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What's in the muscle? Cutaneous Omobrachialis!

I am taking a small detour from the neck, as I got many questions about a specific muscle line in the area of the horse’s shoulder. As such, today’s spotlight is for the:

Cutaneous Omobrachialis The Cutaneous group are the most superficial muscles in the horse’s body. In most anatomy books, not much importance or explanation is given on this muscle grouping. Although mainly considered as fly deterrants, certain cutaneous muscles also assist and provide mechanical functions. For example, the cutaneous coli seems to acts as part of the thoracic sling upon landing – for example during jumps or working down hill. The Cutaenous Coli can be recruited to draw the hind limb forward in case of hind limb – especially stifle – weakness.

The Cutaneous Omobrachialis is located on the lateral surface of the horse’s shoulder and forearm and is often confused as being a ‘fascial line’. However, dissections have taught me that in fact it really should be classified as a separate muscle. Interestingly, there is quite some variation in exact shape or size of this muscle in individual horses.

Despite being a superficial muscle, the Cutaneous Omobrachialis should not be visible from the outside. So why does it pop up in so many horses?

Although there isn’t a definite one-on-one reason / explanation, the most common connections I have seen are two-fold: 1. Shoulder issues / tightness. Most of the time, the lining of the Omobrachialis is combined with a dip in front of the whither, or muscle imbalances in the shoulder area including Biceps, Triceps (Elbow issues!), Deltoid, Iinfraspinatus, Supraspinatus and/or Terres Minor. I also find it quite commonly in horses with a recessed ribcage or sternum issues. When there is a primary weakness, the Cutaneous Omobrachialis can be recruited to assist in shoulder mechanics as a means of compensation.

2. Neurological sensitivity. The skin is a highly innervated organ and some horses are very sensitive to the lightest sensory stimulus. In some cases it could be a seasonal phenomenon - in which case it would have to disappear upon winter. 3. Personal signature. Every horse is different. Hence, these lines can be like identification markers creating their own personal signature. However, in order to come to this conclusion it must be certain that explanations 1-2 can be ruled out - in my personal experience these often still prevail.

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