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Chapter 6 Biomechanics of movement.

Movement of a horse requires the complex integration of several physiological systems. The bones and joints together compose a complex system of levers and pulleys that, combined with the muscular system, imparts the power of motion to the body. Nerves and sensory organs control the movement. Movement is affected by a horse's conformation, or structure.

OBJECTIVES

After completing this chapter, you should be able to:

* Describe muscle contraction

* Describe the nervous control of muscle contraction

* List four functional groups of muscles

* Explain why the heat generated by muscular contraction affects performance

* Contrast aerobic to anaerobic metabolism during muscular contraction

* Name three types of muscle fibers and identify their function

* Name three extensor and three flexor muscles on the hind and front leg

* Describe the two phases of a stride

* Name three factors of a gait that determine a horse's speed

* Define height, directness, spring, regularity, and balance as they relate to gaits

* Describe the walk, trot, gallop, rack, and canter

* Explain the role of conformation in the movement or performance of a horse

* List and describe six common defects in a horse's way of going

* Describe how the center of gravity may affect the movement of a horse

KEY TERMS

abductors

adductors

afferent

amble

balance

beat

center of gravity

collected

cross-firing

directness

dwelling

efferent

forging

gait

interfering

neurotransmitter

pace

paddling

pointing

pounding

proprioceptors

reflexes

rolling

scalping

speedy-cutting

spring

step

stepping pace

stride

stride stance

stride suspension

swing

trappy

way of going

winding

winging outward

NERVOUS SYSTEM CONTROL

A walk, trot, gallop, or any other gait requires the simultaneous contraction and relaxation of muscles. Muscular contraction is a complex interaction of many parts of the nervous system and the muscular system. As Figure 6-1 shows, muscle action starts in the brain, where information received through a variety of sensory inputs is processed. For example, the eyes may sense a jump, the ears hear a cluck, or the sides feel a nudge from the rider's heels. The horse's brain interprets this information along with internal sensory organs like the joint proprioceptors, which give the horse a sense of the positions of its limbs. Next, the brain determines the appropriate muscles to contract or relax. This information is sent down the spinal cord and then to efferent nerves that end on muscle cells. The muscle contracts or relaxes, and the bone and joint respond to produce the action.

After this, the cycle starts again. The afferent nerves send information from the joint proprioceptors back to the brain. As before, the sensory information to the brain is interpreted and another signal is sent back down the spinal cord and efferent nerves to the muscle producing movement in the bone and joint. Of course, this process occurs many times and very rapidly for every movement.

[FIGURE 6-1 OMITTED]

Some signals traveling on the afferent nerves never reach the brain. Instead they go directly to the spinal cord and then back to the efferent nerves and the muscles. These signals are called reflexes. An example is a kick in response to a surprise or a twitch of the skin in response to an insect.

HOW MUSCLES CONTRACT

Figure 6-2 illustrates how muscles are organized starting with the muscle and moving to the muscle bundles, the muscle fibers, and finally the myofibrils. Muscular contraction occurs at the myofibril level.

Figure 6-2 also shows a muscle filament, or myofibril, in cross section at various states of contraction. Each muscle is made up of thousands of these filaments. Contraction or relaxation is controlled by the nervous impulses received by the muscle cells.

When a muscle contracts, a neurotransmitter called acetylcholine (ACH) excites the muscle cells. This causes the release of calcium ions, which bind to a special protein called troponin. In turn, two other muscle proteins, actin and myosin, are free to bind to each other and form bridges. This causes the muscle to contract. When the calcium concentration drops and the muscle is no longer excited by ACH, actin and myosin no longer bind, and the muscle relaxes (Figure 6-2).

The force a muscle can generate is a function of three factors:

1. Short periods of stimulation causing contraction

2. Length of the muscle when stimulated to contract

3. Number of actin and myosin filaments acting

Short periods of stimulation that cause contraction provide for more calcium to be present in the muscle, thus taking longer to overcome elastic elements. If the stimulus is only of short duration, some of the force is taken up in overcoming the elastic elements of the tendons and other connective tissue.

