The Axial Skeleton

THE FUNCTIONAL ANATOMY of the neck and back of the horse (the axial skeleton) is something that is now attracting a good deal of interest and research. Gone (hopefully) are the days when we simply labeled the horse as ‘cold backed’ if he dipped or put his back up when putting on a saddle or mounting. We now understand that cold backed simply means pain, which needs to be addressed by your veterinary surgeon and your physiotherapist.

We also know that what we used to term ‘bridle lame’ is also a pain response to being ridden, and that pathology in the neck and back can cause lameness that may manifest in another area of the body, and often cause behavioural changes. When you have neck or back pain, the last thing you want to do is exercise and flex those areas of your body. And yet some riders and ill-informed instructors will still put these manifestations of pain down to resistance or bad temperament, and will attempt to over-use spurs and/or whips on the horse to ‘adjust his attitude’. Remember that pain is one of the three most common reasons for your horse not doing what you want him to do, so causing him more pain with whips and spurs is unlikely to help the situation. And horses that are normally biddable do not suddenly develop a bad attitude.

If pain is manifesting itself as limb lameness, then it is very often the case that the rider or trainer can see the necessity for treatment that may include rest. So it still remains a mystery why they will very often not accept that their horse is displaying signs of pain in the neck and back which similarly need treatment and rest. In the experience of most vets and physiotherapists this is simply because of a lack of knowledge by the owner or trainer, rather than unkindness. Also, in Chapter 1 it was explained that, as a prey animal, horses will disguise pain until it becomes impossible to continue, and some very stoic horses can disguise pain in the neck and back for a long time before it becomes manifest.

In this chapter we explain the way the back is put together and how it should work because, if it goes wrong, then the whole horse becomes wrong and without understanding the system you could be causing immeasurable discomfort and athletic impairment.

Structure

The equine vertebral column follows the same general structure as all veterinary mammals. There are cervical (neck) vertebrae, thoracic (those to which ribs are connected) vertebrae, lumbar (lower back) vertebrae, sacral (base of spine) vertebrae and caudal (tail) vertebrae (Figure 4.1). Abbreviations for these vertebrae are: cervical – C; thoracic – T; lumbar – L; sacral – S; caudal – CD.

In the horse there are 7 cervical, 18 thoracic, 6 lumbar and 5 sacral vertebrae, and they have functional mechanics that are specific to the horse as a prey animal. It is this structure of the axial skeleton that allows us to ride the horse. If his back moved in the same way as a cat or dog (a predator), then we would not be able to use him for weight carrying. You only have to look at the differences between the flexibility of the spine between a dog and a horse when they are in full flight (Photo 4.2).

There is surprisingly little movement in the joints between the vertebrae in the horse. Much of the movement in any direction comes from the neck, and the central portion of the thorax. It may come as a complete surprise to readers that there is little movement at all behind the saddle in the horse. Because the horse is a prey animal there are very good evolutionary reasons for this in biomechanical terms.

Figures 4.3 to 4.5 (see pages 63, 64 and 65) show the intervertebral joint movement in the equine back. These intervertebral joint movements have been measured in all three planes – flexion and extension (rounding and hollowing); axial rotation (twisting) and lateral flexion (sideways bending).

For the purposes of this book, the vertebral column has been divided into distinct functional sections, so that a complete understanding of the functional anatomy of the back can be given.

Neck

Atlanto-occipital joint

As can be seen, the neck has a lot of movements in all planes except for twisting, but the first two intervertebral joints, the atlanto-occipital joint (C1/2) and the atlanto-axial joint (C2/3) have very specific movement planes. In flexion and extension there is a considerable amount of movement in the former and not much in the latter. As this equates to nodding it is sometimes referred to as the ‘yes’ joint. The muscles responsible for this movement are the splenius and rectus capitus dorsalis, which lift the head up, and the longus capitus and the rectus capitus ventralis which flex the head down.

