Swayback, or equine lordosis, is characterized by a large dip in the spine of a horse, often resulting in a high wither and severe downwards curve to their topline.
Swayback is more common in older horses and broodmares who have carried multiple large babies. Lordosis can also occur in younger horses with a genetic predisposition. 
Equine lordosis is caused by a failure of the ligament support structures along the length of the back. When the ligaments are weak, stretched, or compromised, the spine can sag toward the ground.
Although shocking in appearance, equine lordosis does not usually mean a horse must be retired from all ridden work. With good management, these horses can remain sound and pain-free through their ridden career. 
Swayback horses require appropriate conditioning to strengthen their topline muscles. Special consideration must also be given to ensure proper saddle fit.
Anatomy of Equine Lordosis
Equine lordosis is a ventral deviation of the spinal column towards the ground. It is characterized by a failure of the support structures along the thoracic and lumbar spine.
The support structures holding up the bones of the spine consist of soft tissue, such as muscles and ligaments.
The muscles and ligaments along the length of the spine keep the bony vertebrae suspended between the front and back legs.
You can think of these muscles and ligaments as the cables on a suspension bridge. If the cables of the suspension bridge fail, the roadbed will drop. In horses with lordosis, the roadbed is the vertebrae. 
Supporting Ligaments of the Spine
Lordosis is a partial failure of the ligamentous structures of the spine. The supporting ligaments of the spine can be divided into three groups:
- Nuchal ligament
- Supraspinous ligament
- Dorsal and ventral longitudinal ligaments
Smaller ligaments, called short spinal ligaments, provide support between the individual vertebrae. Working together, these ligaments ensure that the bony spine does not collapse and sag. 
The nuchal ligament runs from a bony point on the back of the skull (the external occipital protuberance) down the neck and into the supraspinous ligament.
This ligament provides passive support for the head when it is held high or dipped low for grazing. 
The nuchal ligament consists of two distinct parts: the funicular part and the lamellar part.
The funicular part is the main chord-like structure that runs down the neck until it becomes the supraspinous ligament.
The lamellar part is sheet-like in structure, extending from the funicular part down both sides of the neck. 
The nuchal ligament is unique in that it contains elastic fibres that stretch and coil, allowing the horse to lift their head off the ground with minimal effort.
This evolutionary adaptation means horses can lift and lower their head without expending energy through muscle contraction. 
The supraspinous ligament provides the main support cable for the suspension bridge-like structure, which is the horse’s back.  It provides support along most of the thoracic region of the spine.
This ligament is a continuation of the funicular part of the nuchal ligament. The supraspinous ligament travels from the wither along the tops of the vertebrae to the first vertebra of the tail.
The supraspinous ligament is made up of non-elastic fibres. The transition from the elastic fibres of the nuchal ligament to the non-elastic fibres of the supraspinous ligament is gradual. 
Horses are large animals with extremely heavy abdominal contents. The supraspinous ligament must be thick and strong to prevent the abdominal contents from dragging the spinal column down. 
Dorsal and Ventral Longitudinal Ligaments
The dorsal and ventral longitudinal ligaments support either side of the vertebral discs, which are crucial for the structure and function of the spine.
The dorsal and ventral longitudinal ligaments reinforce the disks from the top and bottom, preventing the disks from being squeezed out from above or below.
Because of the ingenious design of the longitudinal ligaments, horses rarely experience herniated discs. Even among lordotic horses, rates of disc herniation are low. 
Short Spinal Ligaments
The short spinal ligaments attach the vertebrae to each other, similar to the connecting cables between train cars. These ligaments provide stability between the vertebrae and protect the spinal cord.
The short spinal ligaments include the ligamentum flavum and the interspinous and intertransverse ligaments.
The ligamentum flavum preserves the spine’s normal curvature and resets the spine to its original position after flexion. Like the nuchal ligament, the flavum are slightly elastic and move with little muscular effort. 
The interspinous and intertransverse ligaments are sandwiched between portions of the vertebrae called the spinous and transverse processes. These processes are bony protrusions that provide ligamental and muscular attachment sites.
Ligaments are considered passive support for the spine, meaning they do not require energy to perform their function.
The spine is also supported by muscles. These are considered active support structures because they require energy to perform their job.
Supporting Muscles of the Topline
The topline muscles support the spine to prevent the sagging of the abdomen and support the rider.
The main muscles of the topline are the thoracic trapezius, latissimus dorsi, longissimus dorsi, and multifidus. The muscles of the abdomen also provide support, mainly the rectus abdominus.
Although topline does affect the comfort of swayback horses, equine lordosis is primarily characterized by a failure of the supporting ligaments of the spine.
A horse can have a weak topline but not experience lordosis, and a horse can develop swayback even if they have strong topline muscle
The thoracic trapezius muscle is divided into two sections: the trapezius thoracis and the trapezius cervicus.
The trapezius thoracis falls behind the wither and lies underneath where the saddle would sit. The trapezius thoracis supports the front portion of the spine and keeps the spine from collapsing under a rider’s weight. 
The trapezius cervicus prevents the wither from collapsing. Its primary function is to move the forelimb and it plays less of a role in supporting the topline compared to the trapezius thoracis. 
The latissimus dorsi is responsible for the movement of the forelimbs and provides minor support to the front portion of the back. It acts with the trapezius to support the wither. 
The longissimus dorsi is a major contributor to the topline of the horse. This muscle extends along the horse’s entire thoracic (middle) spine and lumbar (end or near the rump) spine.
