The thoracic sling is one of my favorite topics to explain. It is a fascinating and genius apparatus that is essential for balance control in the horse. Unfortunately, it is often overlooked and compromised in many horses. In this article, I would like to explain what the thoracic sling apparatus is all about as well as why it is often underdeveloped or compromised and how you can (re)optimize its function!
This article is a free shortened summary of my full lectures on Equine Biomechanics explaining the sling systems & recoil system. Click here to access the full lectures.
---------- Basic context Equine biomechanics is the study of both kinematics and kinetics. When comparing human and horses, we can see the horse is build differently. Humans are build two legged in a vertical plane. The horse on the other hand is build four legged in a horizontal plane. To remain balance while standing and moving, there is one constant force that both need to deal with, and that is gravity. Gravity is a vertical force that always acts downwards. Since humans are build vertical, it is relatively easy to defy a vertical force as the two will annul one another to some extent. However, since horses are build in the horizontal plane, they need specialized mechanisms to defy gravity and remain balance. As such, the horse is equiped with two sling systems: the sacro-pelvic sling system in the hind end that is mainly tasked with ensuring stability and producing horizontal forces and the thoracic sling system that is mainly tasked to convert the horizontal forces produced by the hind end and translated through the thoracolumbar spine into vertical upwards forces against gravity. In this article I will elaborate on the basic anatomy & biomechanics of the thoracic sling system and how it promotes balance control in the horse. ------------------------------
Unlike humans, horses do not have a clavicle connecting the front limbs to the trunk. Instead, the horse relies upon strong muscles that act like ‘’slings’’ to suspend the chest between the horse’s front limbs – hence the name: thoracic sling.
The thoracic sling of the horse is primarily made up of the M. Serratus Ventralis, aided by the pectoral group. The M. Serratus Ventralis has both a cervical and thoracic portion. The Serratus Ventralis Cervicis is much thicker than its thoracic counterpart. It extends the horse’s neck and draws the upper part of the scapula forward. It can become hypertrophied or more pronounced in case of lower limb trauma, when extension of the front limb is diverted to the trunk muscles.
The Serratus Ventralis Thoracis attaches the inside of the horse’s scapula to the ribcage and can be intimately linked to the External Oblique. Because of this connection, the SVT may also act as a muscle of forced inspiration.
The two muscles (SVT & SVC) together form an elastic support that suspends the trunk to the scapula. When contracting unilaterally, the weight is shifted to the limb on the side of muscle contraction – which relates to the vertical balance of the horse. When contracting bilaterally, they raise the scapula and lift the horse’s thorax – which relates to the horizontal balance of the horse.
Indirectly, both serratus muscles are also very important for proper functioning of the horse’s stay apparatus as they aid in sustaining the horse’s weight when the muscle bulk relaxes.
For the complete function of the thoracic sling, the Serratus is assisted by the pectoral group. The Cranial Deep Pectorals (in some texts also referred to as ''subclavius''. However i consider that name as somewhat of a misnomer as the hores doesn't have a clavicle) adduct the front limb and draw it backwards. When the limb is fixed, they bring the trunk forward. The Superficial Pectorals – sometimes also referred to as ascending pectorals - lie on each side of the sternum and they adduct and advance the front limb. These muscles are preferably smooth, as they store glycogen. However they often display scar tissue and therefore, palpation is important.
The Transverse Pectorals adduct the front limb. Due to their location, the caudal aspect of the muscle they can be influenced by the girth. They are often also quite asymmetrical upon dissection.
And finally, the Posterior Pectorals – sometimes also referred to as ‘’deep, ascending or descending ‘’ – are influenced by the girth and can be quite tender in girth horses.
Together, all these muscles make up the horse’s thoracic sling apparatus. When the thoracic sling functions properly the horse’s chest is lifted, and the horse can move in good vertical and horizontal balance. On the opposite, when it doesn't function properly the horse will on the forehand and out of balance.
To enhance good posture and biomechanics, it is thus extremely important that the thoracic sling muscles are in good condition. Unfortunately, many horses seem to have weak or compromised sling muscles to some extent. So, how come?
First of all, let’s have a look at general management. Horses are made for dynamic movement. When observing primitive horses in the wild, they graze for about 80% of the time, but they also browse for roughly 20% of the time from bushes and trees. This variation in movement and feeding positions provides a healthy upright posture. Especially during browsing, the horse will use its thoracic sling muscles in order to grab a juicy leaf or berry.
Looking at domesticated horses, they often spend a lot of time standing still in stables. This puts a lot of strain on their stay apparatus that wasn’t designed for such long periods of standing still. As the result, the horse will often start leaning into its chest.
Furthermore, domesticated horses often don’t have to search for their food anymore. They are being fed, predominantly from the ground only. By missing out on variation in feeding positions, their thoracic sling muscles naturally develop less.
As such, offering a horse the choice (not force them!) for various feeding positions will naturally create a better tone of the thoracic sling muscles and a more upright posture in self-carriage. This type of management is referred to as Passive Physio® and is key to develop a healthy thoracic sling because ‘’there where the horse spends most of his time, is where we can gain most benefit.’’ – Jessica Krul
Same horse, different feeding positions. Look at the changes in posture.
