The lead (right) forelimb is the first to take off. The horse is then supported by the leading forelimb which straightens to push the front end into the air (Figs 12.9 and 12.10). Take-off ends at the lift-off of the lead hind limb. To lift the forehand off the ground, the horse has to move the centre of gravity backwards (the experienced jumper does this by raising his head and neck) as well as flexing the hocks.
In Fig. 12.10 the horse raises its head to enable the forelimbs to leave the ground. The rider has brought her hands back and inclined her upper body forwards, closing the angle of the elbow and maintaining a contact on the horse’s mouth. Compared with the loose jumping horse (Fig. 12.9) this appears to have constrained the position of the horse’s head and neck.
Role of the forelimbs in the take-off
The forelimbs play a very important role in the take-off, acting as struts that change forward momentum into the upward thrust of the forehand. This is clearly shown in Figs 12.9 and 12.10. The shoulder and the elbow are extended by the action of the triceps on the elbow joint, and by the biceps brachii and the supraspinatus on the shoulder joint. A large part of the impetus that pushes the horse upwards comes from the extension of the fetlock joint, brought about by the superficial and deep digital flexors. The muscles along the topline of the back, from the neck to the croup also contract, arching the back as much as possible and helping to lift the forehand (Fig. 12.12).
Role of the hindquarters in the take-off
As the horse’s front end rises from the ground, the hindquarters rotate under the body to support its body weight (Fig. 12.14). The hind limbs push off close to the hoof print left by the leading forelimb. The hind limbs are not placed down exactly together (Fig. 12.9) and they do not leave the ground quite together. However, they do appear to be used more in unison in show jumping than when the horse is jumping at speed. The amount the forehand is elevated before take-off influences the trajectory through the air, while increased flexion of the hind limbs increases upwards impulsion.
Straightening the hind limbs
The horse drives its hind feet against the ground, straightening the hips, stifle, hocks and fetlocks (Figs 12.13 and 12.14). This results in the hindquarters and thighs travelling much faster than the lower limbs because the feet remain firmly on the ground until they are lifted by the movement of the body. The hip is extended by the hamstring group of muscles (biceps femoris, semitendinosus and semimembranosus muscles), aided by muscles attaching to the point of hip and the femur. The muscles of the quarters also extend the stifle and hock joints – stifle extension is aided by the quadriceps femoris, and hock extension is backed up by the gastrocnemius muscle (Fig. 12.16). The fetlock joint is straightened by the action of the deep digital flexor muscle.
Folding of the fore limbs
As soon as the front feet leave the ground, the forelimbs begin to flex, especially at the elbow and knee (Figs 12.9, 12.13, 12.14). This folding is helped by contraction of the brachiocephalicus muscle which brings the forelimbs upwards and forwards.
Airborne phase (Figs 12.17–12.28)
The trajectory, impulsion and speed determine the flight over the jump. Most of the joints in the horse’s fore and hind limbs are flexed as the horse is propelled over the fence (Figs 12.21 and 12.22). The hind limbs can be flexed to bring the limbs underneath the body or extended behind the horse (Figs 12.25 and 12.26) in preparation for landing.
The classic parabola over a fence occurs when the horse slightly rounds its back and reaches to get over the fence. The more the horse can use its back, shoulders and hips the better. The less the limbs are flexed, the higher the horse must lift its body to clear the fence; a good showjumper uses limb flexion to full advantage. Some horses kick out behind the body to clear the fence, mainly through the action of the gastrocnemius muscle.
As the horse leaves the ground the rider should give it enough rein to allow the horse to lower its head and bring its withers up. The loose jumping horse in Fig. 12.17 shows the extent to which this may occur, while the ridden horse in Fig. 12.18 is not using its neck and shoulders with nearly as much freedom. The rider maintaining too short a rein while jumping is a common cause of a horse hitting a fence.