When out on a hack one day, a friend of mine suddenly had what can only be described as a philosophical epiphany: we’d been mooching along the lanes in companionable silence for some time when she asked: “Have you ever wondered how bizarre it is how we just jump onto horse’s backs and ride them around? I mean it’s not like any other animals jump on another animal’s back and ride them from A to B. Like you don’t see a monkey riding a dog in the wild, do you?” Well, no, I thought, I’m sure there must be some examples of animals taking advantage of others’ size and speed - hitching rides à la Uber - but nothing to the extent of the human species’ relationship with the horse, particularly when it comes to the bridle and saddle. Outside of those questionable European circuses, I’ve yet to see a capuchin monkey tack up a border collie, climb onto a purpose-made seat and yell “Hi Ho Rover, away!”
Relatively speaking, man’s use of the horse as ridden transport is only a recent development in the long evolutionary line of both species, and the earliest examples of saddles as we would recognise them date to around 500BC. These were created to accommodate a rider who was hunting and fighting, providing them with maximum support to enhance their abilities in battle or in the pursuit of supper, so I would argue that these early pioneers of equestrianism were not wholly interested in what lay under the surface of the horse’s skin!
Thankfully, these are modern times and the developments in our curiosity and knowledge of the horse’s body are pronounced and geared towards welfare. Developments in scientific methods have meant that we are better able to understand the biomechanics involved and this has led to a huge change in manufacturers’ standards and designs to improve efficacy of all saddles. But the proof is in the fit.
In this first part of ‘The importance of saddle fit’, we explore the anatomy and physiology directly and indirectly affected by your horse’s saddle.
Vertebral Column
Forming most of the axial skeleton, the vertebral column not only provides the whole skeleton with a central structure, but also supports the horse’s bodyweight and provides attachment for the ribs in order to form protection for the internal organs. It creates a strong connective bridge for the horse to transfer energy from the powerhouse hindquarters to the forelimbs and provides attachment for ligaments and muscles. It is the last part of the skeleton to fully mature, at around seven years of age. Each vertebra contains a central foramen (see images below, numbered 3) which aligns with those before and after it to create an uninterrupted channel - through this, the spinal cord runs from the brain to the tail.
The saddle sits in an area along the thoracic spine back from around the eighth or ninth vertebrae to as far back as the eighteenth - the whole area is fairly rigid and there is a limited degree of movement. The transverse processes (see above images, numbered 2) of the thoracic vertebrae are relatively short, so a saddle will not (if correctly fitted) impede in any way on these bony structures. However the lumbar vertebrae, which follow on from the thoracic in the vertebral column, have much broader transverse processes; any pressure on these can create discomfort and pain, so a saddle should ideally sit well before the lumbar region of the spine. Above left is an image of the eighteenth thoracic vertebra, to the right is the first lumbar vertebra, clearly showing that although the dorsal spinous processes (numbered 1) are similar in height the transverse processes are incredibly different.
The saddle panels sit over the ribcage, although the area they cover will vary between styles. Research has shown that when a foreleg is in stance and the opposite hindleg is protracted, the ribcage rotates upwards away from the movement of the hindleg, increasing pressure under the saddle on the side of the protracting hindleg (Murray et al. 2019a).
Muscles and Ligaments
A quick note on origin and insertion of muscles: An origin is where the muscle begins, on a fixed bone; an insertion is where the muscle is attached to a bone which is brought towards the less mobile bone.
Longissimus dorsi
The longest muscle in the horse’s body, the Longissimus Dorsi originates from the Ilium, the supraspinous ligament and the dorsal spinous processes of the first to third sacral vertebrae, lumbar and thoracic vertebrae. It runs caudo-cranially (from back to front), gathering small clusters of reinforcing fibres as it inserts at the transverse processes of the lumbar and thoracic vertebrae, before passing beneath the scapula to insert at the last four cervical vertebrae. It is one of the muscles to define top line and is responsible for extending the spine and lateral flexion, as well as being involved in kicking, jumping and rearing. It facilitates the transference of power from the rear of the horse to the forelimbs and stabilises the spine during movement (Groesel et al. 2010), allowing the limbs to move freely.
Serratus Ventralis
Originates from the lateral surface of the first eight to nine ribs and inserts on the medial edge of the scapula, which the muscle helps to retract. It also draws the whole trunk of the horse level when a forelimb is planted.
Latissimus Dorsi
Another muscle which is continuous with the Thoracolumbar Fascia. The Latissimus Dorsi originates from the thoracic portion of the Supraspinous Ligament and inserts at the Humerus and often the Scapula as well. It is responsible for flexion of the shoulder joint and flexion of the back. Along with the Iliocostalis, it is in direct contact with the saddle panels.
Iliocostalis
Originates at the transverse processes of the lumbar vertebrae and lies parallel to the Longissimus Dorsi, running caudo-cranially, attaching to the dorsal portion of the ribs until it finally attaches to the sixth cervical vertebrae. The iliocostalis supports both the ribcage and the Longissimus Dorsi and problems within it will lead to a possibility of rib position altering. The Iliocostalis is directly in contact with the saddle panels.
Multifidus
The Multifidus is the main deep muscle responsible for aligning and stabilising the individual joints of the vertebral column, connecting the spine from the poll to the tail. It consists of composite units which span from between two to six vertebrae at a time and it corresponds to the shape of each vertebral region. Attachments are formed by short fibres extending from the lateral aspect of individual vertebrae and inserting into the dorsal spinous processes of neighbouring vertebrae. The multifidus makes small adjustments along its entire length to support posture.
