Back Conformation
Horses' back shape can vary greatly from horse to horse. The upper curvature of a horse's withers, back, and loin is called the "topline." The line of the belly from elbow to flank is the "under line" or "bottom line." In terms of the back, both are important; a long underine with a relatively short topline is ideal. The underline is where the abdominal muscles are, which, like in humans, can provide tremendous support to the back when well-conditioned. The topline will vary in length and in curvature, with some relationship between the two. When being ridden, a horse's back may either be stiff or relaxed as it moves, depending on the tension and strength of ligaments, muscles or tendons; and is also influenced by training and physical level of condition. The length of the back may affect smoothness of gait, ability to collect and move with agility, limits how much weight the horse can carry, and can impact if a horse might be capble of being laterally gaited. The height of the withers also varies and affects freedom of shoulder movement, length of stride, and is a major area of concern in proper saddle fitting.
A horse's back and ribcage in cross section is often described as "deep" or "narrow" (sometimes "shallow"), depending on the width of the ribs and the depth of the heartgirth. It can be a "pear" shape, an "apple" or inverted "U" shape, may be wide or narrow, short or long, or combinations of these characteristics. Wider but shorter ribs and loins will usually be stronger than long and narrow ribs and loins. If the ribs "fall off" of the back sharply, the back will be narrow, whereas if the ribs are well sprung, the back will be wide.
The average horse can carry up to approximately 25% of its body weight, but body build and, particularly, back structure, may allow it to carry somewhat more or less. Physical condition also plays a role. A horse that is in good physical condition, with well-developed abdominal and back muscles, will be able to carry more weight for a longer time than one that is not in shape.
There are two primary flaws in back conformation, a "too-straight" or "roach" back and its opposite, a too low or "sway back" (lordosis). Horses may also have "well-sprung" ribs or be too narrow, called "slab-sided." A horse may also have very high bony withers, which is not generally a flaw, though they can make a saddle hard to fit. Too low withers, called "mutton withers," can make it difficult to keep a saddle on without rolling or slipping, and may be correlated to a shorter stride.
A roach back is less common, but is characterized by a back that has insufficient curvature. Such animals will have difficulty with flexion and are often rough-gaited. Conformational defects such as straight shoulders often are correlated with a roach back.
When the span of the back dips excessively in the center, it is called swaybacked (other names include saddle-backed, hollow-backed, low in the back, "soft" in the back, or down in the back). It is the far more common condition, particularly in older horses. A low back may make a horse more prone to a stiff head and neck carriage and usually causes stiffness in the back and difficulty collecting. A horse with a long back and loin, while often considered a trait associated with smooth gaits, is prone to developing a sway back sooner than average. Swayback is caused in part from a loss of muscle tone in both the back and abdominal muscles, plus a weakening and stretching of the ligaments. It is often seen in a broodmare that has had multiple foals; an older horse whose age leads to loss of muscle tone and stretched ligaments; overuse or injury to the muscles and ligaments from excess work or loads; or premature work on an immature horse. Less often, a long-backed horse that in poor condition may develop a sway at a younger age simply due to lack of exercise, particularly if kept in a stall or small pen for long periods without turnout. A sway back often makes it harder for the horse to collect, particularly for dressage and any event that involves jumping. A sway back can also cause soreness to a horse's back because most saddles will "bridge," putting the rider's weight only on the front and the back of the saddle, creating abnormal pressure points, especially over the shoulders and loins. A heavy rider may also put additional strain on already weakened ligaments and muscles. A swaybacked horse is less able to achieve rapid impulsion; which may cause problems in such sports such as horse racing, rodeo and polo. With a properly fitting saddle, a swaybacked horse still can be used as a pleasure horse and as a horse for teaching students.
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