SPLENIUS (Figure 4.6)

ORIGIN Spinous processes of T3–5 by means of thoracolumbar fascia; nuchal ligament

INSERTION Nuchal crest and mastoid process of temporal bone; transverse spinous processes C2–5

INNERVATION Dorsal branch of local spinal nerve and dorsal branch of accessory nerve

FUNCTION Extends, lifts or bends neck and head laterally

DEVELOPMENT ISSUES Neck stiff in lateral flexion

RECTUS CAPITUS DORSALIS (Figure 4.7)

ORIGIN Spinous process of axis

INSERTION Nuchal crest

INNERVATION Dorsal branch of C1

FUNCTION Elevates head

DEVELOPMENT ISSUES Horse does not take contact; fights hand

LONGUS CAPITUS (Figure 4.8)

ORIGIN Transverse processes C3–5

INSERTION Muscular tubercle on base of skull

INNERVATION Ventral branch of local spinal nerve

FUNCTION Flexes/bends head laterally

DEVELOPMENT ISSUES Head unlevel, neck tight and stiff; horse does not take contact on affected side; head shaking or unsteady

RECTUS CAPITUS VENTRALIS (Figure 4.9)

ORIGIN Ventral arch of atlas

INSERTION Base of skull

INNERVATION Ventral branch of local spinal nerve

FUNCTION Flexes atlanto-occipital joint

DEVELOPMENT ISSUES As above

Atlanto-axial joint

In axial rotation the atlanto-axial joint has by far the greatest amount of movement. As this allows shaking of the head, it is also called the ‘no’ joint. In lateral bending the atlanto-occipital joint has a reasonably good range of movement but the atlanto-axial joint has barely any. This joint is acted upon mainly by the oblique capitus caudalis, sternocephalicus, and cleidomastoideus (part of brachiocephalic).

OBLIQUE CAPITUS CAUDALIS (Figure 4.9)

ORIGIN Spinous process of axis

INSERTION Wing of atlas

INNERVATION Dorsal branch C2

FUNCTION Rotates atlas (and head)

DEVELOPMENT ISSUES Horse reluctant to make lateral flexion

STERNOCEPHALICUS (Figure 4.10)

ORIGIN Manubrium of sternum and cariniform cartilage

INSERTION Mandible; caudal border of ramus

INNERVATION Accessory nerve ventral branch

FUNCTION Flexes/inclines head and neck

DEVELOPMENT ISSUES Hypertrophied; horse takes pull; lack of control

BRIACHIOCEPHALIC: made up of cleidomastoideus muscle in cranial portion and cleidobrachialis muscle in caudal portion which are divided at the clavicular inscription (Figure 4.11)

ORIGIN Medial deltoid tuberosity

INSERTION Mastoid process of temporal bone; wing of atlas; nuchal crest

INNERVATION Accessory nerve ventral branch

FUNCTION Flexes and turns head (also acts as forelimb protractor)

DEVELOPMENT ISSUES Unlevel gait in front, worse on circles; horse refusing to go forwards, with choppy stride

This complete difference in each joint’s range of movement (ROM), make it diagnostically significant. For example, if your horse has difficulty flexing at the poll, he may have a problem with the atlanto-occipital joint or the muscles responsible for its action. On the other hand, if he carries his head with a tilt, he may have a problem with the atlanto-axial joint or the muscles responsible for its action. If he has difficulty flexing to the left or right, he may well have a problem with the atlanto-occipital joint and muscles of action, but you can virtually rule out the atlanto-axial joint.

Axial rotation of the neck is mainly provided by the cervical portions of the multifidus muscle which also stabilises the neck.

MULTIFIDUS CERVICALIS (Figure 4.12)

ORIGIN Articular processes of last 6 cervical vertebrae

INSERTION Spinous process of preceding vertebra

INNERVATION Dorsal branch of local spinal nerve

FUNCTION Rotates head to opposite side of flexion; extends neck and flexes to side of contraction

DEVELOPMENT ISSUES Resistance in neck action, horse has poor contact with bit

The rest of the intervertebral joints in the horse’s neck are fairly mobile and the transverse processes of C1–C6 can usually be palpated. C7 cannot be palpated because it is in between the shoulders. General movement of the neck is governed by the longissimus (cervicalis), spinalis, iliocostalis, sternocephalic (see above), semispinalis capitus and cleidomastoideus (see above) muscles.

