Chapter 3

The forelimb

Introduction

As the horse evolved into a fleet-footed herbivore, the proportions of the limbs changed. To achieve greater speed the lower limb was kept as light as possible; thus the horse has no muscle below the knee or the hock. The pull of the muscles higher up the leg is transmitted to the bones of the lower leg and foot by long tendons which can easily be felt by running one's fingers down the back of the horse's leg. The length and position of these tendons means that they are susceptible to damage and injury. The design of the forelimb ensures that forces are conducted in a straight line up and down the limb, so that one structure is not stressed more than another.

Design and function (Fig. 3.1)

In reality the modern horse stands on the tip of the middle digit of each forelimb and the tip of the middle toe of each hind limb. The hoof, which is similar to a human nail, has developed to protect the end of each limb; it also acts as a shock absorber. The horse's forelimb is attached to the body by muscles and ligaments; there is no bony attachment or collarbone equivalent. This influences both the horse's movement and the way in which the forelimbs absorb concussion. In the standing horse, 60% of the weight of the body is carried by the forelimbs and they have to with-stand the majority of the impact experienced during movement. Fore-limbs are designed not only to act as shock absorbers; they also have an important role in moving off and propulsion which will be discussed later.

The skeleton (Figs 3.2-3.4)

The forelimb consists of the following bones:

Fig. 3.1 Side view of the forelimb

• Scapula
• Humerus
• Radius
• Ulna
• Knee or carpus (consisting of seven or eight carpal bones)
• Cannon (large metacarpal) bone
• Two splint bones (medial and small metacarpals)
• Long pastern (first phalanx)
• Short pastern (second phalanx)
• Pedal bone (third phalanx)
• Two proximal sesamoid bones
• Navicular bone (distal sesamoid bone).

Scapula

The scapula is a triangular flattened bone which glides over the rib cage. The length and angulation of the scapula (ideally 45°) determine the slope of the horse's shoulder and the length of stride. The thorax is slung between the two scapulae by an arrangement of muscles, tendons and ligaments known as the thoracic sling.

Fig. 3.2 Skeleton of the forelimb - side view

Humerus

The shoulder is a ball and socket joint (the type of joint that allows the greatest movement) between the humerus and the scapula. The humerus is a strong bone and its angulation allows for shock absorption. The shoulder movement is mainly flexion and extension, with some rotation, abduction and adduction.

Fig. 3.3 Skeleton of the forelimb - front view

Radius and ulna

The radius and ulna are equivalent to the bones of the human lower arm but, unlike the human, they are fused together to prevent the horse's foreleg from twisting. The ulna is very small except for the olecranon process, which forms part of the elbow. The elbow is a ginglymus joint between the humerus, radius and ulna, and allows movement in one direction only.

Knee (carpus) (Figs 3.5, 3.6)

The knee or carpus is equivalent to the human wrist and consists of seven or eight small carpal bones arranged in two rows, one above the other.

Fig. 3.4 Skeleton of the forelimb - rear view

• Upper row - radial, intermediate and ulna carpals with the accessory carpal or pisiform bone, which does not bear weight, at the back
• Lower row - first, second, third and fourth carpals.

The joint is designed to absorb shock. It is a hinge (ginglymus) joint because it only moves in one direction, i.e. flexion and extension; as the knee flexes, the hoof moves towards the elbow. There is no lateral or rotational movement.

Fig. 3.5 Knee - front view

Fig. 3.6 Knee - side view

Conformation (Fig. 3.7)

The horse's limbs are not ideally suited to long-term, repeated and strenuous work, but an individual's conformation will make that horse less or more likely to stay sound throughout an athletic career. This means that optimal conformation of the forelimbs is particularly important in performance horses and that a basic knowledge of conformation is very useful when purchasing a horse.

Fig. 3.7 Forelimb conformation

Differences between breeds

Conformational differences exist between the lighter types of horse such as the Arab and Thoroughbred, and the heavier draught horses such as the Shire. Draught horses have more 'bone': the circumference of the cannon bone just below the knee is greater than in lighter horses, this being associated with their superior weight-carrying capacity. Draught horses also tend to be more upright though the shoulder and the pastern, giving them a short, jarring stride, while the sloping shoulder and pastern of the Thoroughbred give a longer, springy stride.

Ideal conformation

Shoulder

The horse should have a good sloping shoulder so that there is 'plenty in front of the rider' and the saddle sits in a comfortable position. A long, correctly angulated scapula will also allow for a longer stride length. Traditionally the ideal shoulder has a 45° slope to the horizontal with a similar hoof-pastern angle. This enables concussive forces to be absorbed equally by all components of the limb. In practice, as long as the shoulder is flat and long enough to ensure a good stride length, it does not matter if it is a little upright.

The humerus is very strong and its angulation, which should be about 60° to the horizontal, allows for shock absorption. The slope of the shoulder should balance the pelvis and hip articulation; it is no good if the forehand has extravagant movement that the hind limbs cannot match.

Elbow

The elbow should be 'free' and allow a fist to be placed between it and the ribs. A 'tied-in' elbow limits stride length. The point of the elbow should be in the same plane as the point of the shoulder, so that it does not turn in or out. The measurement from the withers to the point of the elbow should be about the same as from the point of the elbow to the ground, ensuring adequate depth of chest.

Forelimb

The forearm should be long and well muscled, and the cannon bone should be short with adequate flat bone. Seen from the side and front the forelimbs should be straight. From the front, a plumb-line dropped from the point of shoulder should bisect the limb and hoof. This shows that the bones are arranged in a column, directly on top of each other, giving strength and ensuring that concussive forces spread evenly up the limb. The space between the front feet when the horse is standing square should be large enough to accommodate another foot. The knee should be flat and broad at the front with good depth. Common faults include the following:

• Over at the knee - the knee appears to be slightly flexed
• Back at the knee - the front of the leg appears concave
• Tied in below the knee - there is less bone below the knee than there is lower down the leg
• Calf knees - shallow from front to back
• Offset cannon bones - the bones are not placed directly below the knee in a straight line.

The fetlock joints should be well defined and bony rather than puffy.

Seen from the side, a line dropped from the midpoint of the scapula should run down in front of the forelimb and pass down through the middle of the hoof. If this is difficult to visualise, it may be easier to locate the small projection or tuber on the scapula that lies a little above the mid point. A line dropped from here should pass down through the elbow joint, the knee and the fetlock and hit the ground just behind the heels. In addition, a line dropped from the elbow should run vertically down the back of the leg.

Feet

The shape and proportions of the feet should be suitable for the limb, a pair and 'in balance'.

Hoof balance

• A vertical axis drawn through the centre of the cannon bone should bisect the hoof into two equal halves
• A line running across the top of the coronary band should be horizontal, showing that the hoof wall is at the same angle on both sides
• The wall should not flare out or run under
• The frog should bisect the foot exactly
• The hoof should be the same shape and size on either side of the frog
• The hoof-pastern axis (HPA) should be in alignment. The ideal hoof angle is 45-50° in front and 50-55° for the hind feet. In practice, depending on the individual horse's conformation, the angles tend to be more upright than this
• The angle of the hoof wall at the toe should be the same as at the heel
• The hoof should land level and slightly heel first.

Musculature (Figs 1.4, 3.8, 3.9, 5.4, 5.5)

Trapezius

The trapezius muscle is a flattened triangular sheet of superficial muscle consisting of long muscle fibres which run more or less parallel to its long axis. It is...