[FIGURE 6-2 OMITTED]

Muscle force is also a function of the number of actin and myosin bridges formed. Muscle length alters the relationship between actin and myosin. If the muscle is stretched, the number of bridges decreases. When the muscle is stimulated to contract, the amount of force produced is decreased.

Actin and myosin molecules in the muscle fiber also affect the number of bridges. These filaments align in a parallel formation. The greater the number of filaments that are parallel, the greater the force of contraction. This occurs as either recruitment of more muscle fibers or by actually increasing the number of filaments in cross section. In the latter case, the increased force requirements of an exercise regime produce hypertrophy, an increase in the number of filaments in cross section. The horse will appear "bulked up" or will show larger muscles.

Muscle contraction requires energy. This energy is derived from metabolic processes that produce adenosine triphosphate (ATP). ATP is produced from fats, carbohydrates such as glucose or glycogen, and protein. Oxygen from respiration (breathing) is required to produce ATP. As long as sufficient oxygen is available to produce ATP, muscle contraction is called aerobic. When muscle contraction is of such high intensity or long duration that adequate oxygen is not available, the products of metabolism are converted to lactic acid to produce ATP. This type of muscular work is called anaerobic. Exercise and training can alter the efficiency of the muscles by increasing the animal's ability to deliver oxygen to the tissues.

Figure 6-3 presents simplified chemical reactions for aerobic metabolism and anaerobic metabolism. In the horse's body, each of these reactions involves numerous series of reactions, all linked together and catalyzed by enzymes.

MUSCLE FIBERS

Muscle fibers require nutrients to contract. Different energy sources can be used by horses performing different types of activity, depending on the type of muscle fiber involved in the activity. Three different muscle fiber types are associated with the athletic horse:

* Type I (slow-twitch fibers, aerobic)

* Type IIa (fast-twitch fibers, aerobic)

* Type IIb (fast-twitch fibers, anaerobic)

Type I fibers are in use during relatively slow or light activity and use carbohydrates, fat, or protein. Type IIa fibers are the stamina or endurance fibers used during periods of aerobic work such as jogging or long-distance riding. These fibers can use carbohydrates, fat, or protein for energy. Type IIb fibers are the speed or power fibers used for periods of strenuous anaerobic work such as sprinting, jumping, or cutting. These fibers use carbohydrates only.

For example, the quarter horse is born with a relatively large proportion of type IIb fibers and does best on a diet of carbohydrates--hay and grain. The endurance horse, such as the Arab, is born with a higher proportion of type I and IIa fibers and does best on a diet of both carbohydrates and fat--hay, grain, and oil.

[FIGURE 6-3 OMITTED]

FATIGUE OF MUSCLES

Fatigue of muscles follows continued work, principally due to the accumulation of waste products in the muscle cells. Recovery requires removal of the accumulated waste products by the blood and lymph, and a fresh supply of nutrition brought to the muscles. Hand-rubbing the legs of a horse after exercise stimulates the blood and lymph vessels in the removal of waste products. It also causes the blood to circulate more freely. Fatigue may also be overcome in part by feeding easily digested carbohydrates for a maximum of energy.

An untrained horse, one not accustomed to steady work, fatigues more easily than a trained horse, mainly because the muscles, respiration, and circulation do not operate as efficiently. There is a limit to the amount of continued muscular effort a horse can expend; harmful fatigue can be avoided by working the horse at a moderate rate in order to maintain the proper balance between the products of muscular activity and the ability of the blood to remove waste material. An animal should never be worked until exhausted.

HEAT

Heat is a by-product of muscle contraction. To prevent an excessive increase in core body temperature, heat must be dissipated. In the horse, heat is dissipated through sweating (evaporation) and by air movement across the body. Blood transports heat from the working muscles and the core to the skin, where it is cooled.