LONGISSIMUS (Figure 4.13)

ORIGIN Transverse spinous processes of cervical vertebrae

INSERTION Wing of atlas; mastoid process of temporal bone

INNERVATION Dorsal branch of local spinal nerve

FUNCTION Elevates or bends head and neck laterally

DEVELOPMENT ISSUES Horse tilts head; poor contact in one rein

SPINALIS (Figure 4.14)

ORIGIN Spinous processes of lumbar and last 6 thoracic vertebrae

INSERTION Spinous processes of first 6–7 thoracic and last 5 cervical vertebrae

INNERVATION Dorsal branch of local spinal nerve

FUNCTION Stabilises back and neck. Elevates neck or flexes laterally

DEVELOPMENT ISSUES Inability to flex neck to one side

ILIOCOSTALIS (Figure 4.15)

ORIGIN/INSERTION Between transverse spinous processes of C4 and T1 and first rib

INNERVATION Dorsal branch of local spinal nerve

FUNCTION Extends and flexes (laterally) neck

DEVELOPMENT ISSUES Inability to flex neck to one side

SEMISPINALIS CAPITUS (Figure 4.16)

ORIGIN Transverse spinous processes of first 6 thoracic vertebrae; articular processes of last 6 cervical vertebrae

INSERTION Occipital bone

INNERVATION Dorsal branch of local spinal nerve

FUNCTION Elevates head and neck; flexes neck laterally

DEVELOPMENT ISSUES General stiffness in horse’s neck giving the rider a ‘wooden’ feel

The neck in the horse is elongated not only to enable grazing (Figure 4.17) but it also plays a large part in movement and breathing, particularly at the faster gaits and when jumping. At these times the horse will raise and lower the head and neck set, using it like a pendulum, which affects the balance throughout the body.

When the neck is lifted up it then throws the weight to the back of the horse making it easier to lift the front end. Conversely when the neck is lowered, the weight is transferred to the front of the horse and makes the back end easier to lift up.

When you watch a racehorse in full gallop you can see this movement in the head and neck set. The head comes up when the horse needs to lift the front limbs, and the head goes down when he needs to lift the hind limbs.

Very cleverly this system also regulates the horse’s breathing in the canter and gallop, in what is known as the ‘piston-pendulum’ effect. When the horse’s head goes down and the non-lead forelimb hits the ground, it exerts a decelerative force, but the weight of the viscera keeps moving forwards (like the passengers in a car when the driver slams on the brakes) and compresses the diaphragm, forcing the horse to exhale. When the head comes up, the viscera slide backwards, forcing the horse to inhale. This is another energy saving system in the prey animal because no energy is wasted on muscles being used for respiration.

Figure 4.18 represents both the movement and the respiratory effects of neck raising and lowering very clearly. The neck thus plays an extremely important part in equine locomotion and should therefore be kept in the best condition possible.

Shoulder region

For the purposes of this book the shoulder region is that between T1 and T11. Although this is a completely arbitrary definition in terms of anatomy, the function of this section needs to be appreciated in order to understand its part in overall movement. Indeed ROM in the shoulder region is vitally important for the athletic horse, because not only does it attach the front limb to the body of the horse, the ventral (chest) muscles support the forehand.

The horse, unlike bipeds such as humans, does not have a clavicle and therefore there is no bony joint between the front limb and the body of the horse, which would equate to our shoulder joint. In the horse (as in most of the veterinary mammals) the scapula is ‘strapped’ to the thorax by means of muscles and other soft tissues. The muscles which perform this function are called the extrinsic muscles of the shoulder. These extrinsic muscles are mainly the trapezius, omotransversarius, rhomboideus, and latissimus dorsi.

TRAPEZIUS (Figure 4.19)

ORIGIN Nuchal and supraspinous ligaments from C2 to T10

INSERTION Thoracic portion: dorsal third of scapular spine; cervical portion: entire scapular spine

INNERVATION Dorsal branch of accessory nerve

FUNCTION Elevates scapula, and can move scapula craniodorsally and caudodorsally

DEVELOPMENT ISSUES Underdevelopment noted as dip in front of withers; poor shoulder movement, lack of co-ordination and short choppy cranial phase of forelimb stride

OMOTRANSVERSARIUS (Figure 4.20)

ORIGIN Shoulder fascia/scapula spine

INSERTION Transverse spinous processes of C2–4

INNERVATION Medioventral branch of local cervical nerve

FUNCTION Protracts limb when neck fixed; flexes neck when limb fixed

DEVELOPMENT ISSUES Gait unlevel in front; fused ventrally with cleidomastoideus

RHOMBOIDEUS (Figure 4.21)