Spinalis Thoracis
The Spinalis Thoracis lies medially to the Longissimus Dorsi against the spinous processes of the lumbar and thoracic vertebrae, the latter of which it also inserts into. It is so closely related to the Longissimus Dorsi that it is difficult to pinpoint its origin, although it begins around the dorsal spinous processes of the lumbar region. Its main function is to help extend the thoracic and lumbar spine and due to its proximity to the Longissimus Dorsi is automatically implicated in any painful back.
Other muscles originating from the thoracic region include:
Thoracic Trapezius – this draws the scapula back and up, elevates the shoulder.
Supraspinatus – helps forelimb move forwards and acts as a support.
Infraspinatus – brings forelimb back.
Deep pectoral – retracts Humerus, pulls upper end of scapula forward.
Latissimus Dorsi – lateral flexion of back, flexion of shoulder joint.
Serratus Ventralis – draws the trunk to the proper level when the forelimb is planted, aids in respiration, draws scapula back.
Ascending Pectorals – draws forelimb back.
Don't forget that muscles work in groups and can have an influence on the way others move nearby; there are multiple layers of deeper muscles and these will be affected by any superficial restrictions, too. As I discussed in my last post about fascia, we know such restrictions have a knock-on effect elsewhere in the body and any issues originating from the back are therefore highly likely to affect any or all of the muscles mentioned above, in turn affecting muscles not directly in contact with the saddle, e.g. the superficial gluteals, brachiocephalicus and splenius.
Longitudinal and Nuchal Ligaments
The Supraspinous is the most superficial of the longitudinal ligaments and is easily palpable either side of the vertebral column. It joins every dorsal spinous vertebra from the withers to the sacrum, simultaneously interacting with fascia. Its main function is in stabilising the spine by keeping the vertebrae in place and restricting the movements of their dorsal spinous processes. As it progresses caudo-cranially to the withers it merges with the Nuchal Ligament, which runs all the way to the poll: the Nuchal ligament helps carry the head and neck, reducing the need for muscles to support these weighty structures. Although it does not come into any direct physical contact with the saddle (unless the fit is inadequate - more on that next time!), the Nuchal ligament can be influenced by issues suffered along the Supraspinous.
The Interspinous ligament provides further support and stability to the vertebral column. Its short fibres attach diagonally so as not to interfere with the extension or flexion of the back.
The Ventral Longitudinal is a powerful ligament supporting the thoracic, lumbar and sacral regions of the spine. As its name suggests it attaches to the ventral aspect of the vertebral bodies from T5 towards the tail.
Skin
The horse’s skin (the complete integument) is its biggest organ. It serves as a thermoregulatory, waterproof barrier to infection and its cutaneous sensory receptors detect changes in heat, cold, pressure and pain. Skin is so versatile that whilst its pigmentation protects against sunlight it is also key to the production of Vitamin D. The upper layer – the epidermis – is constantly dying and being replaced by fresh new cells. The deeper layer - the dermis - sits above the layers of subcutaneous fat covering muscle tissue.
Nerves
Nerves are everywhere in the horse’s body and they relay messages to and from the spinal cord and thence to the brain. As each nerve leaves the spinal canal it divides into dorsal and ventral branches which manage muscles above and below the spine respectively. Nerves operate through intermittent impulses and any consistent interruption through pressure (from an ill-fitting saddle, for example) can incapacitate them. The skin is the only boundary between these nerves and direct contact with the saddle.
The Dorsal Rami nerves which innervate the Longissimus Dorsi muscle are located directly under the area where the saddle sits, and the nerves responsible for the intercostal muscles of the ribcage can also be affected.
Fascia
We have explored fascia before, so I will be brief here (for more in depth info on fascia, please see my previous post!) Like nerves, fascia can be found everywhere in a horse’s body – it surrounds bones, muscles and organs, allowing them to work seamlessly together. Fascia has a high density of collagen fibres making it strong and resilient, yet it also means that it has an elastic quality which assists and supports muscle movement. Superficial fascia acts as an insulating shock absorber for muscles, separating them from the skin and internal structures. It also facilitates the entry of nerves, lymph and capillaries into muscles. Deep fascia can be found in and around muscle tissue, supporting and connecting them with bones, tendons, ligaments, nerves and capillaries.
The saddle sits on a large area of the thoracolumbar fascia and any restriction or trauma caused by poor fit will result in it becoming far less supple; but because fascia is practically omnipresent within the horse, any trauma in one focal point can affect other parts of the anatomy and physiology remote from the original injury site!
To sum up...
So as you can see, the back is not just a solid mass: our horses’ backs are like suspension bridges, connecting and transferring power from the hindquarters to the forelimbs. Whilst the area beneath us might appear fairly straightforward, there is in fact a complex structure comprising composite parts performing effortlessly in reciprocal relationships: bones, muscles, ligaments, fascia, nerves, skin – an incredibly intricate system upon which we and our saddle sit.
Next week we shall look at how a saddle should be fitted correctly, and the consequences of a poor fit.
References
Murray, R. – Guire, R. – Fisher, M. and Fairfax, V. (2019a) Reducing peak pressures under the saddle panel at the level of the 10th and 13th thoracic vertebrae may be associated with improved gait features, even when saddles are fitted to published guidelines.
Retrieved from fairfaxsaddles.com/fairfaxdownloads/Fairfax_research_feature_issue5.pdf
Groesel, M. – Zsoldos, R. R. – Kotschwar, A. – Gfoeler, M and Peham, C. (2010) A preliminary model study of the equine back including activity of logissimus dorsi muscle. Retrieved from ncbi.nlm.nih.gov/pubmed/21059036
All images are original illustrations by the author.
Jess Eyres
J. Eyres Equine Physiotherapy
September 2020
Comentários