During exercise in hot environments, the need to control body temperature causes a large shift in blood flow to the skin. This may adversely affect the exercising horse by decreasing the blood flow to the muscles. Fluid losses during exercise in hot environments can also significantly decrease plasma (blood) volume. This too may negatively impact the horse, making it harder to maintain adequate blood flow to the muscles. Finally, exercise in hot environments increases the amount of electrolytes lost in the sweat. These electrolytes are important for fluid balance, acid-base balance, muscle contraction, and nerve function.

MUSCLES INVOLVED IN GAITS

Muscles that the horse uses to execute the various gaits form four functional groups (Figure 6-4):

1. Flexors

2. Extensors

3. Abductors

4. Adductors

[FIGURE 6-4 OMITTED]

Contraction and relaxation of these groups in the limbs and the attachment of the limbs to the body create the horse's gaits and other movements. Flexors decrease the angle of a joint, while extensors increase the angle of a joint. Abductors move a limb away from the center plane of the horse; adductors pull a limb toward the center plane of the horse. Extensor muscles of the front leg include:

* Brachiocephalicus

* Supraspinatus

* Triceps brachii

* Oblique carpal extensor

* Lateral digital extensor

* Common digital extensor

Flexors of the front leg include:

* Teres major

* Latissimus dorsi

* Biceps brachii

* Flexor carpi radialis

* Flexor carpi ulnaris

* Deep digital flexor

Adductors of the front legs are the pectoral muscles. The abductor of the front leg is the deltoid.

On the hind leg, the extensors include:

* Biceps femoris

* Semitendinosus

* Semimembranosus

* Gluteus medius

* Quadriceps femoris

* Gastrocnemius

* Long digital extensor

* Lateral digital extensor

Flexors of the hind leg include:

* Iliacus

* Popliteus

* Deep digital flexor

* Superficial digital flexor

For a review of the muscular system, see Chapter 5.

GAITS AND ACTION

A gait may be defined as a horse's way of going or the way of moving its legs during progression. The horse is more versatile in selecting gaits than is any other four-legged animal, and it uses several gaits unique to the species. A gait is characterized by distinctive features, regularly executed. Action refers to flexion of the knees and hocks, the height the horse lifts his feet from the ground, the speed or rate of movement, and the length of the stride.

An understanding of gaits is important to detect lameness, to train a performance horse, or to signal a horse for a specific gait. Some gaits of a horse are natural, while others are learned or artificial. Most horses must be trained to execute the artificial gaits.

When describing the various gaits, a beat refers to the time when a foot--or two feet simultaneously--strikes the ground. Beats may or may not be evenly spaced in time. A step is the distance between imprints of the two front legs or the two back legs. A stride is the distance between successive imprints of the same foot.

Components of a Stride

The stride has two phases--stride stance and stride suspension. Stride stance is the weightbearing phase, while the stride suspension or swing is the non-weight-bearing phase. The speed of a horse is determined by:

* Length of stride

* Rapidity or frequency of stride

* Overlap time or the time on the ground versus time off the ground

For example, the famous racehorse Secretariat ran faster because he spent less time with his legs in the stance and overlap phases. In other words, Secretariat's legs completed their ground contact quicker, and more time was spent in the airborne (suspension) phase. Other terms used to describe a horse's gait include:

* Directness, or trueness, which is the line in which the foot is carried forward during the stride. A horse that paddles does not carry its feet straightforward during the stride (Figure 6-5).

* Power or the pulling force exerted to create the stride.

* Height, which is indicated by the radius of the arc created from the point of the foot's takeoff to the point of the foot's contact again with the ground (Figure 6-6).

* Spring, or the manner in which weight settles back on the supporting leg at the completion of the stride.

* Regularity, or the rhythmic precision of each stride.

* Balance, which is the ability of a horse to coordinate action, go composed, and remain in form.

[FIGURE 6-5 OMITTED]

[FIGURE 6-6 OMITTED]

Common Gaits

Historically, six gaits were considered natural for the horse--walk, trot, pace, canter (or lope), run, and back. Now all horses are considered to have four natural gaits--walk, trot, canter, and gallop (or run). Any gait that a horse executes without training is natural. Some common gaits are described briefly in this section (Figure 6-7).

