Your horse has no collarbone. That single anatomical fact explains more about why your loose-jointed horse keeps breaking down, drifting, falling on the forehand, and generally refusing to hold any shape you put them in than almost anything else you’ll read about equine training. The thoracic sling is the structure that does the job a collarbone would do — and in hypermobile horses, it is almost never doing that job well.
If you’ve ever had a horse that felt like they were dissolving under you — not resistant, not lazy, just structurally unable to hold it together — you need to understand this.
What the Thoracic Sling Actually Is
The thoracic sling is a muscular and fascial system that suspends the horse’s thorax (the ribcage and everything on it) between the two forelimbs. Think of it as a hammock of muscle. The primary players are the serratus ventralis and the pectoral muscles, working in concert to prevent the chest from dropping between the front legs during weight-bearing.
In a healthy, well-developed horse, the thoracic sling catches and distributes load every single stride. When the horse puts a front foot down, the ground reaction force travels up the limb, and the sling catches that force and transfers it into the thoracic spine and ribcage rather than letting the sternum sink toward the ground.
In a horse with a weak or dysfunctional thoracic sling, that catching doesn’t happen. The sternum drops. The scapulae wing out. The withers sink. The whole front end compresses rather than lifts. And no amount of leg or rein will fix what is fundamentally a structural load-transfer failure.
Why Hypermobile Horses Are Especially Vulnerable
A horse with normal joint stiffness has passive restraints — ligaments, joint capsules, connective tissue — that do some of the work of keeping the structure intact. The muscles don’t have to be as precise because the joints themselves will stop the movement before it becomes destabilizing.
A hypermobile horse has joints that move through a larger range than they should. The passive restraints are lax, or the connective tissue is less stiff than typical. That means the muscles have to do all the work that the passive restraints would normally share. The sling doesn’t get to phone it in. It has to be on, fully, every stride.
When that sling is underdeveloped — which it almost universally is in horses that have been ridden through looseness rather than trained out of it — you see the classic hypermobile horse presentation:
- A front end that looks flashy and free but doesn’t carry load
- A sternum that hangs low between the legs at rest and under saddle
- Scapulae that move beautifully but don’t anchor
- A horse that falls apart the moment the terrain changes, the pace increases, or the exercise demands more stability
- Intermittent forelimb lameness that your vet cannot pin to a single clear lesion
That last one is the one that brings people to me. The horse who keeps coming up sore, keeps swapping out of soundness, but clears on flexions and has clean imaging. The sling is almost always part of that picture.
What Sling Failure Looks Like Under Saddle
You don’t need a biomechanics degree to see this. Once you know what you’re looking for, it’s obvious.
From the ground, watch for:
- The sternum visibly dropping on the landing stride — the belly swings down rather than the ribcage lifting
- Scapular winging — the point of the shoulder moves outward rather than tracking cleanly back and forward in the same plane
- A horse that looks wider through the chest at rest but narrow and collapsed through the base of the neck under load
- Front legs that drift outward (base-narrow movement pattern) even on a horse that stands correctly at rest
From the saddle, watch for:
- The feeling that your horse is falling into your hands on a circle rather than supporting themselves
- A horse that drifts to one shoulder consistently, no matter what correction you apply
- The sense that you’re riding downhill even on level ground
- A horse who is brilliant in a straight line but falls apart the moment you ask for lateral work — because lateral work demands sling engagement the horse doesn’t have
The Sling Is Not the Same as Core
This is where I see the most confusion in riders who’ve done some reading on equine biomechanics. People conflate thoracic sling engagement with abdominal engagement or back lifting. They are related but they are not the same thing.
The core — the abdominals, the multifidus, the thoracolumbar fascia — creates longitudinal stability. It’s the mechanism by which the horse can swing their back and transmit energy from hindquarters to forehand.
The thoracic sling creates transverse stability. It is the mechanism by which the load that hits the front leg doesn’t immediately collapse the structure. You need both. But in the hypermobile horse, the sling is almost always the weaker link, and it’s also the one that nobody talks about, because most training language is built around the hindquarters and the topline.
If you only train core without addressing the sling, you can have a horse with a beautifully swinging back that still collapses the front end under any real demand. I’ve ridden those horses. It’s a very specific and confusing feeling.
How to Start Training the Thoracic Sling
First, the honest answer: you cannot train the sling effectively from the saddle alone, especially in the early stages. Ground work and specific exercises matter here.
Pole work with a purpose
Single raised poles — front legs raised four to six inches — done at the walk ask the horse to actively lift the sternum and engage the serratus to clear the obstacle. This is not about range of motion. It is about loading the sling at the moment of forelimb demand. Three to five passes per session is enough early on. You are not doing a cardio workout. You are asking a very specific muscle group to wake up and do its job.
Correct lateral movement at the walk
Shoulder-fore and leg-yield at the walk, but ridden with a specific intention: you want the horse’s ribcage to shift laterally without the sternum collapsing toward the direction of travel. This sounds subtle. In practice, it means riding off your inside leg into an outside rein that is soft enough that the horse can balance into it rather than hanging on it. If the horse hangs, the sling is not engaging. If the horse organizes around the lateral request and you feel the ribcage lift slightly as they step under, you’re getting something real.
Hill work — uphill only, early
Uphill work on a five to eight degree incline shifts load off the forehand and onto the hindquarters, which paradoxically demands more sling engagement, not less. The horse must lift the chest in order to move uphill efficiently. Start at the walk. Do not do downhill work in the early stages — downhill work loads the forelimb eccentrically and a horse without sling stability will jam through the shoulder on the way down.
What you are not doing
You are not asking the horse to be rounder or lower or looser. You are not doing long-and-low as a sling training strategy. Long-and-low has its place, but it asks the horse to drop the base of the neck and lengthen the topline — it does not preferentially engage the thoracic sling. For a hypermobile horse whose sling is already underperforming, extended long-and-low work can actually worsen the pattern.
What You’ll Feel When It’s Working
The first sign is usually not dramatic. It’s a subtle feeling of the horse organizing differently through the base of the neck — less of that pooling, heavy feeling in front, and more of a sense that the ribcage is actually between your legs rather than hanging below them.
The second sign, usually after several weeks of consistent work, is improved straightness without you having to do anything. The horse stops drifting to one shoulder because the sling is now strong enough to resist that collapse unilaterally. You will notice you’re using your lateral aids less, not more.
The third sign — and this is the one that tells you the sling is genuinely training — is that the horse’s front-end lameness pattern, if they had one, stabilizes. Not because you’ve treated a lesion, but because you’ve changed the mechanical environment that was creating repetitive overload on that limb.
That’s the goal. Not a flashier trot. A sound, stable horse who can do the job without falling apart.
If you want to go deeper on how I assess the thoracic sling in new horses and what the first eight weeks of sling training actually looks like in practice, The Elevated Equestrian podcast is where I get into the mechanics in real detail. The episodes on hypermobility assessment and stability-first training cover exactly this — and if you’ve got a horse who keeps coming up sore in the front end with no clear diagnosis, start there.
Everything I’ve covered here — the sling, the core, the stability-first framework for the hypermobile horse — is the foundation of the book I’m currently writing for everyday riders who are working through these problems without a sports medicine team on speed dial. If you want early access and to be the first to know when it’s available, get on the waitlist at /free-lesson. You’ll also get a free lesson as a thank-you for being part of this from the beginning. This is the resource I wish had existed when I first started putting together what these horses actually need.
