Right diagonal stance
In Fig. 9.2 (p. 101) the left fore and right hind are in contact with the ground. This is known as the right diagonal stance. The diagonal is named from the hind foot on the ground. Figs 9.3 and 9.4 show the skeleton and musculature at this point in the sequence.
Left fore single stance
In Fig. 9.5 (p. 103) the right hind foot has left the ground momentarily before the left fore, which is still in contact with the ground. This is the left fore single stance. Figures 9.6 and 9.7 show the skeleton and musculature at the point of right hind lift-off.
Raising the forehand
To raise a forelimb it helps if the centre of gravity is first moved back. In this way, the horse raises its head and neck and contraction of the serratus ventralis raises the thorax on the side on which the muscles are acting. This places extra weight onto that foreleg and relieves the opposite foreleg of enough weight to permit the elbow to flex and the joint to be moved forwards. At the same time, contraction of the anterior deep pectoral muscles between the front legs also raises the thorax. Flexion in the hocks, which may be almost imperceptible, occurs and the resulting drop of even a few centimetres in the hindquarters is sufficient to move the centre of gravity back and release significant weight from the forehand.
First suspension phase
In Figs 9.8–9.10 (pp. 105–6) the left fore is lifting off the ground just before the first suspension or airborne phase during which the horse is able to cover the ground. Young horses sometimes show less suspension than trained animals. This young horse has natural impulsion and rhythm and clearly demonstrates the suspension phase. The suspension should not lift the horse up without taking it forwards.
Forearm extensor muscles
The muscles that extend the foreleg can be clearly seen in Fig. 9.8. They are the:
- Radial carpal extensor muscle at the front of the leg
- Common digital extensor muscle behind it.
Digital extensor tendon
Only the tendons from the forearm muscles extend down below the knee. In Fig. 9.8 the common digital extensor tendon can be seen at the base of the common digital extensor muscle. This tendon runs down over the knee, joins the lateral digital extensor tendon and attaches to the pedal bone in the foot.
End of first suspension phase
In Figs 9.11 and 9.12 (p. 107) the left fore has lifted off and all four feet are off the ground and the left hind is preparing to make contact. Figure 9.13 shows the left hind limb making contact with the ground slightly before the right fore lands, so that the horse’s weight is taken on one limb momentarily. This is left hind single stance. The fetlock is extended so that the digits are in a straight line with the leg when the foot lands (Fig. 9.14).
Collection and suspension
As the trot becomes more collected there must be sufficient impulsion to maintain the suspension so that the freedom of the trot is not lost. The sequence remains unchanged in the variations of the trot but the length of stride and duration of the suspension alter. A slow trot or jog has little or no suspension, while in the dressage horse the suspension should be obvious.
Left diagonal double stance
In Fig. 9.15 the right fore has made contact and the horse has the left diagonal feet on the ground. This is followed by lift-off of the left hind, leaving the right fore the only foot on the ground for a short period of time. Fig. 9.16 shows the skeleton at this point in the sequence.
When the foot is on the ground the fetlock sinks, as shown in Fig. 9.15. The suspensory ligament, then the superficial flexor tendon, then the deep flexor tendon and finally the check ligament control this movement. These tendons and ligaments help to reduce shock (by absorbing the energy of the impact) and to add smoothness to the action. The fetlock recovers when the leg has passed over the vertical. In this way the limb is shortened and lengthened, helping to keep the body at nearly the same level throughout the stride.
As the horse’s leg leaves the ground, deep flexor muscles pull the pedal bone back with such energy that the rotation of the coffin joint and the upward movement of the fetlock add forward propulsive force to the stride. This is assisted by the elasticity of the suspensory ligament.