Walk. This is a slow, even, four-beat gait. The sequence of hoofbeats for the walk is (1) left hind, (2) left fore, (3) right hind, and (4) right fore. This sequence of beats is considered lateral because both feet on one side strike the ground before the feet on the opposite side strike the ground (Figure 6-7).

[FIGURE 6-7 OMITTED]
Horses Had Role in Development of Moving Pictures

Because of his fame, his success at publicizing
his activities, and his habit of patenting
machines before actually inventing them,
Thomas Edison received most of the credit for
inventing the motion picture. As early as 1887,
he patented a motion picture camera, even
though it could not produce images.

Actually, many inventors contributed to the
development of moving pictures, and horses
helped too. Perhaps the first important contribution
was the series of motion photographs made
by Eadweard Muybridge between 1872 and
1877. He was hired by the governor of
California, Leland Stanford, to capture on film
the movement of a racehorse. Stanford had bet
someone $25,000 that when a horse is at a fast
trot, all four of its feet are off the ground. To
prove his point, he hired Muybridge to make a
photographic study documenting animal
motion. At an elaborately designed experiment
station on Stanford's farm (later site of Stanford
University), Muybridge set up a series of
stereoscopic cameras.

Muybridge tied a series of wires across the track,
connecting each wire to the shutter of a still
camera. The running horse tripped the wires and
exposed a series of still photographs. Muybridge
mounted the photos on a stroboscopic disk and
projected them with a magic lantern to reproduce
an image of the horse in motion. Stanford won
the bet. Muybridge continued his research into
various forms of animal locomotion, from crawling
infants to elephants. For more information about
Eadweard Muybridge and to view his photos of
a horse in action go to http://en.wikipedia.org/
wiki/Eadweard_Muybridge.

[ILLUSTRATION OMITTED]


Trot. The trot is a two-beat gait with the diagonal fore and hind legs acting together (Figure 6-8). A period of suspension in which all four feet are off the ground occurs between each beat. The road horse trot is a fast-stepping trot characterized by length and rapidity and executed with extreme degree of extension, or length of stride. Heavy harness trot and hackney trot are high-stepping gaits with a high and springy stride, very collected (controlled), and executed with each step showing extreme flexion and precision (Figure 6-7).

Canter. The canter or lope is a three-beat collected gait. The sequence of beats is (1) the right rear hoof, (2) left rear and right front hoofs striking simultaneously, and (3) the left front hoof (Figure 6-7). When cantering, the horse carries more weight on its haunches, or rear quarter. The gait is executed in a slow, animated, collected, rhythmic way in which the lead changes on command. If moving to the left, the horse should lead with its left leg, and vice versa. If a horse is cantering to the right and leading with its left foot, the horse is exhibiting what is known as a counter canter.

[FIGURE 6-8 OMITTED]

Gallop. The gallop or run is a fast, four-beat gait (Figure 6-7). One hind foot makes the first beat, followed by the other hind foot. The diagonal forefoot is the third beat, and the remaining forefoot is the fourth. A period of suspension follows the four beats. If the horse changes leads, it will do so in the period of suspension. The run is the gait of a racehorse.

Pace. This is a two-beat, lateral (side-to-side) gait with the fore and hind legs on the same side moving together (Figure 6-7). A period of suspension occurs between each beat. Since the horse is shifting its weight from side to side, the gait has a rolling motion. It requires a smooth, hard footing and a minimum of draft. Trotting downhill will cause some trotters to pace; pacing uphill will cause some pacers to trot. The pace is a speed gait. The amble is a lateral gait distinguished from the pace by being slower and more broken in cadence. It is not a show gait.

Slow Gait. The slow gait or stepping pace is a show gait. This is a lateral, four-beat gait done under restraint in showy, animated fashion with the forefoot on the right followed by the hind foot on the right. In the stepping pace, the break in rhythm is between the lateral fore and rear foot.

