IN THE NEXT THREE chapters there are some of Alexa’s brilliant illustrations to enable you to understand the anatomy of the horse and how it relates to function. Muscles have effects on joints and they can either move a joint through its range of movement (ROM) during locomotion, or stabilise a joint to facilitate standing. Very rarely does one muscle work alone and it will usually have ‘synergists’, which are other muscles that have a similar effect, and ‘antagonists’ which are muscles that have the opposite effect. For example, one muscle may flex a joint and another will extend the same joint. Although they both have an effect on the same joint their function is the opposite of the other.
During stance and unridden movement, the horse takes about 60 per cent of his weight on his front limbs. This is a general principle of veterinary mammalian movement in that the forelimbs are mainly for support of the trunk, whilst the hind limbs act to propel the animal forwards.
In this chapter we describe the major muscles of the forelimb and shoulder, giving their anatomical origins, insertions, innervation, function and any developmental problems associated with that particular muscle (or group of muscles).
We also describe what is known as the ‘stay apparatus’, which is the system by which the horse is able to sleep standing up.
ORIGIN Proximocaudal on 3rd metacarpal bone (MC3) and palmar carpal ligament
INSERTION Proximal sesamoid bones. Tendons bifurcate from insertion to insert on common digital extensor tendon
INNERVATION Deep ulnar nerve
FUNCTION Prevents overextension of fetlock
DEVELOPMENT ISSUES Lameness
Figure 2.17 The straight and cruciate ligaments associated with the fetlock and proximal sesamoid bones, known as the sesamoidean ligaments.
Below the knee there is no muscle (the interosseus has evolved to be a passive elastic spring). However, there are important ligaments associated with the fetlock, namely the straight and cruciate ligaments (Figure 2.17 – the cruciate ligaments are in orange and the straight ligaments are in brown).
These sesamoidean ligaments are clinically important in athletic horses. They are a common site of injury (desmitis) and result in a low to moderate degree of lameness.
COMMON DIGITAL EXTENSOR
a. radial head (not shown)
ORIGIN Lateral epicondyle of humerus
INSERTION Proximodorsal on proximal phalanx
INNERVATION Radial nerve
FUNCTION Carpal and digital extensor
DEVELOPMENT ISSUES Stumbling or dragging of toe
Figure 2.18 Common digital extensor. Only the humeral head of this muscle is illustrated because the radial and ulnar heads are deep to this muscle and are not shown in this illustration.
Figure 2.19 Lateral digital extensor (shown here in the hind limb, but the soft tissue architecture is almost identical to that in the forelimb with the exception of the origin. Refer to the description on page 36).
INSERTION Short tendon on accessory carpal bone; long tendon proximally on 4th metacarpal
INNERVATION Radial nerve
FUNCTION Flexes carpus
The horse is perfectly capable of dozing whilst standing up, but to truly rest he lies down. Why should he have evolved to sleep standing up? Again, the answer is that he is a prey animal, and if he is already standing up when he senses a predator, he can just gallop away without wasting time getting to his feet. If you have observed a horse rising from a recumbent position, you will know that it is not a particularly speedy process, and should he have been lying down when the predator approaches, he may well have become someone’s supper before he even gets on his feet. Therefore, horses only tend to lie down at night when they are unobserved.
Figure 2.22 The stay apparatus
The four legs of the horse are angulated bony columns, which normally if not under conscious control, would collapse. In Chapter 1 we explained about elastic energy and how tendinous structures within certain muscles of the horse are there as simple passive springs to aid locomotion. However, they also have another function within what is known as the ‘stay apparatus’ (Figure 2.22). The stay apparatus is the anatomical means by which the horse can rest in a standing position without the joints collapsing due to relaxed muscle tone. Many of the structures within the stay apparatus contain tendinous tissue and because they act as tendinous springs, once they have reached their maximum length, they become stable. Also, because they are non-fatiguing tissue no real energetic input is required.
The first major muscle involved in the stay apparatus is the serratus ventralis muscle (see Chapter 4Figure 4.23) which is the principle muscle of weight bearing of the thorax. It is heavily interlaced with tendinous tissue so that, once limbs are stabilised, the horse can simply settle the weight of the thorax into the serratus ventralis.
Stablising front limbs starts with the shoulder joint, which is largely restricted to flexion and extension because of the stabilising effect of the supraspinatus and infraspinatus laterally, and the subscapularis medially. The primary stabiliser of the shoulder is the biceps brachii, the tendon of origin of which presses against the front of the shoulder joint. As the horse relaxes his body, the shoulder joint flexes, but as it does so the tendon of the biceps tightens. As this happens, the first insertion of the biceps (just below the front of the elbow joint) pulls the elbow cranially, i.e. it pulls it forwards to stabilise the joint, and prevents it from flexing.
Another tendinous portion of the biceps brachii fuses with lacertus fibrosus and extensor carpi radialis, which then stabilises the knee.
The elbow is principally stabilised by the carpal and digital flexors which also contain much fibrous tissue. Whether the triceps also play a part in the stay apparatus is still widely contentious. Many believe that because of the lack of increased tonus in this muscle whilst the rest of the stay apparatus is in use, then it cannot be playing any part in shoulder stabilisation.
The knee is prevented from flexing by the biceps brachii (see page 27) acting on the extensor surface of the knee, but also by the flexor carpi ulnaris and ulnaris lateralis (which attach to the accessory carpal bone) that come under tension by the weight of the trunk via the fixed shoulder joint. This exerts a caudal pull on the flexor surface of the knee to prevent knee flexion. This is aided by the accessory (check) ligaments of the deep and superficial digital flexors.
The suspensory apparatus is all tendinous/ligamentous tissue with no muscle fibres to fatigue. The suspensory apparatus is also present in the hind limb (see Chapter 3). The fetlock needs to be supported to prevent overextension when the horse relaxes and brings the stay apparatus into action. Therefore the fetlock needs to be ‘suspended’ and this is where the suspensory ligament comes into play. Technically the suspensory ligament is a muscle (interosseus – see Figure 2.16) but it has evolved into a passive tendinous spring. The branches of the suspensory ligament are really the tendons of insertion of the interosseus muscle. As the horse’s weight settles into the fetlock, it is prevented from overextending not only by the suspensory ligament, but also by the tendons of the superficial and deep digital flexor muscles.
The equine forelimb is therefore a highly complicated functional structure designed to support the majority of the horse’s weight, whilst at the same time providing optimum, energy-saving performance.