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What's in the muscle? Trapezius

In the last post I described muscle linings of the Rhomboid. Today’s spotlight is for the Trapezius muscle. This post will be a bit longer than usual since it is a quite complicated one and it covers both the neck as well as the beginning of the thoracic spine. The Trapezius muscle consists of a cervical and thoracic portion. The cervical part of the muscle originates from the Nuchal Ligament whereas the thoracic part originates at the Supraspinous ligament from T3-T10. The muscle inserts at the spine of scapula.

The Trapezius Cervicus functions to draw the scapula upward and forward whereas the Trapezius Thoracis draws the scapula upward and backward. Together, they raise the shoulder. So let’s zoom in a bit more on both parts of the muscle. 1) The Trapezius Cervicus is considerably thinner than it’s thoracic counterpart and upon dissection it is revealed that there can be considerable variation in size, shape and thickness in this part of the muscle from horse to horse. Furthermore, on the superficial surface there are fibrous strands and muscle fibers that are difficult to define and these can extend as far as to the Brachiocephalic muscle and in some horses even over it. This can have significant impact on front limb locomotion as the Brachiocephalic acts as an antagonist to the Trapezius Cervicus. The Brachiocephalic draws the front limb forward by pulling on the lower part of the scapula forward, whilst the cervical part of the Trapezius moves the upper part of the scapula forward. As such, they are working in opposition towards each other. To enable free movement, it is functional to have some distance between the two muscles so they influence each other a bit less. When the space between these muscles is narrowed, the Brachiocephalic also pulls on the Trapezius muscle and basically counteracts its own action on the scapula and thus limiting front limb movement.

Furthermore, as mentioned in last post, together with the Rhomboideus, the Trapezius Cervicus works in opposition to the deep muscles of Long Colli and Scalenus.

When talking muscle linings, the Trapezius Cervicus can often show atrophied or hypertrophied in the horse. An atrophy of Trapezius Cervicus usually shows in a dip in front of the wither and is usually well recognized. It can be caused by poor training practises such as a rider holding the reins or putting the horse’s neck in a forced position.

On the opposite, a hypertrophy of the Trapezius Cervicus is evident when the muscle can be clearly defined through palpation or when it shows up in movement – especially in turns.

Remember, this portion of the muscle should be quite thin, so if you can clearly distinct or palpate it from the outside, this suggests a hypertrophy. This is not to be confused with fascia linings common on the shoulder area – this usually is the Cutaneous Omobrachialis. Furthermore, a hypertrophy is also distinct to healthy development. In my previous post I explained the mechanism that raises the base of the neck: contraction of the Thoracic Sling, Longus Colli and scalenus support the neck from below giving leverage to Rhomboideus and Trapezius to assist in the telescoping ability of the neck. However, while doing so, Trapezius Cervicus should develop smoothly and in harmony with other muscle and still NOT SHOW in any clear muscle DEFINITION or LINING. So you need to know your anatomy really well and refine your palpation skills to assess Trapezius Cervicus correctly.

So when trapezius cervicus appears to be hypertrophied, what is the most likely cause you might ask? Dissections have learned me that by far, the most common cause for a hypertrophy of the cervical portion of the trapezius is because of a problem ( lameness) in the front limb from the knee below – so feet included! This is because the muscle acts as lever upon movement. Always take in mind that this is an INDICATOR and not a DIAGNOSIS.

So taking last week’s post into account, it can be summarized that a hypertrophy of the rhomboid can be used as an indicator of hind end lameness whereas hypertrophy of the trapezius cervicus can be used as an indicator for front limb issues from the knee below. These issues could either be caused by injury, genetics or poor training.

It is more common than generally known so it will be an interesting area for further research as I myself find this part of the muscle especially tricky so I am on a quest to enhance my knowledge every day through palpation of as many horses as I can and all future dissections.

2) So now it’s time to also have a quicklook at Trapezius Thoracis. Since the Trapezius Thoracis lies directly under the gullet of the saddle, it is in a vulnerable position to be damaged. Naturally, this portion of the muscle can become quite sensitive to poor fitting saddles – especially those with narrow gullets – as well as to the rider’s seat.The overlap between Trapezius Thoracis and Latissimus Dorsi is right under the rider’s seat. As such, weight shifts of a rider forwards-backwards will hinder proper harmony of those muscle by favouring contraction of either muscle over the other and as such stiffen either the thoracic or lumbar spine of the back.To not go off too much off topic, remember that the horse’s back doesn’t swing and it certainly doesn’t move all up or all down (string-and-bow theory). Yes, the spine is a very dynamic entity since each individual vertebrae rotates around each other. However, they do so within a very limited range of motion as the main job of the spine is to protect the spinal cord. We always thing of the back and its muscles as something that should move, but it should also limit movement to protect the spine’s integrity and stay within natural range of motion.

When talking muscle linings, it is quite common for the Trapezius Thoracis to be atrophied with the most common causes already mentioned such as saddle fitting and poor riding practises. A horse that is ridden without the base of the neck supported and the front limbs able to provide vertical impulse will move as if the withers get pushed into the shoulders and as such it will put strain on the Trapezius Thoracis. On the opposite, a hypertrophy is less common – but can also appear. However, take into account that certain horses just have a lot of fat in this area so that it might not always be the muscle we’re dealing with.

When horses have a lot of fat on the muscle it can appear as hypertrophy.

So by now it should be clear that the trapezius altogether is a very important muscle for ensuring proper biomechanics in the horse. I hope you can use some of this knowledge to analyse and feel your horse’s muscle linings and better understand them. Want to learn more? Join one of the dissection case study webinars by Zefanja Vermeulen from Equinestudies. For more info check:

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