Rack. The rack is an even, fast, flashy, four-beat lateral gait. It is sometimes called a single foot and is characterized by quite a display of knee action and speed. The rack is hard on the horse but easy on the rider. Its excessive leg movement increases the amount of concussion and trauma to the forelegs.

Running Walk. The running walk is the fast walk of the Tennessee walking horse. It is faster than the ordinary or flat-foot walk. It is a single-foot or four-beat lateral gait with a break in the impact or rhythm occurring between the lateral fore and hind feet. The horse travels with a gliding motion because it extends the hind leg forward to overstep or overreach the forefoot print.

Back. When a horse backs, it is actually trotting in reverse. Backing is a two-beat gait in which the diagonal pairs of legs work together.

CONFORMATION AND ACTION

Conformation, the form or structure of a horse, has a bearing on how well it functions or performs. While Figure 6-9 illustrates desired traits, irrespective of breed, this does not mean the illustration is a true representation of all breeds.

How a horse stands is indicative of how it will move. The normal stance, with width between the legs in proportion to the width of the chest, and feet placed straight, results in the legs and feet moving in a straight line. A base-wide horse, particularly if it also toes out, wings inward or moves its feet and legs in with each stride. If the condition is severe, the horse is apt to strike one leg with the other (interfering), resulting in injury and even unsoundness. Base-narrow, with toes pointing in, results in a horse that paddles. This is unsightly and results in excessive hoof wear on the outside quarters and excessive strain on the knee, fetlocks, and tendons.

If a horse stands straight, it is likely to move straight and true. If the legs are set properly, it is better able to move with collected action (Figure 6-10). A horse with crooked legs cannot move true. Regardless of a horse's excellent head, neck, shoulder, top, and general balance and conformation, if it is crooked on its legs, it is not a top horse.

Unsoundnesses in the pasterns, cannon bones, knees, and especially the hocks also affect movement.

[FIGURE 6-9 OMITTED]

[FIGURE 6-10 OMITTED]

The following conformation features affect action and gaits and may predispose an animal to certain unsoundnesses:

* A long forearm contributes to a long stride.

* Sloping shoulders and pasterns are associated with a springy stride. Straight shoulders are associated with a rough ride.

* A calf-kneed (back at the knees) posture is associated with hard concussion or a pounding gait; it predisposes a horse to bone chips.

* Low, rounding withers are associated with a defective gait called forging. A horse with low withers commonly hangs in the bridle, moves with its head low, and handles its front feet awkwardly.

* A pigeon-toed horse will paddle or wing out. Conversely, a splayfooted (heels in, toes out) horse will wing in, and the striding leg may actually strike the supporting leg. Further, its hooves will wear unevenly (see Figure 6-5).

* Short, steep ankles and pasterns result in a stilted stride, hard concussion, and a tendency to cocked ankles and unsoundness.

* Front legs out at the corner or legs set too far apart in front are a structural defect associated with a rolling motion when the horse moves.

* A short, thick, bulky neck too often goes with a straight shoulder and reduces neck suppleness and mobility and the rider's ease in controlling the horse.

* A short, straight shoulder and forearm, accompanied by steep pasterns, results in a short stride and a tendency toward sidebones.

* Buck knees and long toes cause stumbling.

For more information on conformation, refer to Chapters 7 and 8.

COMMON GAIT DEFECTS

How the horse moves its feet and/or legs while executing the gaits may involve defects. Some defects cause limb interference and may be severe enough to cause injury. Other defects are not serious, but they prevent top performance from the horse. Defects and peculiarities in the gait include forging, interfering, brushing, striking, paddling, winding, scalping, speedy-cutting, cross-firing, pointing, dwelling, trappy, pounding, and rolling. These defects can be related to conformation, injuries, or improper shoeing and trimming of the feet.

Forging

Forging is striking the end of the branches of the hoof or the undersurface of the shoe of the forefoot with the toe of the hind foot (Figure 6-11). This is the diagonal foot in pacers and the lateral foot in trotters.

