The horse’s head is the heaviest part of the body. It’s centre of gravity is not situated halfway between the nose and the tail, but just behind the withers.
In a natural position, the horse carries 60% of its weight on the forehand and 40% on its hind legs. Because the muscles over the hind quarters are big and strong, we need to educate the horse to be able to carry more weight on these muscles. This will result in the horse carrying 40% on the forehand and 60% on the hind quarters.
The nerves through which we communicate with the horse are situated in the skin. These send messages to the brain via the spinal cord and back to the muscles, causing contraction of the relevant muscles. This nervous impulse is called a reflex, which causes a quicker reaction in the muscle. There are two types of reflex – natural reflex (fright, flight and moving away), and conditioned reflex (trained reflex). We use the natural reflexes to bring about reactions that enable us to teach the horse to respond in a certain way, becoming a conditioned reflex.
The elevation of the neck causes a reflex reaction to the positioning of the hind leg, while the flexion at the poll causes a parallel reflex reaction to the flexion of the hock and stifle. The back muscles are also involved in the active role of the hind legs. A tight back leads to a stiff hind leg and high croup so the hind leg doesn’t come as far forward under the horses centre of gravity. The flexor muscles in the hind quarters are weaker than the extensors, which provide the propulsion and control the flexion of the hind limb when the leg is loaded. The hind quarters, assisted by the abdominal muscles, cause the loin to round, allowing the hind leg to come further forward under the body.
There are 4 types of reflex relationships between the head, neck and hind quarters :
Neck down and nose out – hind leg out behind with an extended hock and stifle. Similar to how a race horse stretches out in a race.
Neck down and poll flexed – commonly seen when the horse is ridden ‘long and low’. The hind leg is out behind but with hock and stifle flexing.
Neck elevated and nose forward – hind leg drawn forward under the body but with relatively extended or straight hock and stifle joints.
Neck elevated and poll flexed – seen in advanced schooling and highly collected work. The hind leg engaged with flexed hock and stifle.
The ‘effect of the aids’ is the result of a positive response, brought about through education and training. The ‘engagement of the hind leg’ is the effect of a positive response to the riders leg aid, causing the hind leg to step forward and under the body. This is also the reason why dressage test sheets include marks for the rider on ‘effectiveness of the aids’, ‘position and seat of the rider’ and ‘correct application of the aids’.