ORIGIN Nuchal and dorsoscapular ligaments from C2 to T8

INSERTION Scapular cartilage

INNERVATION Medioventral branch of local thoracic and cervical nerve

FUNCTION Moves scapula dorsally and cranially; elevates shoulder

DEVELOPMENT ISSUES Poor shoulder activity; affects co-ordination of front limbs

LATISSIMUS DORSI (Figure 4.22)

ORIGIN Supraspinous ligament from T3 caudally via thoracolumbar fascia

INSERTION Teres major tuberosity of humerus together with teres major muscle

INNERVATION Thoracodorsal nerve

FUNCTION Retracts limb; flexes shoulder joint; when limb fixes draws trunk cranially

DEVELOPMENT ISSUES General forehand stiffness seen predominantly on circles; gives wooden feel to rider

However, here must be mentioned a very large muscle that supports the weight of the neck and thorax of the horse from its attachment on the inside of the shoulder blade: the serratus ventralis.

SERRATUS VENTRALIS (Figure 4.23)

ORIGIN Transverse spinous processes of C4–7; lateral surfaces of 1st–8th ribs (inserted with tendinous sheaths)

INSERTION Scapular cartilage medial aspect of scapula

INNERVATION Ventral branch of local spinal nerves; long thoracic nerve

FUNCTION Lifts body in relation to scapula, suspends trunk between scapulae.

DEVELOPMENT ISSUES General forehand stiffness seen predominantly on circles; can be noted to hold trunk in a tilted position if one side is hypertonic

This is the major part of what is known as a ‘synsarcotic’ connection, which simply means that it is a joint formed of soft tissues. The extrinsic muscles of the shoulder together with the serratus ventralis support nearly all the weight of the thorax, and as 60 per cent of a horse’s weight is taken on the front limbs then these soft tissues have to be in the best possible condition. They are also responsible for much of the forelimb movement in all planes, so you can begin to understand the importance of this shoulder movement in correct movement and athletic ability.

The structure of the vertebrae in this shoulder area reflects the power and need for support for these muscles to act against the skeleton for support and movement. These thoracic vertebrae have extremely long dorsal spinous processes (DSPs) and these can clearly be seen in the illustration of the axial skeleton at the beginning of this chapter (see Figure 4.1). On the surface anatomy these thoracic vertebrae represent the basis of the withers (Photo 4.24).

However, the ventral muscles of this portion of the body, which lie between the horse’s front legs, also play a huge part in the support of the thorax and in movement. These are the pectoral group of muscles which comprise the subclavius, deep pectoral, transverse pectoral, and superficial pectoral muscles.

SUBCLAVIUS (Figure 4.25)

ORIGIN Sternum and costal cartilages 1–4

INSERTION Supraspinatus and shoulder fascia

INNERVATION Cranial pectoral nerve

FUNCTION Suspends trunk and stabilises shoulder

DEVELOPMENT ISSUES General forehand stiffness and incoordination

DEEP PECTORAL (Figure 4.26)

ORIGIN Sternum; distally on ribs 4–9; abdominal fascia

INSERTION Major and minor tubercles of humerus; tendon of origin of coracobrachialis

INNERVATION Cranial and caudal pectoral nerve

FUNCTION Suspends trunk between forelimbs; retracts limb; stabilises shoulder

DEVELOPMENT ISSUES Short stride; inability to stretch over fence; reacts to tightening of girth

TRANSVERSE PECTORAL (Figure 4.27)

ORIGIN Costal cartilages 1–6 and adjacent sternum

INSERTION Forearm fascia

INNERVATION Cranial and caudal pectoral nerves

FUNCTION To connect forelimbs with trunk; adducts and resists abduction of forelimbs

DEVELOPMENT ISSUES Shortens stride and gives uncoordinated forelimb movement

SUPERFICIAL PECTORAL (Figure 4.27)

ORIGIN Manubrium sterni

INSERTION Deltoid tuberosity of humerus

INNERVATION Cranial and caudal pectoral nerves

FUNCTION Protracts and retracts forelimb

DEVELOPMENT ISSUES Short stride

The H frame

Functionally it can be imagined that the front legs, the shoulder blades, the pectorals, the extrinsic shoulder muscles and the serratus ventralis act like an H frame with the thorax suspended between the two top uprights of the H (Figure 4.28).

