Kissing spines.

Adams’ ‘Lameness in Horses’ (1987)1 describes kissing spines as a condition of the vertebral column in horses caused by overlapping and/or impingement of the dorsal spinous processes (DSP) in the thoracic and/or lumbar vertebrae. 

Kissing spines lead to pain in the back with the associated avoidance behaviour, such as reluctance to be saddled, and inability to perform in the intended sport of equitation or racing. The etiology (origin of the condition) in this edition of Adams’ is mainly observed to be due to injury in the back, with some ambiguity as to other causes. As yet, it does not mention training as one of the potential causative factors. 

More recently, this condition has been established as one of the underlying osseous changes of the vertebrae, causing primary back pain in horses, together with spondylosis (fusion of vertebral bones), degenerative joint disease (DJD) and intervertebral disc disease, and is more frequently seen in show jumping and dressage horses.

The location is frequently observed between T13 to L2, and riders report the feeling of loss of power from the hindquarters ‘not coming through the back’, lack of clarity in the flying changes, especially tempi changes, in downward transitions and during quick changes of direction, or simply the horse not accepting the rider on their back.

On palpation, horses suffering from impinged DSP (kissing spines) will exhibit pain over the area of the processes, as well as over the para-spinal muscles, which are typically very sensitive to touch.

Radiographs at times are inconclusive, because some clinically normal horses can show diminished spaces between the dorsal spinous processes without the horse showing any symptoms.

A large field study on a population of 805 horses failed to establish a clear causative correlation between back pain and lameness, and between lameness and back pain2.

However, kissing spines or DSP impingement is clearly seen more frequently in performance horses, and should be considered in all cases where back pain is leading to changes in ‘rideability’ and the horse’s general performance.

Questions need to be asked as to the increased occurrence of this type of pain and skeletal change, particularly today, when the quality of performance horses specifically bred fit-for-purpose has increased significantly.

Anatomical considerations

The skeleton of the horse shows clearly the horse was not made to carry a rider on their back.

The construction of the vertebral column, supported by front and hind legs, is that of a bridge with sufficient stability (mainly passively via the nuchal ligament and superspinous ligament dorsally, and the white line ventrally) to support weight suspended underneath (GIT tract, foetus) and the horse during straight forward movement (see Image A).

This passive stability is supported by the direction of the dorsal spinous processes along thoracic and lumbar sections of the vertebral column. The first 14 DSP of the thoracic spine are directed toward the back, effectively functioning as a lever for this area, when the neck is lowered during grazing.

The 15th thoracic vertebrae is the area of a directional change (anticlinal vertebrae), after which the DSP are directed upwards and slightly forwards, before changing again in the sacral area. The latter arrangement, again, aiding in the ‘lifting of the back’ from behind, when the haunches/hind legs step under the body (see Image B).

When placing weight upon this structure from above, on an untrained horse, the horse automatically will brace the top line of their body, resulting in a contraction of the musculature, followed by a sinking of the back.

Due to the above described anatomical make-up of this area, this contraction will result in a narrowing of the intervertebral spaces, bringing the DSP closer together, tilting the pelvic area upwards and forwards, setting the hind legs further backwards and away from the body mass. In the front, the thorax (chest) will sink relative to the shoulders and the neck will come up, contracting the area at the base of the neck, in front of the horse’s shoulder blade.

In the training of the riding horse, it is important this passive mechanism is supported and strengthened to counteract the downward forces of the organs suspended within the abdominal cavity and the weight of the rider.

In order to achieve this, the horse has to learn to activate two major muscle chains. The ventral muscle chain is made up of the abdominal musculature, including the psoas group, and the flexor muscles running along the underside of the neck (omohyoid and sternohyoid muscles), and their associated subcutaneous fascia on the shoulder and the tensor facia latae around in the stifle area. This muscle chain has to be activated.

The dorsal muscle chain is formed by the neck and back extensors, including the multifidi muscles and the brachiocephalic muscle on the underside of the neck. Interestingly, the functional and anatomical connection of these two muscle chains is in the area of the hyoid bone (forming part of the base of the horse’s tongue).

This is important to understand because only with a relaxed and ‘chewing’ tongue base (i.e. hyoid) is the horse able to relax their jaw and poll, in turn coordinating supportive tension and relaxation along the ventral and dorsal muscle chains.

Achieving and maintaining this reaction to the bit is one of the cornerstones in classically correct equitation.

Images C, D and E (scroll down to download the illustrated version of this article), show the interplay of the dorsal and ventral muscle chains in three situations. Image C depicts a horse at rest in a neutral position. In this position, both muscle chains are relaxed and utilising a minimal amount of energy. The back is relaxed, and the horse is resting and not carrying a rider.

Image D shows a horse with a head and neck position that allows for the hind legs to step under, due to an engaged ventral muscle chain which, in turn, causes the dorsal muscle chain to function with the rhythmical ‘relaxed’ activity, which will show a swinging, oscillating back of a well engaged and ‘through’ horse – the aim of early dressage training.

Note, the neck is relatively long, the underline of the neck and jaw are open, the nose is just in front of the vertical, but with the nose at a level of the point of the shoulder. The engagement of the ventral muscle chain, and its connection via the abdominal musculature and facia, allows for a backward tiling of the pelvis and a stepping under of the hind legs.

Due to the connection of the long back muscles to the dorsal aspects of the last cervical vertebrae, the ‘telescoping’ forward of the base of the neck from in between the shoulder blades is vitally important to the correct positive tension of the dorsal muscle chain, allowing for a lifting of the back.

Image E shows the reaction of an untrained horse having to carry a rider or that of a badly trained horse, with neither muscle chain working correctly, the back legs camped out behind, tensed muscles and a sunken back.

