To remove some ambiguity, here, I've added the decription of the Military Seat from the Manual of Horsemanship and Horsemastership, Vol. 1, Education of the Rider. Here is Chapter 2, The Miliary Seat, in its entirety.
Due to length, it is in two parts.
The Military Seat
3. THE MILITARY SEAT.‑-a. General principles. Seat is that quality which permits the rider to remain master of his equilibrium, whatever may be the actions of his horse. The seat for military purposes must be secure in itself, that is, independent of other means of security, and must provide ease and comfort for both rider and horse. Such a seat is dependent upon balance, supplemented by suppleness and muscular control.
The military seat, while obligatory for military purposes, is also admirably adapted to all kinds of riding, requiring only an appropriate adjustment of stirrups.
In order that the detailed discussion of seat, which follows, may be clearly understood, it is necessary first to consider the seat from a purely mechanical standpoint. The principal elements entering into this consideration are, the rider's upper body, his base of support, his legs and the horse.
The upper body isconsidered to mean that part of the body from the hip joints up.
The base of support is formed by those parts of the rider's body in contact with the saddle and horse, from the points of the pelvic bones down along the inside of the thighs, to and including the knees, legs, and stirrups. The fleshy parts of the buttocks are to the rear and in no case form part of the seat.
The leg is that part of the limb between the knee and ankle.
The horse is the active element, and supplies the varied impulses to which the different parts of the rider must react.
It is quite evident that the rider's body, receiving impulses from the horse, is constantly tending toward a state of unstable equilibrium, and can remain stable only by balance and the clinging of the knees and thighs, reinforced by a sufficiently strong leg grip. The use of some outside means, such as the reins is wrong. Balance obviates the necessity for leg grip, saves the legs from undue strain and fatigue, and is therefore the principal requisite of a secure military seat.
Balance requires that the center of gravity of the rider's upper body remain as nearly as possible over the center of its base of support. When in motion this center of gravity should be advanced to compensate for the movement of the horse. The more the center of gravity departs from the center of its base of support, the more unstable becomes the rider's equilibrium. When the center of gravity passes outside the limits of its base of support, the rider's balance is lost and he regains it by gripping with his legs, supplemented by the clinging of theknees and thighs. A poor rider makes the gravemistake of pulling on the reins as an additional means of regaining balance. The seat must be entirely independent of the hands.
b. Position mounted.‑(1) The rider sits squarely in the middle of the saddle, his weight distributed from the points of his buttocks forward upon his crotch, inner thighs, knees, and stirrups.
At the halt, the upper body must be erect. Its center of gravity is slightly in front of the points of the buttocks, facilitating the correct placing of the thighs, and the proper distribution of weight. When in motion, to be in balance, it is inclined forward from the hips. Inclining the upper body to the rear, or convexing the loin to the rear, places the center of gravity of the upper body in rear of its base of support, and causes the rider to sit on the fleshy parts of his buttocks. This faulty position raises the thighs and knees, weakens the seat, concentrates the weight toward the cantle, is unmilitary in appearance, injurious to the horse's back and places the rider behind his horse.
(2) The thighs extend downward and forward, their inner sides resting without constraint on the saddle. With the buttocks and upper body placed as in (1) the thighs are naturally forced down, and the throat of the saddle comes well up into the rider's crotch; the large fleshy muscles of the inner thighs are thus forced to the rear, and the flat of the thigh is permitted without muscular contraction, to envelop the horse. Thus seated the proper proportion of the rider's weight is distributed along his thighs, and the tendency to grip with them is avoided.
If the thighs are turned inward too much, so as to pinch the saddle with the knees, the heels are forced out, and the legs assume an incorrect position and loose their proper contact. .
If the thighs are turned outward excessively, contact of knee and lower thigh with the saddle is lost; the rider bas neither the correct distribution of weight nor the proper base of support; lack of security and instability result.
(3) The knees are forced down as low as the adjustment of the stirrups will permit. Flexed and relaxed, they rest with their inner sides in continuous contact with the saddle.