Interfering

Interfering is striking the supporting leg, usually at the fetlock, with the foot of the striding leg. Interference commonly occurs between the supporting front leg and a striding front leg or between a supporting hind leg and a striding hind leg (Figure 6-12). Brushing is a slight interference. Striking is a severe interference resulting in an open wound.

Paddling

Paddling or winging outward is an outward deviation in the direction of the stride of the foreleg (Figure 6-13). It is the result of a narrow or pigeon-toed standing position. Winging outward is exaggerated paddling and very noticeable in high-stepping horses. Paddling almost always causes interference.

Winding

Winding is twisting the front leg around in front of the supporting leg as each stride is taken. Sometimes it is called threading, plaiting, or rope-walking. Wide-chested horses tend to walk in this manner. Winding increases the likelihood of interference and stumbling.

[FIGURE 6-11 OMITTED]

[FIGURE 6-12 OMITTED]

[FIGURE 6-13 OMITTED]

Scalping

Scalping occurs when the hind foot hits above or at the line of the hair (coronet) against the toe of a breaking-over (beginning the next stride) forefoot (Figure 6-14).

Speedy-Cutting

Speedy-cutting occurs when a trotter or pacer traveling at speed hits its hind leg above the scalping mark against the shoe of a breaking-over forefoot (Figure 6-14). In trotters, legs on the same side are involved. In pacers, diagonal legs are involved.

Several faults in conformation predispose a horse to scalping and speedy-cutting: short backs and long legs, leg weariness or hind legs set too far under the body, short front and long back legs, and toes too long on the forefeet.

Cross-Firing

Cross-firing is essentially the same as forging in a pacer in which the inside of the fore and hind foot strike in the air as the stride of the hind leg is about completed and the stride of the foreleg is just beginning (Figure 6-15).

[FIGURE 6-14 OMITTED]

[FIGURE 6-15 OMITTED]

Pointing

Pointing is a stride in which extension is more pronounced than flexion. A horse with a pointed stride breaks or folds its knees very slightly and is low-gaited in front. Thoroughbreds at the trot are pointy-gaited. The term pointing is also used to indicate the standing position pose a horse frequently takes when afflicted with navicular bone disease or injury to the foot or leg: It stands on three legs and points with the fourth.

Dwelling

Dwelling is a perceptible pause in the flight of the foot, as though the stride had been completed before the foot strikes the ground. It may occur either front or rear and is particularly common in heavy harness horses, heavy show ponies, and some saddlers.

Trappy

Trappy is a gait that is a short, quick, choppy stride. Horses with short and steep pasterns and straight shoulders tend to have a trappy gait.

Pounding

Pounding is heavy contact with the ground, usually accompanying a high, laboring stride. Faults in conformation that shift the horse's center of gravity forward tend to create pounding.

Rolling

Rolling describes excessive side-to-side shoulder motion. Horses wide between the forelegs and lacking muscle development in that area tend to roll their shoulders. The toe-narrow fault in conformation can also cause rolling.

CENTER OF GRAVITY

The center of gravity, where the mass of the horse is centered, is another important feature affecting the gait. Even though the center of gravity will vary with the horse's shape, it is most commonly located in the middle of the rib cage just caudal to the line separating the cranial and middle thirds of the body. Because the center of gravity is located more cranially, the forelimbs bear 60 to 65 percent of the body's weight. This puts increased stress on the forelimbs, resulting in an increased incidence of lameness in those limbs. The horse that is taller over the croup than in the withers has an additional disadvantage because this shifts its center of gravity even further forward. Young horses that are growing may be higher in the withers, but this will change as they develop.

SUMMARY

The nervous, muscular, and skeletal systems work together to produce movement. The nervous system gathers information about the internal and external environment and provides the stimuli causing muscle contraction. As muscles contract and others relax, they act on the joints and bones to produce movement. Muscle contraction requires oxygen and energy in the form of ATP. Muscle contraction produces waste products and heat, both of which must be removed from the muscles.