This functional arrangement endows the horse with many movement and athletic benefits. For example, it allows the thorax to swing between the scapulae by abducting (moving away) and adducting (bringing towards) each front limb (Figures 4.29a and b). The body of the horse can therefore ‘swing in the cradle’ of the scapulae to enable him to bend around corners and work on a circle.

Inability to bend through the body

If you review the ROM of the thoracic vertebrae in the shoulders it can be seen that there is very little, if any, lateral movement in this area. Indeed, that is of necessity because the forelimbs have to be anchored to a stable structure. We have all seen the classic drawing in equitation texts apparently demonstrating that the horse can bend equally from poll to tail when on a circle (Figure 4.30). Unfortunately this is nonsense; we know from our studies of anatomy that there is very little bending behind the saddle and, indeed, the sacral vertebrae are fused together and cannot flex.

So how did we become so confused about this apparently vital, but completely impossible, function of the equine spinal column? How can the horse appear to bend equally around the rider’s inside leg, when we know that he cannot?

The answer is the aforementioned ‘swinging in the cradle’ function of the back. The neck can bend as seen in the diagram but, as we found out in this section, there is little movement in the shoulder area. The apparent ‘bend’ is caused by the horse drawing the inside forelimb towards his thorax (adducting) but moving the outside forelimb away from the thorax (abducting).

Scapular movement

One further important aspect of the shoulder area is known as ‘scapular glide’. We have seen that the scapulae are strapped to the sides of the body of the horse by soft tissue structures rather than with a bony joint. This allows the scapula to move and rotate against the thorax.

Figure 4.31 shows an image taken from computer-aided gait-analysis images. An appreciation of the shoulder movement can be gained from this image, and throughout the small amount of stride phase that is illustrated, the scapula has not only moved backwards and forwards but it has rotated slightly. Bear in mind that this is just at an in-hand trot speed and that the faster the gait the greater the shoulder movement has to be to accommodate the larger stride lengths that need to be achieved. Therefore a greater shoulder movement is positively correlated with longer strides, so the more supple and free the shoulder, the greater the limb protraction and retraction. Speed and extended gaits are, therefore, very much affected by shoulder movement.

Folding of the forelimbs

Jumping is another athletic activity which needs maximum shoulder movement. When the horse flexes his forelimbs to negotiate the top rail, the complete ‘folding’ of the limbs has a great dependence on shoulder joint flexion. That shoulder flexion is enhanced by the scapula rotating backwards. In Figure 4.32 we see the skeletal reproduction of the horse jumping, and how the front legs need to fold up to clear the fence.

However, in the elite jumping horse, that limb folding function has to work to extremes, and therefore the shoulder flexibility becomes of paramount importance. As an example have a look at Photos 4.33a and b, which show Sarah Stretton and Lazy Acres Skip On negotiating the Burghley show-jumping and cross-country courses.

By making a comparison between the front limb folding of the jumping skeleton and that of Skip On, it is clear that the elite jumping horse has to make extreme use of the shoulder movement. Because of the soft tissue nature of shoulder attachment to the body, this is something that equine sports physiotherapists can enhance considerably.

The more that the horse can wind up those front limbs, the less of a jumping effort he has to make, and the less energy he has to expend in the jumping effort. If the front limbs were dangling in any way, he would have to jump much higher to clear the fence and that would expend a lot more energy.

We can therefore take from this that free and unencumbered movement of the shoulder is vital to sustain fluid movement and athletic ability. Dressage horses are, in part, selected for freedom of the shoulder. In Photo 4.34 we see a quite extraordinary shoulder movement in a yearling colt by Totilas. The scapula has rotated to allow forelimb protraction.

One way of checking that the shoulder is free is to see if you can get your fingers between the front of the scapula and the thorax (Photo 4.35). Generally, the horse with shoulder freedom will find this enjoyable and relaxing.

Clearly, if shoulder movement is such an important part of movement and athleticism, then anything that would impede shoulder movement should be avoided. The most likely cause of shoulder impingement is a poorly fitting saddle, and this will be discussed later on in this book.

Mid-back and lumbar region (the thoracolumbar bow)

As with the shoulder region, the section that comprises this region, from T12 to T18 (mid-back) and L1 to L6, is purely arbitrary and its selection is based on biomechanics rather than on anatomy. Indeed the anatomical differences between the structure of the thoracic vertebrae and the lumbar vertebrae are marked.

