By Alexa Linton, Equine Sports Therapist

Today, we get to hang out in one of the most thrilling, complex and well-built skeletal structures in the body, the thoracic cage. While thrilling might be a stretch unless you’re an anatomy geek like me, suffice it to say that this region and its resilient, flexible function has potential implications in respiration, saddle fit, behaviour, body control, bending, collection, energy levels, the ability to be symmetrical and free in movement, and more. And I’m not just talking about your horse. If you, the rider, cannot move well through your thoracic cage, you’re going to be impacting your time in the saddle more than you realize. 

The main function of this incredible feat of nature that is the thoracic spine and cage is to protect the heart, lungs, spinal cord, and the many essential vessels moving through this region.

There are a few key things we need to highlight before we dive into the anatomy of this region. 

The Horse

First, the healthy functioning of the thoracic diaphragm is dependent on the mobility of the thoracic cage. The thoracic diaphragm is a large structure of muscle that separates the abdomen and thorax and connects to the ribs, thoracic vertebrae and sternum, and is crucial in thoracic respiration, drainage, pressure regulation, digestion, and overall vitality.

Second, our equine friends have no clavicles (collarbones), but humans have two, and these bones have a very special function in our ability to exist in verticality.

And third, when we ride our horses, we’re on top of and potentially compressing this whole unit — it  is all interconnected. The mobility (or lack thereof) of the thoracic spine will impact the mobility of the rib cage, and will impact the mobility of the sternum, and will have an effect on the heart and the lungs, on breathing, on the surrounding muscles, and on all the many places this region attaches via fascia, dura, and circulation, including the pelvis, cervical spine, and cranium. So yep, we’re in big territory here, especially if things aren’t moving.

When we ride the horse, we are sitting on top of and potentially compressing the entire interconnected thoracic area. Photo: Shutterstock/Decade3d

Now on to the geeky stuff! Horses have a lot of ribs, 18 to be exact compared to our 12, with most articulating (connecting) at the head of the rib with two thoracic vertebrae, once with the rib tubercle at the transverse process of the vertebrae (the costotransverse joint), and once at the meeting of the two vertebrae (the costovertebral joint). The sympathetic ganglion chain sits in front of the rib heads. The ganglia of the sympathetic nervous system deliver information to the body about stress and impending danger and are responsible for the fight-or-fight response, meaning that a well-functioning thoracic cage contributes to a relaxed and resilient horse.

In horses, the ribs vary in shape and size depending on function, lengthening and shortening, and the curve increasing and decreasing. The shape of a horse’s barrel, or even how the saddle fits, depends largely on rib shape. If there is a rib issue, such as a subluxation or an embedded rib, the side-bending and rotation mobility of the cage will be impacted, breathing will be affected, and compression of the ribs, even during saddling, will generally be quite uncomfortable.

Let’s not forget about the sternum, which is technically one bone made up of seven separate bones called sternabrae, which connect by cartilage and then fuse. Because of the clavicle, the human sternum also includes the manubrium (the long, flat bone that forms the front of the rib cage). In horses, the lack of clavicle means the connection between their front legs and torso is muscular, with the serratus ventralis thoracis, pectorals assisted by the external obliques, and brachiocephalicus, acting like a sling and suspending the chest between the front legs and lifting the thoracic cage. If this mechanism is not working well, the result can be a dropping of the torso and a sense of the horse being downhill on the forehand. 

The shape of a horse’s barrel, and fit of the saddle, depends largely on the shape of the ribs. Photos: Canstock/Zuzule

Getting back to the sternum, it can, not surprisingly, be negatively affected by pressure from a tight girth. The first eight ribs are attached by the costal cartilage to the sternum, with the cartilage of the remaining ribs attached together to form the costal arch and connected to the cartilage of the eighth rib and therefore, the sternum. In humans, our last two ribs tend to be “floating” ribs, with no cartilaginous attachments to the sternum, but this tends not to be the case in horses and can be a source of discomfort.

The equine thoracic cage attaches to the spine through the 18 thoracic vertebrae. You’ll feel the spinous processes of the vertebrae when running your fingers along your horse’s spine, with the transverse processes buried under muscle tissue to the side of the spine. As you can imagine, this area is susceptible to saddle fit issues, particularly if the gullet of the saddle is not wide enough or if there are pressure points.

As riders, we can often feel an issue in this area from the saddle. First, if this region of the horse lacks mobility, it may feel like you’re riding the equine version of a piece of plywood on one or both sides, or a freight train heading downhill. On careful observation, you will notice the horse having difficulty standing square in the front, a discrepancy between the two sides of the thorax, or you will feel tension or reaction when you palpate around this area. Second, if there is a problem here, such as a vertebra out of alignment, your horse can be very grumpy under saddle, which is understandable. Third, the growth plates in the vertebrae and ribs fuse close to last (usually between five to seven years of age), so there should be no rush to start your youngsters, especially under saddle.

Kissing spine syndrome, also known as spinous process impingement, is where the long upward projecting spinous processes are touching. Most cases of kissing spine are seen between T13 and T18, directly under where the rider sits. The horse may exhibit symptoms including anxiety, discomfort during grooming, difficulty mounting, back stiffness, unwillingness to go in collection, and a loss of muscle mass over the topline. If you suspect this might be a problem for your horse, contact your veterinarian for a thorough exam and x-rays.

The Rider

As a rider, you may underestimate the impact of your own thoracic spine mobility on your effectiveness in the saddle. Stiffness in this area can restrict the easeful movement of your spine, including the sacrum and pelvis, can increase tension or mobility in shoulders or arms, can create a change in your lumbar or cervical curve, or can lead to asymmetry. In response, our horses may need to compensate or even act out to address our immobility, which is not comfortable for them.

Heather Nelson helps a horse restore resilience and functionality in the thoracic cage. Photo courtesy of Alexa Linton

To use myself as an example, the rigidity in my thoracic spine and cage was a major contributing factor to issues within my lumbar and cervical spines, pain in my sacroiliac joint, and even migraines. The more open I was in this region, the more able I was to give subtle and independent aids with my body position and have clear communication with my horse.

Working Together

The best and most lasting changes to my riding have occurred with regular osteopathy sessions. I recommend that every horse and rider work with their favourite manual therapist on restoring resilience and functionality to this area, and include exercises to mobilize both of their thoracic cages in their regular routine. 

I love working with classical training on the ground (my favourite local coach is Heather Nelson who has trained in the Academic Art of Riding) to help horses become freer and more symmetrical through the thoracic cage, and improve their ability to lift up and through, and power from the hindquarters. Personally, I work daily with thoracic rotation, cat pose, and downward dog pose, and optimally attend a dance or yoga class every week. 

More by Alexa Linton

Photo: Thinkstock/GlobalP