Properly placed thighs, as in (2) above, naturally and correctly place the knees. Knees placed too high, or excessively turned in or out, produce the same faulty results mentioned for similar incorrect positions of the thighs.
(4) The legs, ankles, feet and stirrups are disposed as follows:
The legs extend downward anal backward with the calves in light, elastic contact with the horse.
Stirrup leathers are approximately vertical.
The heels are well down, the ankles flexed and relaxed. The feet, turning out naturally, rest with their broadest part upon the stirrup tread. The rider must not normally support his weight in the stirrups. They should sustain a minimum of weight, sufficient only to enable him to easily maintain them.
The length of stirrup is approximately correct when the tread hangs opposite the lower edge of the ankle bone, the rider being seated as described, his feet out of the stirrups, with the legs hanging naturally.
(a) Legs‑The calves naturally fall into tie contact with the horse when the knees are flexed and relaxed, and the ankles, feet and stirrups are disposed as described. This contact is a means of communication‑ between rider and horse and assists security. When the legs are not in contact, communication is lost. The resultant swinging of the rider's legs confuses a well‑trained horse and irritates a nervous one. Correct adjustment of stirrups assists materiallyin preserving leg contact.
(b) Stirrups.‑Stirrup leathers for special forms of riding may be longer or shorter than described. For schooling, a longer stirrup should be used. For show jumping, steeplechasing, and racing, the stirrups should be shortened. Too long a stirrup diminishes the rider's base of support, renders balance difficult, reduces stability and interferes with the proper use of the legs. Too short a stirrup elevates the knees excessively and may either place the rider behind his horse, with his weight on the cantle, or cause him to stand in his stirrups in order to keep in balance.
All the requirements of military riding may be met by adjustingthe stirrups as described above.
(c) Feet and ankles.‑Ankles should be relaxed, in order that the downward thrust on the stirrup may cause the heels to sink below the level of the toes, and the ankle joint to flex freely with the movements of the horse. Ankles which are stiff or not relaxed cause the rider to have the heels too high. Stiff ankles result in unsteady legs, frequent loss of stirrups and restrict the rider in the proper employment of his legs in the control and management of his horse. "Toes turned in" stiffen the ankles, cause loss of contact of proper part of the calf of the leg, and throw the heels out, thus reducing the security of the rider and making the proper use of the leg difficult. "Toes turned out" (excessively) stiffen the ankle, put the knees out of contact, place the rider on the back of his thighs and lower legs, and may cause unintentional use of spurs.
The toes should not press down on the sole of the boot, but on the contrary should be relaxed thus aiding materially in obtaining a relaxed ankle.
(d) Without stirrups.‑The legs hand in a natural manner; knees and ankles are relaxed; feet are turned out naturally; toes are lower than heels. When the horse is in motion, the flexion of the knees is increased, and the legs come into light, elastic contact with the horse.
(5) The upper body is naturally erect without stiffness.
When seated correctly the rider maintains his back without stiffness in a position identical with that of the naturally erect dismounted soldier at attention. The hip joints are relaxed to enable the rider to remain in balance. If the upper body is not erect, but is inclined too far forward, an improper distribution of weight results, the rider becomes unbalanced and his legs slide too far to the rear. On the other hand, the center of gravity of the upper body should never be in rear of the points of the buttocks.
(6) The chest should be lifted, the shoulders square, without stiffness, and carried back evenly.
Lifting the chest, with the shoulders as described, facilitates the maintenance of an erect posture of the upper body. Rounded shoulders cramp the chest, invite a general slumping of the back and loin, and cause the elbows to fly out from the body. Shoulders forcibly carried back result in general contraction.
(7) The arms are free and relaxed, the elbows falling naturally.
A natural relaxation of the arms insures freedom and quietness in the use of the hands. Contraction quickly communicates body movements through the hands to the horse's mouth, resulting in the loss of, that calm confidence which the horse should always have in his rider.
