originator of bascule (“If I could get his head down then he’ll be collected”), which is a common fallacy. Headset is a result of bascule and not visa-versa. Also, the elevation of the poll isn’t relevant to a “correct” head set, only the raising of the base of the neck. Indeed, the rider can “put the poll” wherever she wants as long as the base of the neck remains raised.
• On the Bit or On the Bridle: Originally a metaphor for moving in collection it’s now commonly misinterpreted as having constant contact with the bit (typically with tension on both reins), a common culprit in “push-pull” riding. There’s no need for a bit to achieve collection nor is collection maintained by pulling on the reins to maintain "contact." Indeed, a horse can maintain self-carriage even on a loose rein.
• Frame: This term was originally a metaphor for how an individual horse was moving according to his build, whether in a collected frame, extended frame, etc. However, the term today has been corrupted to facilitate "push-pull" riding and justify ideas about "suitability." 
• Elastic Ride: The springiness of the strides as a result of lightness. However, the elasticity of a ride is often attributed to the suspensory mechanisms of the horse’s legs or the slope of his pasterns, neither of which is correct. His suspensory mechanisms weren’t designed that way, plus many horses with steeply sloped pasterns have produced wonderful elastic rides. In fact, elasticity is a product of the suppleness and relaxation of the Shoulder Sling as a result of proper schooling. 
• Suitability: The idea that some conformational builds are "suitable" for certain disciplines, most notably dressage. The truth is, there's no such thing as suitability. All horses can do all horse motions. Granted, some conformational types do certain motions with a certain style, but skilled horsemanship can take even a cart horse to haute ecole or Grand Prix level. 
• Arched Tail: Sometimes called a “Rainbow Tail," this term refers to the desireable arch to the tail bones as a passive response to bascule.
• Flagged Tail: When the tailbone is held upright or curled over, which often happens when the animal is excited.
• Schwung: A German word describing the swinging motion of the ribcage that's caused by the oscillations of the spine in bascule. A loss of Schwung creates a pacey, hurried, unbalanced, or stiff gait due to a stiff spine.
• Spiritual Movement: Movement that's joyful and enthusiastic due to proper bascule.
• Mechanical Movement: Movement that's uninspired and forced as a result of false collection.

Conclusion to Part II

It's a lot to digest, isn't it? We deal with a lot of horse terms every day, and they're used to describe any number of aspects about our subject. So familiarize yourself with them then, and next time we'll discuss tissues. Then after that, we'll travel back in time to see what makes a horse a horse—and why. 

Now one might wonder why we need a discussion about evolution for a discussion about equine anatomy and biomechanics, and the truth is: it's all about context. It's not enough to know how the equine is built and moves, but perhaps even more importantly, we need to know the why. Once we understand that, the natural logic of his body prevents us from exaggerating type or movement to unhealthy degrees we so often see in life or in art so we can portray the subject responsibly and compassionately.

So until next time...tease yourself with terminology!

"Nature puts leaves on trees and they are, none of them, exactly alike; like people, endless variations on a theme. In that sense, nature does the art thing better than we do."

~ Eva Kosinski

Equine Anatomy and Biomechanics: A Primer of Equine Engineering Part II, Terminology