Science & Learning Nerd Alert: The Mystery of the “Fly Twitch” Muscle

It all started with a sliding saddle pad.

I’d place it on Spoon, walk away to grab the saddle, and by the time I came back, the pad had shifted several inches toward his hind end. At first, I thought it was behavioral. Then I blamed the pad itself. But the more I watched, the more I wondered:

Was he fidgeting? Was it voluntary? Was it involuntary? Or was he reacting to something I couldn’t see?

His skin would twitch — sometimes even before I touched him and with no flies in sight. That little quirk of his sent me down a research rabbit hole:

What is this skin twitching all about? Why does Spoon react to even the lightest touch, while my other horse Rosie barely reacts at all?

Meet the M. Cutaneous Trunci

This thin, sheet-like muscle lies just under the skin (in the superficial fascia layer) and runs along both sides of the horse from just behind the shoulder/wither region to the flank and top of the hip. It does not extend up the neck or down the limbs.

Its primary function? To power the panniculus reflex — aka the “fly twitch.”

This reflex is:

• Involuntary

• Protective

• Triggered by light touch, insects, or even subtle irritation

• Essential for areas the horse can’t reach with a tail swish or bite

Areas it affects:

• Thorax (behind the scapula, over the ribs)

• Flanks

• Upper abdomen (upper sides)

Think of it as the horse’s “skin-shifting sheet” — moving the skin independently from deeper muscles.

What’s the Panniculus Reflex?

The panniculus reflex is an automatic contraction of the cutaneous trunci muscle in response to light touch, pressure, or irritation.

How it works:

1. Sensory nerves in the skin detect a stimulus.

2. The signal travels through spinal sensory nerves to the spinal cord.

3. The spinal cord processes it and triggers the lateral thoracic nerve.

4. The lateral thoracic nerve activates the cutaneous trunci, twitching the skin.

A light touch in this zone can trigger a twitch across several inches — sometimes even creating a visible “wave” along the horse’s side.

Primary roles:

• Protection from insects and irritation

• Sensory feedback about skin sensitivity, nerve health, and spinal cord function

• Diagnostic value for vets in assessing spinal cord and nerve integrity

Spoon’s Story: Hypersensitive and Hyper-Responsive

I’ve had horses for over 30 years, and Spoon’s reaction is unlike anything I’ve seen before.

What I noticed:

• Extremely sensitive — sometimes twitching before I even touched him

• Moves the saddle pad back toward his hips if the cinch isn’t tightened immediately

• Reacted to a lightweight dish towel

• Twitched the BEMER cuff off when it wasn’t running, but stayed calm once the session started

My thoughts:
Spoon’s nervous system seems highly reactive — possibly from fascia tension, old compensation patterns, or nerve irritation. His history includes a left stifle injury, arthritis, and years of being a performance cutting horse.

Could it be:

• Fascial tightness/adhesions from old injuries?

• Nerve hypersensitivity (possibly involving the lateral thoracic nerve)?

• Saddle pressure points or spinal arthritis?

Interestingly, his reflex calms during BEMER sessions, which makes me wonder if it’s more about nerve irritability than structural damage.

Rosie’s Story: Almost No Response

For comparison, I tested Rosie. I expected some reaction — but got almost none.

What I noticed:

• Minimal to no twitch on either side

• Same method as Spoon

• Completely different outcome

Possible reasons:

• Metabolic challenges (IR/EMS)

• Medications (Ertugliflozin, formerly Thyro-L)

• Chronic inflammation or laminitis history

• Nervous system “freeze” or energy conservation

• Thicker skin or coat density (she’s 17)

My gut tells me her diminished response is linked to her metabolic health and possibly her medication, but it could also be her body conserving energy due to her chronic inflammatory state.

What This Means for Me as an Owner

I wasn’t expecting to uncover two completely opposite responses in my horses — and while it’s unlikely either is facing a major neurological crisis, these findings matter.

• For Spoon, it affects tack fit, comfort, and potentially performance.

• For Rosie, it’s a clue into her systemic health and nervous system status.

Subtle changes — even in something as simple as skin twitching — can be worth noticing and exploring.

And What could these opposite reactions mean to me and my horses?

I honestly don’t expect Spoon’s strong panniculus response – or Rosie’s opposite lack of one – to be anything significant.  But this experience reminded me that even small changes in a horse’s behaviour, whether voluntary or involuntary, can signal something worth noticing.  The truth is, our horses can’t speak to us, so it’s up to us to pay attention to the subtle ways they communicate.

This blog was a hard one for me to write. Honestly, I almost didn’t publish it because it felt a little heavy. Even though I don’t believe either horse has a serious condition, my horses are my world, and I’ve always been proactive about their care. They’ve both faced health issues that had nothing to do with neglect or poor management. Spoon’s stifle injury, for example, couldn’t have been prevented—he was in great shape and only nine years old when it happened. I sometimes wonder if it was a re-injury, but I’ll never know.

And Rosie—how could I have known she’d develop a chronic condition so quickly? She was twelve when I bought her, in great condition and not overweight, but within three months she had her first bout of laminitis. My vet thought it was heat-related. I guess my point is, no matter how careful you are, sometimes things just happen.

If you want to check your own horse’s panniculus reflex, there are plenty of licensed veterinarians on YouTube demonstrating the test. It’s simple to do for a quick check, but if you notice anything unusual, your vet can do a full neurological exam. I like to do as much as I can within my skill set to assess and support my horses, but I will never skip professional veterinary care when it’s needed.

My next step is to share my videos with my vet and follow his guidance on what to do for each horse. And as always—please remember, I’m sharing this from the heart. I’m self-taught in many areas, have taken multiple equine science classes, and I’m also a registered nurse, but I am not a veterinarian. Always work with your vet for your horse’s medical needs.

Sometimes, learning more about our horses brings answers we weren’t expecting—and sometimes those answers aren’t easy to hear.

Till next time,

Wendy