(8) The reins are held in either, or both hands, fingers relaxed, knuckles about 30 degrees from the vertical.
(a) The reins in both hands.‑The hands, fingers relaxed, are well separated, and held normally above the withers.The wrist is straight and supple, and the forearm, wrist, hand, and rein form one straight line from point of elbow to horse's mouth, the elbow being slightly in advance of the point of the hip. This position will vary from time to time in guiding or controlling the horse, but, with reins properly adjusted, the elbow should never pass in rear of the hip. Unsteady hands quickly communicate unintentional impressions to the horse's mouth, making him nervous and difficult to control. Relaxed elbows, permitting a soft and elastic opening and closing of the elbow joint, enable the rider to follow the movements of the horse's head and neck which is essential to good hands.
The inexperienced rider should guard against turning the knuckles up and using a play of the wrist joints to follow this movement. The straight line existing from the elbows to the relaxed fingers should not be broken.
When riding with a single snaffle bit, this softness is further facilitated if the reins are taken into the hands between the third and fourth fingers.
(b) If only one hand is used, the free arm hangs naturally
(9) The neck is erect without stiffness, headand chin up
The naturally erect position of the upper body is continued in the neck. Stiffness of the neck quickly communicates itself to other parts of the body and must be avoided. If the neck is carried forward the resulting tendency is to round the shoulders and back to the rear, which is faulty, unsightly and unmilitary. The chin is held up without being thrust out.
(10) The eyes look to the front.
Eyes looking to the front, besides being soldierly, enable the rider to avoid many difficulties, and also to take the greatest advantage of constantly changing conditions of the foreground. A horse's movements are sensed by "feel", not sight. Therefore, the rider does not fasten his eyes on his horse. This bad habit results in hanging the head and humping the back, making balance difficult and often destroying the whole seat.
(11) Balance.‑When a rider so disposes hisweight as to require the minimum of muscular effort to remain in his seat, and when the weight distribution interferes least with the horse's movement and balance, the rider is commonly said to be "with his horse" or "in balance". This condition ofbeing "with the horse" is the keynote of riding.
When passing from the halt to motion, and when the horse is moving, the seat undergoes certain modifications. The rider must assume positions which assure his retention of balance and which keep him "with his horse". The knees, legs and to a great extent, the thighs, remain fixed in position. The upper body, the unstable part of the rider's mass, remains in balance over its base of support by appropriate variations in its degree of inclination in the direction of movement, and thus overcomes the disturbing effects of the horse's movements.
Any change in body inclination modifies the distribution of weight on the various parts of the base of support. As forward inclination increases, the center of gravity is carried forward and downward; there is a decrease in weight borne by the rear of the seat, and a corresponding increase in that borne by the thighs, knees and stirrups, until finally, in certain phases of racing and high jumping, the knees and stirrups support almost the entire load.
(12) Inclination of the upper body.‑‑(a) In forward movement the degree of forward inclination of the upper body varies with the speed of the horse. It should always be such that the rider remains in balance over his base of support. When the inclination of the upper body is not sufficient to maintain this balance, the rider is not "with" but "behind" his horse. When it becomes excessive, the rider is not "with" but "ahead" of his horse."
(b) The upper body as a whole is inclined forward from the hip joints. The back should not break to the rear at the loin, but retains its normal posture. The chin is lifted in order that the back may retain its unbroken line and the field of vision be not reduced. To allow the back to break rearward at the loin and to permit the shoulder and head to drop forward, places the weight on the fleshy part of the buttocks and usually tends toward loss of balance o to the rear, and to “cantle pounding.”
(c) Suppleness, muscular control, and the resultant opening and closing of the joint‑angles, supplement the inclination of the upper body and enable the skilled rider "to be and to remain with his horse". In the case of unforeseen movements, such as shying, which tend to unbalance and unseat the rider, security is provided by an increased grip of the lower thighs, knees and legs, until balance has been restored.