This muscle contributes to the motion of the back and provides stiffness to the spine, supporting the abdominal contents below the back.
A weak longissimus dorsi means the horse cannot support their weight or the weight of a rider. Horses with a weak longissimus dorsi will develop back pain and possible kissing spine. 
The multifidus muscle consists of many small muscles along the length of the back from wither to tail. This muscle provides stability for the spine and prevents excess movement between the sections of the spine.
The multifidus supports the abdomen of the horse and prevents the sagging of the back. 
The large muscles of the abdomen help to raise up the back, support the load of a rider and support the contents of the abdomen. 
Structural Implications of Equine Lordosis
When the ligaments around the vertebrae and down the length of the back are slackened, the spinal column and vertebrae will sag towards the ground with the weight of the abdominal contents.
This swayback can look shocking, with affected horses exhibiting an extreme dip in their backs and abnormally low stomachs. This condition is made worse when weight is carried in the abdomen (such as during pregnancy). 
Although extreme looking, horses with lordosis can be ridden comfortably and rarely experience pain.  Lordotic horses can have strong topline muscles that support a rider while still exhibiting a low dip to their backs.
Many horse owners wonder whether a lordotic horse will have a higher risk of kissing spine or overriding dorsal spinous processes.
Horses with lordosis tend to carry their heads higher, hollowing their backs more and bringing the spinous processes of their vertebrae closer. This could be a risk factor for kissing spine.
However, no research is currently available to evaluate whether horses with lordosis are more prone to kissing spine than other horses.
Causes of Equine Lordosis
There are five main causes of equine lordosis: genetics, age, conformation, pregnancy, and excessive strain on the back at an early age.
In these cases, the inheritance is an autosomal recessive pattern. This means that both parents of the affected horse must carry and pass down a copy of the lordosis gene for the offspring to become lordotic. This results in the horse being homozygous or having two copies of the gene.
One study examined the genome of 40 Saddlebred horses, half of whom were diagnosed with lordosis and half of whom were not. 
Researchers looked at a specific gene that tends to make the vertebrae wedge-shaped rather than square-shaped. This makes the vertebrae more likely to collapse together.
17 out of 20 lordotic horses tested homozygous for the lordosis gene, while only 7 of the 20 unaffected horses tested homozygous.
A further study examined the genome of 13 additional horses affected by lordosis and 166 unaffected horses. Of affected horses, 70% were homozygous for a specific genetic variant, 21% were heterozygous (possessed only 1 copy of the gene), and 9% did not possess the gene. 
Among the unaffected horses, 15% were homozygous, 47% heterozygous and 38% did not possess the specific genetic variant.
The researchers concluded there is a strong genetic link to equine lordosis, specifically in American Saddlebred horses. The estimated prevalence of lordosis in American Saddlebred horses is 7%, and early-onset lordosis has a prevalence of 5% in this breed. 
Senior horses are more likely to be affected by equine lordosis. As horses age, structures in the back start to slacken due to gravity.
The slackening effect weakens the entire spinal structure, sometimes resulting in lordosis. 
Some horses are born predisposed to equine lordosis because of their conformation. Horses born with long backs are more likely to develop lordosis.
When the back is excessively long, the supporting pillars of the shoulder and hind end are further apart, meaning the back is weaker and more susceptible to gravitational forces. 
Carrying a large fetus for 11 months can significantly affect the structures and ligaments of a mare’s back. The problem is compounded in broodmares who have foaled several times, sometimes leading to a dramatic swayback appearance. 
Excessive Strain on the Back in Early Age
Although not well studied, it is believed that sitting on the back of a young horse before they are mature can cause lordosis.
The ligaments of the spine take time to develop and become strong enough to carry a rider. Riding a horse too early could stretch the ligaments and increase the risk of lordosis. 
Caring for the Lordotic Horse
The best way to support a swayback horse is to ensure their topline muscles are strong and their saddle fits appropriately.
Horses with equine lordosis are often rideable but need a saddle that fits their abnormal back shape.
To ensure appropriate saddle fit and prevent back pain under saddle, contact a professional saddle fitter and have them assist you in your saddle purchase.
Strengthening your horse’s topline is important for any horse, but especially the lordotic horse to ensure they remain pain-free and can carry a rider without further damaging their back.
You can help strengthen your lordotic horses topline with specific exercises to make the back and abdominal muscles contract. Incorporate the following exercises into your routine:
Long and low work: Asking the horse to drop their head low will stretch the topline muscles and contract the abdominal muscles.
Transitions: High-quality transitions will raise the horse off their front end and encourage them to soften and engage through their entire back while tucking their hind end.
Pole work: Asking your horse to walk and trot over poles will encourage them to raise their back, engage their abdomen, and round their topline.
Rein back: Asking your horse to rein back will encourage them to tuck their hind end underneath their body, load the hind end, and engage their abdomen and topline muscles.
Lateral work: Asking your horse to perform lateral work (such as leg yield, turn on the forehand, and turn on the haunches) will engage your horse’s adductors and abductors (the muscles that move the legs out and in). This will also encourage the horse to soften their topline while carrying the rider.
Hill work: Asking your horse to walk up and down hills engages their hind end, stretches their topline and makes them push and reach with their hind legs, contracting the back muscles.
You can learn more about how to build your horse’s topline muscles here.
With a strong topline and properly fitted saddle, your lordotic horse can have a happy and pain-free ridden career.
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