It is in the mid-back section that the second most intervertebral joint movement is possible (after the neck), particularly in lateral flexion. However, it also just happens to be the part of the back upon which the saddle and rider sit. Clearly therefore, any interference in back movement during riding is going to have a marked effect upon the mechanics of this mid-back area. It will thus come as no surprise that poor saddle fit and a leaden rider are the likely culprits in mid-back movement restriction.

It is also this part of the back that has to support the weight of the saddle and rider and, again, the mechanics by which this is achieved by the horse is unique.

If you sit on the middle of a rectangular table with a leg at each corner, then not only will the table bend in the middle but the legs will splay out in front and behind (Figure 4.36).

It would be unfortunate if we sat on the horse and his legs shot out in front and behind him. But not only can he resist these forces, he can also gallop at full speed and jump with a rider and tack on his back. This is especially ingenious when you consider how thin and light his lower limbs are, compared to the weight and muscularity of his back.

So how does the horse achieve this? By the mechanical analogue for the horse’s back known as the ‘bow and string theory’.

The bow and string theory

Imagine using a longbow, like the type that Robin Hood used (Figure 4.37); when you tighten the string the bow flexes (arches more) and when you loosen the string the bow flattens out.

So now try thinking of the bow as the horse’s back (dorsal line) and the string as his abdominal muscles (ventral line), particularly the rectus abdominus muscle (Figure 4.38).

RECTUS ABDOMINUS (Figure 4.39)

ORIGIN Lateral surface of costal cartilages 4–9

INSERTION Prebubic tendon and (via accessory ligament) on head of femur

INNERVATION Local intercostal and ventral branches of lumbar nerves

FUNCTION Flexes lumbar spine and lumbosacral joint

DEVELOPMENT ISSUES Inability to bring hind limb sufficiently underneath to support rider and go forwards

EXTERNAL ABDOMINAL OBLIQUE (Figure 4.40)

ORIGIN Lateral surface of ribs 4–18; thoracolumbar fascia

INSERTION Abdominal tendon: linea alba and prepubic tendon; pelvic tendon: tuber coxae and prepubic tendon

INNERVATION Local intercostal and ventral branches of lumbar nerves

FUNCTION Flexes the trunk

DEVELOPMENT ISSUES Inability to bring hind limb sufficiently underneath to support rider and go forwards

INTERNAL ABDOMINAL OBLIQUE (Figure 4.41)

ORIGIN Tuber coxae and adjacent inguinal ligament

INSERTION Last rib; cartilages of last 5 ribs, linea alba and prepubic tendon

INNERVATION Local intercostal and ventral branches of lumbar nerves

FUNCTION Flexes the trunk

DEVELOPMENT ISSUES As above

TRANSVERSE ABDOMINUS (Figure 4.42)

ORIGIN Medial surface of costal cartilages 7–18; lumbar transverse processes

INSERTION Linea alba

INNERVATION Local intercostal and ventral branches of lumbar nerves

FUNCTION Flexes the trunk

DEVELOPMENT ISSUES As above

When the horse contracts the rectus abdominus (and other abdominal muscles), it pulls the pelvis closer to the sternum and changes the angle of the lumbosacral joint, just like the string of the bow pulling the two ends together. This effectively shortens the ventral line (the underline) of the horse and at the same time the dorsal line (the topline) must lengthen and flex, and the hind limb can be placed further underneath the horse.

LONGISSIMUS (See Figure 4.13)

ORIGIN Spinous processes of sacrum, lumbar and thoracic vertebrae; wing of ilium; transverse processes of thoracic and cervical vertebrae

INSERTION Transverse spinous processes of vertebrae; tubercles of ribs; wing of atlas; and mastoid process of temporal bone

INNERVATION Dorsal branch of local spinal nerves

FUNCTION Stabilises and extends vertebral column; elevates or bends head laterally

DEVELOPMENT ISSUES Sore, tight neck and/or back, giving rider a stiff and wooden feel; reluctance to engage and go forwards

SPINALIS (See Figure 4.14)

