The equine back is gaining popularity as research topic, but it has a long way to go. It’s difficult to research, diagnose and treat because it’s so inaccessible and so complex. The most popular current model of the back is the bow and string introduced in ’46 by Slijper. In this theory the vertebral column (spine) is a bow held in tension by the ventral abdominal wall – the part we call the stomach. Contraction of the abdominal (stomach) muscles, particularly the rectus abdominus, will tense the bow flexing/rounding the back, as will the engagement of the legs underneath the horse (retraction of the forelimbs and particularly protraction of the hind limbs). The string will be tensed (i.e. the back extended/hollowed) by protraction of the forelimbs and retraction of the hind limbs – when the forelimbs flash out in front or the hind legs trail behind. The string is also tensed by the weight of the abdominal organs, hence the many old brood mares with their often sunken backs.
The bow and string theory puts a lot of emphasis on head and neck movement, a subject that people already get very excited about. Theoretically, if the head is lowered, the nuchodorsal ligament will pull on the withers and flex/round the spinal column. Vice versa, lifting of the head will extend/hollow the back.
In dressage the desired position of the head and neck is described by the Fédération Equestre Internationale (FEI) as: “The neck should be raised, the poll high and the head slightly in front of the vertical”, implying a much more upright position than in the natural situation. Both in academia and on forums across the internet the welfare implications of training techniques using neck positions strongly diverging from the natural position are always a popular topic. Studies carried out by an international group have established that there is a significant influence of head/neck position on spine movement. Positions with an elevated neck tend to induce extension (hollowing) in the thoracic region and flexion (rounding) in the lumbar region. Lower neck positions produce the opposite. High neck positions generally lead to reduced movement of vertebrae, especially in the lumbar area, but low-neck positions increase motion. A very high neck position seems to greatly disturb normal motion, much more than a strongly flexed position (Gómez Álvarez et al., 2006; Weishaupt, 2010, Rhodin et al. 2005). The 2006 workshop organized by the FEI in Lausanne discussed the acceptability of the Rollkür or hyper-flexion training technique, in which the horse is ridden with a strongly flexed mid-cervical region that brings the head almost down between the front limbs. The preliminary outcome was that “there was clearly no evidence at the present time that any structural damage is caused by this training exercise, when used appropriately by expert riders” (Jeffcott 2006). The effects on vision, respiration and psychology, I’ll leave to another post…
The great majority of the muscles that attach to bony elements of neck or back run from one part of this skeleton to the other, not attaching to the limbs, prioritising active, internal stabilisation to compliment the passive, internal stabilisation provided by the ligaments. The back will try to compensate abnormal or one-sided loading of it (e.g. by lameness or rider) by increased muscle tension. As a result, painful muscle spasms are common and early clinical signs of back problems even though they are generally secondary in nature.
The different gaits have characteristically different movements of the spine and muscle activity. The walk is largely under the influence of passive mechanisms, where movement of the head, neck and limbs affect the spine kinematics. At the walk the back does not experience the twisting of the thoracolumbar junction seen in the trot and canter. The trot shows a very stable back with a reduced range of movement, and with major restraining influence of the muscles. At the canter, the back is influenced by passive mechanisms as in the walk and is restrained by the muscles in the total range of movement as in the trot. Muscle activity has a restraining function instead of an initiating function. Diagonal support of the trot and canter sees extension and twisting of the spine in the areas where pathologies are often found. Abdominal muscle strength, as well as hip extensors, are important in stabilising the back and preventing these injuries. There are clear relationships between back conformation and movement that are likely to be important to the orthopaedic health of the horse. Another topic for a future post…
