Key points at a glance:
- The horse’s back is not naturally designed to carry a rider—it must be trained for this task
- The thoracic sling (abdominal, chest, and wither-lifting muscles) is responsible for carrying weight, not the back muscles
- Without a collarbone, the rib cage is suspended only by muscles between the shoulder blades and can drop
- Only when the trunk is lifted and the back can round is the horse truly able to carry weight
- A weak thoracic sling leads to tension, pain, and serious long-term health problems
A structure that was not designed for riding
Evolution did not design the horse to be a riding animal. In the wild, a horse never carries sustained weight on its back—at most briefly, when rolling or pushing through dense undergrowth. The equine body is optimized for endurance movement on level ground, not for carrying weight. Every rider should be aware of this fact, as it explains why so many horses develop back problems when they are not trained and ridden correctly.
The good news is that, through targeted training, a horse can indeed be conditioned to carry a rider in a healthy way. To do so, however, one must understand how the carrying system works and which muscles are responsible for it.
The rib cage hangs in a “muscular hammock”
As most riders know, horses do not have a collarbone. This has far-reaching consequences for the body’s entire structure. The rib cage is not rigidly connected to the shoulder blades but is suspended solely by muscles, tendons, and ligaments between the forelimbs. It can be imagined like a heavy basket hanging from ropes.
This construction is known as the thoracic sling or carrying system. The thoracic sling musculature consists mainly of the abdominal muscles, the chest muscles, and the so-called wither lifters. These muscles must work actively to lift and stabilize the rib cage. If they are too weak or do not function correctly, the rib cage sinks down between the shoulders.
This is precisely where the problem lies for many riding horses: if the thoracic sling is insufficiently trained or the horse is ridden incorrectly, the rib cage drops. As a result, the spine is pressed downward, the back hollows, and the horse’s entire posture and balance change. In this state, the horse cannot carry the rider’s weight in a healthy way.
The back muscles are not weight-bearing muscles
A widespread misconception is that the back muscles carry the horse. Many riders focus on building muscle along the back while overlooking the far more important thoracic sling musculature. In reality, the long back muscle running alongside the spine is a movement muscle, not a weight-bearing one.
The role of the back muscles is to enable movement. They contract alternately as the limbs move forward and should work elastically and fluidly—rhythmically tightening and releasing. If the back muscles remain constantly tense and try to support the trunk because the true carrying muscles are too weak, serious problems develop.
A tense back cannot swing freely. The horse appears stiff, shows rhythm irregularities, and gradually develops painful muscle hardening. The spine is overloaded, intervertebral discs are compressed, and inflammation occurs. Over time, such incorrect loading can lead to kissing spines, spondylosis, or other degenerative conditions.
How the back is able to round
For a horse to become capable of carrying weight, the back must be able to round upward. This rounding results from a complex interaction of different structures. When the horse lowers its head and stretches forward and downward, the nuchal ligament is stretched. This stretch is transmitted to the supraspinous ligament that runs along the spinous processes of the vertebrae. At the same time, the abdominal muscles contract and lift the rib cage from below.
Through this combined effect, the spine is stabilized and can arch upward like a bridge. In this state, the vertebrae can absorb the load from above and distribute it downward into the supporting structures. The spinous processes now have sufficient space between them, the back muscles can work freely, and the horse can move with swing.
It is crucial to understand that this rounding does not come from strength in the back muscles themselves, but from abdominal activation and ligament stretch. The back must be able to let go, not brace. A horse moving with a high head and tense under-neck cannot round its back. In this posture, the nuchal ligament is shortened rather than stretched, the spine is not stabilized, and the rib cage drops.
The hindquarters must be involved
The carrying system only functions if the hindquarters also do their part. The hind legs must step actively under the center of gravity and take on load. Only then is the abdominal musculature activated and the trunk lifted. A horse that trails its hindquarters behind and carries most of its weight on the forehand cannot stabilize its trunk.
The connection between hindquarters and trunk runs through the pelvis and lumbar spine. When the hind leg swings forward and lands, the muscles of the croup and lumbar region are activated. This activation is transferred to the abdominal muscles. In a correctly moving horse, one can clearly see a wave traveling through the entire body with each step of the hind leg—from the pelvis through the back to the neck.
What happens when the carrying system is weak
When the thoracic sling is insufficiently developed, a vicious cycle begins. The rib cage drops, the back hollows, and the shoulder muscles tense as they try to support the sinking trunk. The back muscles are forced to carry weight and become rigid. The horse no longer moves through its back, and movement becomes blocked.
Externally, these horses often show characteristic signs: a low wither, a hollow saddle area, prominently visible shoulder blades, pronounced under-neck musculature, a “hanging belly,” and a tense croup. They appear stiff, struggle with transitions, and often show resistance under saddle because every movement with rider weight causes discomfort or pain.
In the long term, a weak carrying system leads to serious health problems. Constant incorrect loading causes changes in the spine, tendon damage in the forelimbs (which are forced to carry weight that should be borne by the hindquarters), and chronic muscle tension. Many cases of thoracic sling fatigue and kissing spines originate in a carrying system that was never properly developed—or was destroyed through incorrect riding.
Training for a strong carrying system
The good news is: the carrying system can be trained. Young horses must be systematically prepared before carrying a rider. Even older horses with a weakened thoracic sling can improve through targeted training, provided no irreversible damage has occurred.
The key lies in activating the abdominal muscles and working correctly through the back. Exercises such as rein-back, lateral movements, transitions, and pole work challenge the thoracic sling. Particularly important is work in a stretching posture, where the horse learns to stretch forward and downward while lifting the back. Groundwork without rider weight—such as lunging or long-lining—is also valuable for developing the necessary musculature.
What matters most is that the horse is not simply exercised, but taught to move correctly. A horse that is chased around the arena for 20 minutes in rising trot with a high head does not train its carrying system—it develops compensation patterns and tension. In this case, quality clearly outweighs quantity.
The rider’s responsibility
Anyone who understands that the horse’s back is not naturally designed to carry weight will treat their horse differently. It becomes clear that a young horse needs time to develop the necessary musculature before being ridden consistently. It also becomes evident that every rider bears responsibility for riding in a way that allows the thoracic sling to work—rather than causing the back to suffer.
The carrying system is not an abstract anatomical theory; it is the foundation of every healthy life under saddle. Those who ignore these relationships and simply use their horse without regard for biomechanics will sooner or later face the consequences—lameness, back problems, or behavioral issues. Those who systematically develop their horse and allow the carrying system to strengthen will gain a healthy, capable, and long-lived partner.