ORIGIN Spinous processes of lumbar and last 6 thoracic vertebrae

INSERTION Spinous processes of first 6 thoracic and last 5 cervical vertebrae

INNERVATION Dorsal branch of local spinal nerves

FUNCTION Stabilises back and neck; elevates neck or bends it laterally

DEVELOPMENT ISSUES As above

MULTIFIDUS (Figure 4.43)

ORIGIN Articular and mammillary processes of all vertebrae from C2 to sacrum

INSERTION Spinous processes of preceding vertebrae

INNERVATION Dorsal branch of local spinal nerves

FUNCTION Stabilises and twists the vertebral column

DEVELOPMENT ISSUES As above

As the back lengthens and flexes, the dorsal spinous processes of the vertebrae in the shoulder and thoracolumbar bow open up and exert a backwards pull on the withers, which lifts the forehand. At the same time the scalenus muscle which connects the first rib to the cervicothoracic joint contracts and this enables the head and neck set to elevate, and the horse to work forwards in complete self-carriage. It is this anatomical function that gives the rider the glorious feeling of being lifted up and floated forwards as represented in Figure 4.44.

This then leads us to the concept of dorsal (topline) and ventral (underline) muscular chains. When looking at Figure 4.44 it can be appreciated that to perform in this perfect self-carriage, the horse has to make the dorsal chain (extending from the poll to the back of the stifles) as long as possible whilst making the ventral chain (extending from the throat, along the abdominals and ending at the insertion of the rectus abdominus onto the pelvis) as short as possible. However, just like any chain, it is only as strong as its weakest link, and it only takes one muscle within this whole process to be injured or painful, and the whole self-carriage concept will fail.

When training the young horse, or when rehabilitating a horse after injury, there is only one ultimate goal and that is to train for this balance of dorsal and ventral chains, as this will form the complete postural and biomechanical base from which you can begin to build/rebuild the athlete. So whilst this abdominal suppleness is being achieved, the young or damaged horse should be worked in the position we know as ‘long and low’ when the head is lower than the withers and the neck is reaching out and down so that the length between the poll and the back of the stifles is as long as you can make it. Establishing this correct posture from the beginning will also have a profound effect on your horse’s weight carrying and athletic ability.

If this system fails to function because, for example, the long muscles in the back are sore or in spasm and cannot lengthen, then this is the start of a potentially long downward spiral into poor posture, loss of performance and behavioural changes due to pain. The effects of this will be demonstrated in Chapter 5, and schooling strategies to rehabilitate following these effects will be discussed in Chapter 8.

Energy saving

At this stage it is necessary to discuss another function of the lumbar vertebrae. You can see from the graphs at the beginning of this chapter that there is hardly any intervertebral joint movement in the lumbar region of the horse and, again, the reason for this is rooted in the evolution of the horse as a prey animal.

The horse’s major means of defence is to outrun a predator. Therefore as much energy as possible has to be put into the systems that propel him forwards and away from whatever is chasing him. Any energy that is used for body movements not associated with forward propulsion is wasted energy.

The principles of elastic energy were discussed in Chapter 1, but this energy-saving system needs more than elastic energy. When you look at a horse walking, you are looking for a nice fluid action through the back, with the quarters swaying from side to side. But swaying from side to side wastes energy that could more usefully be spent on propelling the horse forwards and away from the predator.

The reason why the vertebrae behind the saddle in the horse do not allow much movement is that they are designed to act as a mechanical strut against which the hind limbs can exert the power created by hind limb retractor muscles. This ensures that all the power generated to propel the horse forwards is not wasted in any other type of movement. Therefore, as the horse’s gaits get faster he loses the sideways sway in the back that is seen in the walk, and if you watch racehorses from behind on the gallops, you will see that there is absolutely no sideways motion at all, every ounce of energy is being used to drive the horse forwards.

Lumbosacral joint

In Figure 4.3 it can be seen that normal movement in the lumbosacral joint is about 30 degrees in the flexion and extension planes. We have demonstrated that lumbosacral joint flexion and extension is vital in correct movement. Therefore, the greater amount of movement in the lumbosacral joint, the more the angle of the pelvis can change and the greater the athletic ability. This is because the more flexion in the lumbosacral joint, the more the horse can bring the hind leg underneath him and this is important for speed, jumping, and extended and collected movements. In changing the angle of the lumbosacral joint, as the ‘string’ shortens, the back lifts and flexes and the dorsal and ventral muscular chains function in harmony.