Horses travelling like this will show lack of rhythm and short steps. In this position, the DSP will be forced closer together, laying the foundation for potential impingement and pain.In addition, when looking at the skeletal development of the horse, one can see whilst some of the long bones supporting front and hind legs are maturing from six months to four years, the fact is the vertebrae of the back do not mature fully until the horse reaches seven years of age or older in some individuals3.

Not taking any other factors into account other than skeletal maturity, it becomes obvious that adding a rider’s weight on the back of a young horse puts un-physiological strain onto as yet insufficiently mature bone tissue, and further sets the horse up for premature health issues.

Furthermore, bone growth in the long bones of the limbs is directed upwards, with the bony precursor cells that later develop into solid bone being aligned along the area of greatest stress, i.e. directed more vertically. So, whilst adding a rider’s weight is still not ideal, at least the stress is directed in line with the developing bone. This is not the case with the vertebral column, where bone growth is directed more horizontally and any weight added from the top will act as a force that is perpendicular to the direction of growth.

Most performance horses are backed at the age of two, three or four years old, possibly due to the pressures of their planned performance careers, which forces the riders, trainers and owners into a set time frame to achieve each competitive performance milestone4.

In order to ensure the proper development of a riding horse, it becomes apparent horses have to be trained and conditioned, both physically and mentally, in a knowledgeable, careful and patient manner, in order to avoid musculoskeletal issues; resulting in a horse which is capable to excel in equitation for many years, without taking undue damage to their body or mind.

It is to this end, the understanding of the principles of dressage training, as described by the Old Masters, becomes vitally important. Even prior to the advent of scientific study in equitation, and with possibly a limited knowledge of functional anatomy, but based on decades, and possibly centuries, of observational experience, a training system and philosophy was developed, which takes into account the above-mentioned challenges of the horse’s skeletal maturation and the need for complete re-training of its natural way of going, having to carry a rider without taking damage.

Gustav Steinbrecht said in his book ‘The Gymnasium of the Horse’: “The training of the horse is based on gymnastic exercises according to the laws of nature, through which the [horse’s] whole muscular system is being conditioned and put into a direction, which will be useful to the rider.” If you exchange the term ‘laws of nature’ with ‘functional anatomy’, it becomes apparent any horse needs to be trained with sufficient knowledge of functional anatomy and biomechanics.

To train a horse to be ridden without suffering damage, according to the Old Masters, a period of basic training of up to two years consisting of lungeing, in-hand work and ridden work is scheduled.

This training will teach the horse gradually to adopt and maintain a posture of ‘artificial equilibrium’, or balance, which results in its ability to carry a rider with ease. To achieve such balance, the trainer uses a system of varying amounts of pressure and pressure-release signals with legs, hands and the rider’s weight – collectively called ‘the aids’.

Once the horse has understood and is able to maintain this new balance, they are able to shift their centre of balance further toward their haunches and can take more weight onto their hind legs by learning to flex the large joints (the coxofemoral joint and the stifle joint), which is needed for collection. In addition, they can travel relatively straight (i.e. their hind legs follow the track of their front legs on straight lines, as well as on a circle, and take evenly-sized steps and push with similar force), and they are able to work in self-carriage, whilst carrying a rider through various exercises in all three basic gaits5.

The Old Masters understood this process cannot be rushed, it is different for each horse, and the training schedule has to be individualised according to the conformational and temperamental challenges given by each horse, and should not be submitted to the constraints of competition schedules.

As Heuschmann writes: “The back [of the horse] has been regarded as the critical centre of movement in the horse since 1896” and “The whole trunk musculature in a correctly ridden horse is designed for movement – his back was not built for direct weight-bearing”.

A recent study at UC Davis using radiographic imaging of the equine vertebral column compared the position of the DSP of a horse with their back in a neutral position with that of a horse mimicking an ‘engaged’ core (using their dorsal and ventral muscle chain). The x-rays clearly add visual and quantifiable evidence the spaces between the DSP are enlarging significantly along the thoracic and lumbar vertebrae, and especially at mid thoracic level.

Knowing the horse will not be able to protect their body naturally when carrying a rider, correct basic training following the above outlined principles becomes the responsibility of each trainer and rider for any horse destined for a long and healthy ridden life.

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List of references:

  1. Stashak T.S. Adams’ Lameness in Horses. 4th edition. (1987) Lea & Febiger
  2. Landman M.A.A.M. et al. Field Study of the Prvalence of lameness in horses with back problems. (2004) Veterinary Record. 155, 165-168.
  3. Bennett D. Timing and Rate of Skeletal Maturation in Horses. With Comments on starting young horses and the state of the industry. (2008).
  4. Dyson S. Lameness and Poor Performance in the Sports Horse: Dressage, Show Jumping and Horse Trials (Eventing) (2000). Proceedings of the Annual Convention of the AAEP. 308 – 315.
  5. Zich A. & Ohms D. Calme, En Avant, Droit. Ruhig Vorwaerts, Gerade (2007). WuWei Verlag.

Further reading about kissing spines:

  • Heuschmann G. Balancing Act. The horse in Sport – An Irreconcilable Conflict? (2011). Trafalgar Square.
  • Lin Y. Lumbar disease and Lameness in the Horse. (2010) In: Yang Z, Xie H. ed. Traditional Chinese Veterinary Medicine. Reddick FL Jing Tang. 253-262.
  • Henson F. Equine Back Pathology. (2009) Oxford, UK Wiley Blackwell
  • Developing your horse’s back. The biomechanics of engagement. Equitopia.
  • Racinet Jean-Claude. Falling for Fallacies. Misleading Commonplace Notions of Dressage Riding. (2009). Cadmos.
  • Stodulka R. et al. Medizinische Sattellehre. (2013) Olms.

This article about how to avoid kissing spines was published in Horses and People March 2018 magazine.