Back (horse)

The back is the area of horse anatomy where the saddle goes, and in popular usage extends to include the loin or lumbar region behind the thoracic vertebrae that also is crucial to a horse's weight-carrying ability. These two sections of the vertebral column beginning at the withers, the start of the thoracic vertebrae, and extend to the last lumbar vertebra. Because horses are ridden by humans, the strength and structure of the horse's back is critical to the animal's usefulness.

The thoracic vertebrae are the true "back" vertebral structures of the skeleton, providing the underlying support of the saddle, and the lumbar vertebrae of the loin provide the coupling that joins the back to the hindquarters. Integral to the back structure is the rib cage, which also provides support to the horse and rider. A complex design of bone, muscle, tendons and ligaments all work together to allow a horse to support the weight of a rider.

The structure of the back varies from horse to horse and varies a great deal by breed, age and condition of the animal.

A horse has an average total of 18 thoracic vertebrae, with five located in the withers. Each thoracic vertebra is also associated with a rib. A horse also has, typically, six lumbar vertebrae. Some breeds, such as the Arabian, will sometimes, but not always, have five lumbar vertebrae and 17 thoracic vertebrae. There appears to be little correlation between back length and number of vertebrae, as many horses with short backs do have the typical number of vertebrae. The length of each vertebra in the lumbar region seems to have the greater influence on the strength of the horse's back.

The horse has no collarbone. Hence the entire torso is attached to the shoulders by powerful muscles, tendons, and ligaments. The spine of a horse's back is supported by muscles, three ligaments, and abdominal muscles. The Spinalis Dorsi originates on the fourth cervical vertebra and inserts beneath the thoracic section of the Trapezius. The Longissimus dorsi originates from the last four cervical vertebrae, and courses along the spine, inserting eventually into the ilium and sacrum. This muscle contracts the spine and also raises and supports the head and neck, and is the main muscle used for rearing, kicking, jumping, and turning. It is the longest and strongest muscle in the body, and is the muscle the rider sits on. The Intercostal muscles begin at the spaces between the ribs and aid in breathing. The external and internal abdominal obliques are attached to the ribs and pelvic bones, and support the internal organs. The Supraspinous ligament is a continuation of the nuchal ligament, which begins at the poll and inserts on the withers. The supraspinous ligament attaches to the tops of the remaining thoracic and lumbar vertebrae. These two ligaments in combination provide a link throughout the topline of the horse, aiding in the elastic movement of the body.

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 underline 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 capable 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.