Do Horses Bend?
The short answer is yes indeedy they do! The longer answer is that bending is only one of the movements the spine/rib cage/trunk/neck can perform. The longer answer is HERE.
The rib cage is an ingenious spring which can move in three dimensions limited by ligaments (tie bone to bone) and cartilage (sternum and bottom portions of ribs shown below in bright yellow).
In the picture BELOW, the spine/rib cage/elastic ring is being shaped by the horse to store and then release enough potential energy to launch a rider at a rodeo performance.
The bend in the thoracic spine is evident from approximately the fourth through the 18th (19th) thoracic vertebrae, where the cartilage portion of the ribs (costal arches) plus the sternum could serve to "spring load" the bend with elastic strain. Ligaments also play a role in mediating bend and rotation.
The articulated model I have developed for biomechanics (in color) showed interference of ribs and vertebrae if too much more positioning was asked. Another investigator has worked with equine cadavers to quantify bend, and there is at least 2 degrees of movement available between each thoracic vertebra in a deceased horse. Still other workers have produced a series of papers making direct measurements on the three dimensional activity of the horse's spine during walk, trot and canter and found about 5 degrees of motion available between vertebrae of the thoracic spine in living horses (direct mesurement by markers attached to vertebrae with stainless steel pins rather than skin markers).
Note that the equine spine has many joints. Why? If there were to be no bend, why have joints? Motion between vertebrae is usually measured in cadaver material with ribs removed because of the difficulty in making measurements with the bulky rib cage attached. Rib cage joints to the spine and sternum are limited in motion to a few millimeters or so. A slight shift of position in one is translated to all behind it and the next one in line adds to the shift, and so on until there is a major change along the whole thoracic spine.
Dressage riders note: this horse has engaged (taken weight on the hindquarters) the inside hind leg very effectively!
(Original grayscale background photo from Rodeo Magazine 1989.)
What are general functions of a vertebrate rib cage? It keeps the heart and lungs from being whacked too badly when an animal moves. It may also contribute dynamics of storage of potential energy and release of kinetic energy to portions of the striding.
Among vertebrates, birds and dinosaurs have stiff "boxes" for rib cages: Some dinosaurs (carnosaurs and sauropods included) had sternal ribs for added structural stiffening. Neat-o piece of trivia: horses, carnosaurs and ostriches have surprisingly similar hind legs! There is a question about relative speed and anatomy among tyrannosaurs, ostriches and horses. That is outside the subject on this page but is argued intensely by McNeill Alexander, Gregory Paul and others. A long series of papers by Milton Hildebrand looks at anatomy gait pattern and speed.
Restricted joints and many of them is part of the ingenious equine rib cage and spine design. Another part of the structure is the amount of cartilage (springiness for absorbing gravitational potential energy as a leg is grounded in the stance phase of a stride). Each time a foreleg is on the ground, it allows compression of the rib cage and a swing phase leg may benefit from the conversion to kinetic energy in the amplitude of the next step. Rooney in The Lame Horse discusses some relations between leg position and the energy of gaits.
In technical papers, authors talk about "clockwise and counterclockwise" in terms of movement around an axis (x, y or z). However, judges will talk about "right" or "left" bend. If the horse rotates in a clockwise direction around the z axis, it is concave on its right side (left bend on left rein) and convex on its left side (rider facing the same direction as the head of the horse).
Behavior of the rib cage is illustrated and discussed in detail in terms of concave/convex shape to right or left during bending of the spine in an article by J-M Denoix (Spinal biomechanics and functional anatomy. Veterinary Clinics of N. Am.: Equine Practice Vol. 15 No. 1 April 1999, pp 27-60). The point is made that, because of the shape and linking of vertebrae, a lateral bend is also accompanied by some rotation of the spine. The two illustrations below indicate how this motion occurs.
You can get a feel for this behavior of a rib cage by putting your hand along your own ribs and bending to one side. In contrast to a horse, you have floating ribs and very small lateral processes on your lumbar spine.
You can feel how the bend works by crawling (really a 4 beat genuine tetrapod walk) on hands and knees. Note how your spine/rib cage swings from side to side alligator style. You can prevent bending by locking the ribs with the muscles between them (intercostal muscles) and clamping your jaw. This is exactly what a stiff horse does! Many gymnastic excercises of dressage are aimed directly at strengthening, coordinating and routinizing the various spinal postures that the horse needs to perform test movements.
J-M Denoix Spinal biomechanics and functional anatomy. Veterinary Clinics of N. Am.: Equine Practice Vol. 15 No. 1 April 1999, pp 27-60.
Hilary Clayton 1999 (February) The Mysteries of the Back, Dressage Today, p. 28.
Faber, Schamhardt & van Weeren Johnston. 1999. Determination of 3D spinal kinematics withouth defining a local vertebral coordinate system . Journal of Biomechanics 32: 1355-1358 (Technical Note).
Faber, Schamhardt, van Weeren Johnston, Roepstorff & Barneveld. 2000. Basic three-dimensional kinematics of the vertebral column of horses walking on a treadmill. American Journal of Veterinary Research 61(4): 399-406 (DIRECT measurements on vertebrae of Dutch warmblood horses in walk.)
Faber, Schamhardt, van Weeren Johnston, Roepstorff & Barneveld. 2001. Basic three-dimensional kinematics of the vertebral column of horses trotting on a treadmill. American Journal of Veterinary Research 62(5): 757-764 (DIRECT measurements on vertebrae of Dutch warmblood horses in trot.)
Faber, et al. 2001. Three-dimensional kinematics of the equine spine during canter (on a treadmill). Equine Veterinary Journal Supplement 33: 145-149 (DIRECT measurements on vertebrae of Dutch warmblood horses in canter.)
Schusdziarra, H., and V. Schusdziarra. 1986. Reitergesprache: Der Weg zum Unabhängigen Sitz. Pareys Reiter un Fahrerbibliotek, Berlin.
(Conversations with Riders: The Way to an Independent Seat)
This small paperback volume is still in German, but has a series of questions and answers from riders about their seat mechanics generated from the original book, An Anatomy of Riding. There are illustrations showing an unmounted but saddled horse in the walk with the stirrups swinging side to side as the ribcage/spine moves during gait. They offer this as a route to understanding lateral bend.
Schusdziarra, H., and V. Schusdziarra. 1985. An Anatomy of Riding. Breakthrough Publications, New York.
Translated from German by Sandra Newkirk, this book by father and son physicians focuses primarily on humans as they are challenged by the need to coordinate biped structure and function with equine structure and function. There is a thoroughly described set of seven exercises toward the end of the volume. It is intended for instructors and students. The "spiral seat" advocated by these doctors is explained in detail. It is illustrated with technical drawings and photographs of horses ridden with versions of the equestrian seat.