Muscles that produce the gaits in horses can be grouped according to their function--flexors, extensors, abductors, and adductors. All horses are capable of the four natural gaits: walk, trot, canter, and gallop. Other gaits require training and are said to be unnatural. Gaits can be described by the number of beats and the characteristics of the stride. Conformation can affect the gait, as can the horse's center of gravity. How the horse moves its feet and/or legs while executing the gaits may involve defects.

REVIEW

Success in any career requires knowledge. Test your knowledge of this chapter by answering these questions or solving these problems.

True or False

1. An adductor muscle decreases the angle of a joint.

2. Heat is a by-product of muscle contraction.

3. During a gallop, all four of a horse's feet are off of the ground at the same time.

4. A pigeon-toed horse will exhibit a trappy gait.

5. ATP is a neurotransmitter that excites muscle cells.

Short Answer

6. List four functional groups for muscles.

7. Contrast aerobic to anaerobic metabolism during muscle contraction.

8. List four natural gaits common to all horses.

9. What three factors of a horse's gait determine its speed?

10. Name three extensor and three flexor muscles on the hind leg.

11. Name the two phases of a stride.

12. Identify three types of muscle fibers.

Critical Thinking/Discussion

13. In relation to muscle contraction, why do working horses sweat?

14. Describe the sequence of events during muscle contraction.

15. How can conformation affect the movement or performance of a horse?

16. Describe six common defects in a horse's way of going.

17. Compare the walk, trot, canter, gallop, and rack.

18. Explain how the center of gravity affects the movement of a horse.

STUDENT ACTIVITIES

1. View videos of horses in motion showing different gaits. Show the video in slow motion and describe phases of the stride.

2. Attend a horse show or view a television broadcast of a horse show and learn to identify the gaits.

3. Using a drawing of the horse skeleton, draw in the muscles of the limbs and attach them to the proper location on the bones of the legs.

4. Research muscle contraction and make several drawings showing how the myofibril contracts. Specifically, show how ATP and calcium (Ca) are involved in contraction.

5. Learn about proprioceptors by closing your eyes or putting on a blindfold. Move your arms or legs to new positions and describe these without looking. Knowing where your arms and legs are in time and space without looking is proprioception.

6. Visit with a horse trainer and discuss how horses are trained to perform gaits.

7. Using numbered hoofprints for the left and right and front and hind legs, diagram the hooves on the ground through one complete cycle of a walk, trot, canter, and gallop.

8. Develop a rapid-fire game using the gaits and common defects matched to their descriptions.

ADDITIONAL RESOURCES

Books

American Youth Horse Council. (2004). Horse industry handbook: A guide to equine care and management. Lexington, KY: Author.

Back, W., & Clayton, H., Eds. (2000). Equine locomotion. Philadelphia, PA: Saunders.

Evans, J. W. (2000). Horses: A guide to selection, care, and enjoyment (3rd ed.). New York: Owl Books.

Frandson, R. D., Fails, A. D., & Wilke, W. L. (2003). Anatomy and physiology of farm animals (6th ed.). Philadelphia: Lippincott Williams & Wilkins.

McCracken, T. O., & Kainer, R. A. (1998). The coloring atlas of horse anatomy. Loveland, CO: Alpine Publications.

Pilliner, S., & Elmhurst S. (2002). The horse in motion: The anatomy and physiology of equine locomotion. Oxford, UK: Blackwell.

Internet

Internet sites represent a vast resource of information, but remember that the URLs (uniform resource locator) for World Wide Web sites can change without notice. Using one of the search engines on the Internet such as Yahoo!, Google, or About.com, find more information by searching for these words or phrases:

biomechanics

nervous system

muscle contraction/

relaxation

neurotransmitter

acetylcholine

metabolism

anaerobic

aerobic

muscle fibers

muscle fatigue

gait of a horse

horse conformation

Table A-18 in the appendix also provides a listing of some useful Internet sites that can serve as a starting point for further exploration.
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Author:Parker, Rick
Publication:Equine Science, 3rd ed.
Date:Jan 1, 2008
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