Second, underdevelopment or dysfunction in the thoracic sling muscles can also occur due to trauma. Many horses have scar tissue at the area of the pectorals. This area is prone to any form of injury as it is at the same height as fences and gates.
Same horse. Left picture was taking during life. Right picture during its dissection
Furthermore, certain disciplines also predispose the horse more to injury in this area because of the athletic demand placed on the horse.
Third, breeding also plays a major role. Unfortunately, horses are often bred for fashion rather than function. They are bred for excessive front limb action and flexibility in the neck which interferes with the horse’s naturally capability to balance itself. Incestuous breeding leads to poor confirmation with lots of structural asymmetries – such as Equine Congenital Vertebral Malformation.
Anatomical imbalances also affect the horse’s biomechanics. For example, many modern horses show a negative Diagonal Advanced Placement in the trot which means that the front limb is still on the ground while the diagonal hind limb is already off the ground.
Fourth, tack can impact the functioning of the horse’s thoracic sling. As mentioned earlier, the posterior and transverse pectorals are affected by the girth. It is important that the girth has the right length, is placed at the right spot and isn’t too tight.
During the dissections, I research the effects of various girths and their placements
Finally, training plays a major role into the (under)development of the thoracic sling muscles. In most training methods, there is a lot of emphasis on concepts as ‘’engage the hind limbs’’, ‘’ride the back of the horse’’, ‘’activate the core muscles’’ and ‘’control the head and neck position’’. However, little is said about the importance of the thoracic sling.
When looking at the horse’s anatomical design, the hind limbs are designed to produce horizontal forces that results in forward movement. The front limbs on the other hand are mainly designed to produce a vertical incline against gravity and thus balance control. So, this is why the thoracic sling is so crucial in ensuring good posture and biomechanics. Put simply, the thoracic sling is inextricably linked to balance control in the horse. Horses weren’t made to be ridden. As such, in the beginning, in response to the unusual added forces of a saddle and a rider, the horse will slightly hollow its back and divert the weight to the trunk. Knowing this, it is our duty to properly prepare the horse for the demand at hand in order to minimize this natural reaction. We can do this through postural groundwork in which we teach the horse artificial balance control in self-carriage through activation of the thoracic sling muscles. Unfortunately, this phase is often skipped and thus the horse gets broken in without being prepared for the demand at hand. The front end succumbs under the additional forces which compromises the thoracic sling and the ability to provide a vertical incline against gravity. The front limbs end up too far behind the shoulder and the joints are at risk – especially the elbow.
Same horse. Ridden on the forehand with clear DAP on the RF/LH diagonal. Upon dissection, the elbow joint showed worn out lesions.
Because of a blocked and dropped front, the hind legs simply have no space to come forward too. They will literally hit a wall. If they can’t go forward, they will go backwards and thus they will increase the extension phase and the push, resulting in fast and choppy strides with pelvic extension as the loin-coiling capacity at the lumbo-sacral joint is compromised. Due to the connection of the Serratus Ventralis Thoracis to the External Oblique, the core of the horse that functions to limited extension in order to support the back, is also compromised. The breathing sequence also becomes disrupted, and the heart rate could potentially be affected as well. The horse might display nervous and tensed behavior. It will be heavy in the reins and the rider usually holds on firmly as otherwise the horse would collapse.
The right hind limb is blocked by the late take-off from the right front limb. There is negative DAP as the right front is still on ground whereas the left hind is already off the ground.
Put simply, when the chest is blocked and the thoracic sling compromised, the hind legs simply can’t engage forward to the midline. The problem is usually worsened by long low and over flexed head and neck positions as well as continuous driving aids in a quest to create more ‘’impulsion’’ that often literally drives the horse into the ground. These practices stem from the outdated ‘’String and Bow’’ theory and should be discarded. A compromised sling system will start a cascade of dysfunction throughout the body as the horse is robbed from the ability to provide a vertical incline against gravity and thus balance control. The recoil system will suffer, and movement becomes heavy instead of light and efficient.
Only through thoracic sling engagement can the horse lift and free it’s chest, support the base of the neck, create good core stability and spinal alignment and allow the hind legs to step forward. It is only then that the horse’s back can carry a rider in self-carriage with minimal effort and maximal coordination and balance.
Same horse. From dysfunction to almost full function in the thoracic sling
In order to (re)train the thoracic sling muscles, training should be considered from neuromuscular perspective. The Serratus Ventralis is innervated by the 5th – 8th cervical nerves and the long thoracic nerve. The pectorals are innervated by the brachial plexus or nerves that branched from the brachial plexus.
Through these nerves the brain signals the muscles whenever their movement is needed. As such, we need to establish or re-set new neuromuscular pathways to unlock the horse’s potential for optimal balance control.
Once established properly, the pathways will remain forever, locked in the horse’s subconscious. Like the pianist whose hands can play the most complicated piece without conscious effort because the body has ‘’remembered’’ the actions by storing it in the subconscious mind.
Establishing or re-setting neuromuscular pathways requires a very delicate and refined approach as essentially, it is working with the horse’s brain. It is all about small, refined movements with conscious effort and free breathing. If you take the time to do this work, you will be amazed by your horse’s potential.
If this article has spiked your interest and you want to learn more about how to optimize the function of your horse’s thoracic sling apparatus, we would like to refer you to Online Support Program.