Understanding the Muscles of Your Horse's Back
- Gillian Higgins
- 9 hours ago
- 7 min read
When we look at a horse's back, it's tempting to think of it simply as the place where the saddle sits. However, underneath that saddle lies a fascinating, complex system of muscles that work together to allow movement, support posture, and help carry the rider. In this article, we're going to take a closer look at three of the key muscles in this region, and once you understand what they do, you'll never look at your horse's back in quite the same way again!
Throughout this article, I suggest various other resources that will help you to learn more. Also, there's an exciting opportunity to join us for a practical day course looking at saddle related anatomy during the our Inside the Horse anatomy exhibition this August.
Why The Back Matters
Before we dive into the muscles themselves, it's worth making one important point: back movement isn't created by the back alone, in fact there’s very little movement in this area. The movement we see in a horse's back is driven by the movement of the limbs and the head and neck. This is a theme we'll return to throughout this article, and it has real implications for how we train and manage our horses.
Muscle 1: The Thoracic Portion of The Trapezius
The trapezius muscle is one you've probably heard of, but it's often misunderstood — particularly the thoracic portion.
The trapezius actually has two parts: a large cervical (neck) portion at the front, and a smaller thoracic portion that attaches onto the upper third of the spine of the scapula (shoulder blade). The thoracic part is notably smaller than the cervical portion, and because it's the most superficial (closest to the surface) muscle in this area, it's one you can actually feel if you palpate along the back part of the shoulder.
What Does It Do?
The thoracic trapezius is part of the forelimb protraction chain, a group of muscles that work together when a forelimb reaches forward (protracts). Horses don't have a collarbone like we do; this means the shoulder blade is able to slide and glide over the structures beneath it. As the leg is reaches forward, the thoracic trapezius helps to stabilise the back part of the scapula, holding the rear part of the top of the scapula back.
Now, here's what makes it especially interesting: this muscle isn't only part of the protraction chain — it also connects to the back muscles through the extensor chain. Try this yourself: lift your arms above your head and notice how your back muscles contract. Then lower them and feel your abdominal muscles shorten. This same principle applies to the horse. As the forelimb comes forward (protraction), the back muscles and extensor chain are stimulated.
A Role In Lateral Stability
The trapezius also plays an important role in lateral stability, working alongside a deeper muscle called the rhomboideus. While the trapezius is a relatively thin, sheet-like muscle, the rhomboideus is more chunky and substantial beneath it.
Together, these muscles, move the scapula relative to the withers while the infraspinatus and deltoid muscles create external rotation in the shoulder, enabling the horse to perform lateral movements like leg-yield, half-pass, and shoulder-in. They are also vital in protecting the horse's body during any work involving circles or turns.
It's worth remembering that horses evolved primarily to escape predators by galloping in a straight line — they're not natural sideways movers! This means many horses genuinely need specific work to build their lateral stability before we ask them to perform these movements safely and comfortably.
Hollows Behind the Withers
You may have heard people discussing the hollow that sometimes appears just behind the scapula. It's often attributed solely to atrophy of the trapezius muscle, but this is actually a misconception. Given how thin the thoracic trapezius is, even significant atrophy wouldn't create a substantial hollow. The more likely cause is a change in the position of the thorax between the front legs (poor posture) and loss of muscles in the bulkier muscles underneath the scapula.
I've written a detailed article on this subject if you want to understand more about it and what you can do to improve it - have a read. Taking the Mystery out of Equine Anatomy – Part 6: Hollows Near the Withers
Muscle 2: The Dorsal Serrate Muscle
This one is less well-known, but once you understand its role, you'll appreciate why cardiovascular fitness matters so much for all horses.
The dorsal serrate muscle has two portions: a cranial (front) part and a caudal (back) part. It originates from the ribs and attaches into the thoracolumbar fascia (a broad sheet of connective tissue). What makes it unusual is the direction of its fibres — they run at roughly 90 degrees (transverse) to the extensor chain muscles like the longissimus dorsi.
Think of it this way: the longissimus dorsi runs along the length of the back, while the dorsal serrate runs across it. The dorsal serrate creates a tunnel which longissimus dorsi runs under.
Why Does This Matter?
