What's in the muscle? Cutaneous Trunci

I am starting off a new short series explaining muscle linings and their possible meanings. Today I'd like to start with one of the largest superficial muscles: The Cutaneous Trunci

The Cutaneous Trunci is a so-called 🦟 fly muscle as it activates [twitches] when a fly - or any other annoying insect, lands on the horse's body.

Through the Cutaneous Trunci there is a direct connection from the stifle apparatus all the way to the hyoid and its surrounding structures.

In some horses, the Cutaneous Trunci shows as a line under the skin with a slightly raised egde. This is often mistakenly interpreted as a metabolic issue or a sign of an

unidentified inflammation. So what are probable explanations for this phenomenom? 1️⃣ Dehydration. In case of dehydration, fascia and muscles tighten and muscle linings start to show. However, this is usually more an exception than plausible cause. 2️⃣ Hyper sensitivity. The skin is a highly innervated organ and some horses are very sensitive to the lightest sensory stimulus. Hence, as a result, the Cutaneous Trunci hypertrophies during summer. However, if this is the case, the muscle lining should disappear over winter. If the muscle line stays visible throughout all seasons, the most likely explanation is the third cause below.

3️⃣ Assisting in drawing the hind limb forward. If the horse has a weakness in the hind end, it can recruit the the Cutaneous Trunci to assist in drawing the hind limb forward. This is the most common explanation I find in my practice. The weakness in the hind end can be anything, but in my experience it mostly involves the stifle apparatus.

By restoring functionality in the hind end - as much as possible, I have seen the muscle line of Cuteanous Trunci gradually disappear.

4️⃣ 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 explanation 1-3 can be ruled out - in my personal experience these often still prevail.

Want to know more? Come join us for a dissection of biomechanics assessment module listed under bookings!