Movements of the Horse's Back in 3 dimensions:
Lateral Bend, Flexion/Extension and Axial Rotation
Dressage makes particular requirements of the horse's whole spine in terms of coordination, engagement and balance. Those requirements demand great strength and range of motion founded on confidence and harmony between horse and rider. That discussion is HERE.
Here is a pair of movies of Raynyday Maximillian (Morgan gelding, at 14 years) and Vulkan (Württemberger gelding, at 23 years) performing a soft piaffe without any tack so you can see the activity of their backs and rib cages. Once the movies have played through, use your mouse or keyboard arrow keys to step through the movies for a frame by frame view. Movements of these back structures are usually hidden by saddles or may even be inhibited by incorrectly placed or fitted saddles.
For working without tack see Dancing with Horses: Collection on a Loose Rein by Klaus Ferdinand Hempfling.
This movie below (large version 897 K) shows alternating (side to side) movement of the horse's back/rib cage from the rear: Max is working on a square volte and performing haunches-in on four tracks (left rein). Haunches-in offers particularly interesting view of the middle body/rib cage (axial rotation plus bend) and hindquarters/hips (axial rotation of the lumbar spine dominates the motions).
More movies of classical exercises that affect bend and relative elevation postures are HERE.
DEGREES of motion of selected intervertebral joints
The coordinate system developed by Faber and co-workers that does not use a specific vertebra for reference is below the graph from Dr. Clayton's work.
The graph shows measurements made on equine cadavers for the amount of motion (flexion and extension) available for bending. A small amount of flexibility between thoracic and lumbar vertebrae can produce increasingly large displacements of structures from front to rear. This is because each bit of movement at a joint gives a new position for the next vertebra, its joint allows the next vertebra a slight shift, and so on toward the hindquarters. In this way, sections of equine spine can have some advantages of semirigidity (weight bearing, transmission of energy from limbs) as well as advantages of flexibility (flexing and bending for agility or mobility). Adapted from: Hilary Clayton 1999 (February) The Mysteries of the Back, Dressage Today, p. 28.
Another series of studies with direct measurements from living horses is cited in Faber, et al. 2001 and in Haussler, et al. 2001. This group of five papers (the first sets up a coordinate measuring system) documents remarkably complex motions for thoracic, lumbar and sacral areas during each stride cycle. The degrees of motion between thoracic vertebrae in the live horses are larger than in the cadaver experiment.
Click on Diagram above to show at full size.
Colors indicate some regions of the spine.
The relatively flexible neck or cervical spine is blue, the spine of the back (thorax) is red and the loin (lumbar) region is green.
Click on diagram above to show at full size. The author's experiment with computer programmed repositioning of successive vertebrae to produce incremental lateral displacement only in the thoracic spine. An interesting feature of this is that it that it produces the impression of an "even bend from poll to tail." The experimental vertebrae are shifted slightly to the model's right so they do not fall directly on top of the model vertebrae.
Coordinate system (BELOW) for measuring verterbral movement in three dimensions without reference to a particular vertebra. Dressage judges refer to bend as "the laterally arced position in which a horse's body appears to form an even curve from poll to tail" This would correspond to C (yaw in Space Shuttle lingo) in the image below.
"Overbent" is a condition where the neck is displaced relative to the center line of the body and its evaluation usually is in the context of a specific gymnastic exercise such as shoulder-in. "Flexion" is usually used by judges to refer to the neck/cervical spine, especially near the poll: authors in the biomechanics literature talk about this in reference to the whole spine (B). "Overflexed" means behind the vertical with the profile of the face. It may occur with a poll position lower than the crest of the neck or with a contracted or constricted neck carriage.
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. 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.)
Faber, Schamhardt, van Weeren, and Barneveld, (2001B) Methodology and validity of assessing kinematics of the thoracolumbar vertebral column in horses on the basis of skin-fixated markers American Journal of Veterinary medicine, 62, 301-306. Work with skin markers is most accurate with flexion-extension movements.
Haussler, Bertram, Gellamn and Hermanson. 2001. Segmental in vivo vertebral kinematics at the walk, trot and canter: a preliminary study. Equine Veterinary Journal Supplement 33: 160-164. (DIRECT measurements on vertebrae of the sacrum, lumbar spine and thorax using Steinman pins and strain gauges.)
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,
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.