Also the further underneath him the horse can place his hind limb, the longer the stance phase (when the foot is on the ground). Greater stance time equals greater propulsion forwards (for speed) and upwards (for jumping). It is also vital for collected movements in dressage as the hind limb must come well under the body. Take a look at Photos 4.45 and 4.46 and compare the angle of the hind limb in the first picture with the hind limb angle in the second. A lot of this difference in angle comes from lumbo-sacral joint movement. Bringing the hind limb underneath (limb protraction) is a function of lumbosacral joint flexibility, suppleness in the back muscles and the elastic energy released by the ilipsoas muscle causing protraction of the hind limb (see Fig.1.2 in Chapter 1). It also requires the part of the dorsal muscular chain between the lumbosacral joint and the back of the stifles (the hamstring group – see chart on page 96 and illustrations on pages 46 and 47 Chapter 3) to be able to lengthen and contract to the maximum.

BICEPS FEMORIS (See Figure 3.6)

ORIGIN a. Vertebral head: spinous and transverse processes of last three sacral vertebrae, sacrosciatic ligament and tail fascia; b. pelvic head: tuber ischia

INSERTION a. Patella, lateral and middle patella ligaments; b. cranial border of tibia and crural fascia; c. via long calcanean tendon to calcaneus

INNERVATION Caudal gluteal and sciatic nerves

FUNCTION a. Extends hip and stifle; b. with caudal division flexes stifle; c. abducts hind limb

DEVELOPMENT ISSUES Shortening of cranial portion of stride; resists lateral movement; discomfort in hind joints

SEMITENDINOSUS (See Figure 3.7)

ORIGIN a. Vertebral head: last sacral and first two caudal vertebrae, tail fascia and sacrosciatic ligament; b. pelvic head: ventral tuber ischia

INSERTION a. Cranial border of tibia, crural fascia; b. via long calcanean tendon to calcanius

INNERVATION Caudal gluteal and sciatic nerves

FUNCTION a. During weight bearing: extends hip, stifle and hock; b. during non-weight bearing: retracts and adducts limb

DEVELOPMENT ISSUES As above

SEMIMEMBRANOSUS (See Figure 3.8)

ORIGIN a. Vertebral head: first caudal vertebra, sacrosciatic ligament; b. pelvic head: ventromedial aspect of tuber ischia

INSERTION Medial condyles of femur and tibia

INNERVATION Caudal gluteal and sciatic nerves

FUNCTION a. During weight bearing: extends hip and stifle; b. during nonweight bearing: retracts and adducts limb

DEVELOPMENT ISSUES As above

Once the hind limb is on the ground, the powerful action of the gluteal and hamstring groups retract the hind limb, and again, change the angle of the pelvis to propel the horse forwards and/or upwards (see chart opposite and illustrations on page 44 of Chapter 3)

MIDDLE GLUTEAL (See Figure 3.3)

ORIGIN a. Longissimus lumborum; b. gluteal surface of ilium; c. sacrum; d. sacroiliac and sacrosciatic ligaments

INSERTION Greater trochanter at head of femur

INNERVATION Cranial gluteal nerve

FUNCTION a. Extends hip; b. abducts hind limb

DEVELOPMENT ISSUES Reduced step length; resists lateral movement

DEEP GLUTEAL (See Figure 3.5)

ORIGIN Spine of ischia

INSERTION Greater trochanter at head of femur

INNERVATION Cranial gluteal nerve

FUNCTION Abducts hind limb

DEVELOPMENT ISSUES As above

Unfortunately it is the very same combination of unencumbered neck movement, balance of dorsal and ventral muscular chains and exceptional lumbosacral movement that allows the extremely athletic horse to buck very extravagantly!

Congratulations go to Team GB rider Sarah Stretton for staying onboard as Lazy Acres Skip On decides to prove the point in the warm-up arena (Photos 4.47a and b).

The functioning of the equine axial skeleton is, therefore, of prime importance in keeping a horse sound and at full athletic performance. Because there is such a heavy reliance on muscular function throughout, very careful consideration must be given to its development right from the start of your conditioning programme. Also, as many of the structures are reliant on other structures in the system functioning properly, any injury or incorrect development affects the whole system and your horse will never reach his true athletic potential and will always be prone to injury.