When a muscle works, it expands. Try measuring your calf before and after a run — it genuinely gets larger as blood and fluid move into the working tissue. The same happens in the horse's back. For this to happen comfortably and efficiently, the muscles need space to expand, and the layers of tissue need to be able to slide and glide against each other.
Pressure plays an important role here too. Healthy soft tissue depends on the right balance of fluid — not too compressed, not too slack. Think about squeezing your fist tightly: you can feel how that pressure affects the tissues. This is why saddle fit, saddle pressure, and the movement of the rib cage under the saddle are all so significant.
Breathing and Rib Cage Posture
The dorsal serrate muscles attach to the ribs on both the left and right sides, which means they influence breathing, rib movement, and rib cage posture. In fact, the main function of the cranial portion is inspiration — breathing in.
This leads us to a really important question: does your horse need to be fit to be ridden?
Even for light hacking or a gentle walk down the road, if a horse is unfit, their dorsal serrate muscles may not be strong enough to adequately support rib cage posture under the weight of a rider. This affects not just breathing, but the health and function of all the back muscles sitting above and around it.
The answer to strengthening these muscles is perhaps simpler than you might expect: cardiovascular fitness.
Other things that can support this area include correctly fitting tack and encouraging horses to move well through a full range of motion and feeding from the ground to promote a good natural head carriage and neck and back stretching.
Muscle 3: The Multifidus (Deep Back Muscles)
The multifidus sits beneath the longissimus dorsi, nestled between the articular and spinous processes of the vertebrae. It has many small fascicles (bundles of muscle fibres) — which is where its name comes from — and it is highly innervated, meaning it’s packed with nerve endings and very sensitive.
What Does It Do?
Unlike the longissimus dorsi, which is a large, powerful muscle designed to create big movements, the multifidus is far more subtle and arguably more important: individual vertebral stability. It fine-tunes the position of each vertebra relative to the next, acting as a precision stabiliser for the spine.
When a horse experiences back pain, the multifidus muscles often stop working effectively which leads to atrophy. When this happens, the longissimus dorsi tends to step in and take over the stabilising role — but it's not well-suited to this job. The result is usually increased muscle tension which can further compromise back health. This is one of the reasons why addressing back pain early matters so much.
The Connection To The Ribs
There's a fascinating anatomical link worth mentioning here: the multifidus connects to the costotransverse ligament (a structure associated with the ribs). This might seem surprising, but it illustrates that the back and the ribs are deeply interconnected. This is part of why movements that mobilise the rib cage, such as lateral work and turns, also affect and mobilise the horse's back.
Putting It All Together
What can we take from all of this? Here are some practical takeaways:
Fitness is back care. Cardiovascular fitness directly supports the muscles that maintain rib cage posture under a rider. A horse doesn't need to be an elite athlete to benefit, even horses in light work need a level of fitness appropriate to what we're asking of them.
Help the lateral stabilisers. Try Pilates style exercises to strengthen the lateral stabiliser muscles. The Wither Rock stimulates the lateral stabiliser muscles of the limbs as well as the core muscles which are important for maintaining posture. The lateral stabiliser muscles support the limbs when turning and moving sideways.
Here's a free video demonstrating the wither rock:
If you are not already a member you will need to sign up to the Horses Inside Out Academy – it’s FREE to join and you can access the FREE Tutorial videos. In this library your will find the Wither Rock Exercise, where Gillian explains how to do this exercise correctly for maximum benefit for your horse.
Pole work is a wonderful tool. Poles encourage horses to flex the limbs, but also, when done correctly they encourage head and neck carriage that stimulates the back muscles through those chain connections. Watch the leg on the ground as much as the leg in the air; the stabilising work happening there is just as important as the movement.
This autumn I will be doing a series of webinars dedicated to pole work. The Power of Poles series will cover in detail the impact pole work has on the horse, plus there will be lots of exercises that you can try with your own horse.
Saddle fit is about more than where it sits on the back. The layers of muscle in the back need to be able to expand, hydrate, and slide freely. The position and fit of the saddle affects all of this.
Understanding the anatomy of the horse's back transforms the way we see movement, training, and management. Everything in anatomy is connected to everything else — and the more we appreciate that, the better equipped we are to support our horses' health and well-being.
Want to learn more? Join us for a practical day studying anatomy related to saddle fit.
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