Direct Equilibrium.—This relates to the balance of the horse when the center of gravity is moved forward or backward. The fore legs of the horse are used for translation, the hind legs for propulsion. The horse naturally bears more weight upon his fore legs than upon his hind. The center of gravity, then, is nearer the fore quarters than the hind, which is therefore favorable to the forward movement of the horse.
If the weight of the horse, by changing the position of parts of the body, brings the center of gravity towards the rear, it is more favorable for the backward movement.
When the center of gravity is forward, the fore legs control the movement of the mass, and the hind legs merely propel. When the center of gravity moves backward, the hind legs become more and more masters of the control of the movement of the mass, and their action then is from the ground up as well as from rear to front as before. In other words, the movements gain in height.
Position of a Horse for the Forward Movement.—
1. When a horse wants to move forward, he naturally puts himself in the most favorable position; consequently he throws his weight forward to his shoulders, and, to do this, stretches out his head and neck.
2. To obtain the forward movement and the accelerations in gait then, the rider must let the horse extend and lower his head and neck; similarly he must raise them to obtain the slower gaits.
3. A horse must always be ready to carry himself forward. This is the most important rule in equitation. The tendency to move forward is called “impulsion.” Without impulsion we can have no influence over the direction. (De Saint-Phalle.)
The horse must go into the bridle. Nearly every vice and resistance offered by a horse is preceded and made possible by the animal getting behind the bit; that is, he refuses to face the bit, or take the pressure of the bit, so that the rider’s hand can find nothing by which he can enforce his demands. (Anderson.)
Lateral Equilibrium.—This relates to the balance of a horse when the center of gravity is moved sideways. It is a case where the horse is made to load one shoulder or one haunch or all one side more than the other. It is used for changes of direction, parallel displacements, etc. The horse being in motion, if we force him to carry the weight of his forehand to one side, the entire forehand tends to be displaced to the same side. This displacement of the horse is obligatory if that of the center of gravity is sufficiently accentuated.