🐎 Muscular Disorders in Athletic Horses: Vet Advice & Genetics for 2025 🧬💪

By Dr Duncan Houston BVSc

One of the most well-known and misunderstood equine conditions is “tying up”—a general term used to describe painful episodes of muscle cramping and stiffness that often affect performance horses. While once considered a singular disorder, we now know that tying up represents several different muscular conditions, some of which are hereditary and breed-specific. In 2025, with better diagnostics and a deeper genetic understanding, managing these disorders is more achievable than ever. 🧠🐴

💥 What Is “Tying Up”?

Also called exertional rhabdomyolysis, tying up refers to a group of muscular conditions where the horse’s muscles break down following exercise. This results in:

• ⚠️ Painful cramping, usually in the hindquarters
• 🚶 Stiff or reluctant movement
• 😰 Sweating and signs of distress
• 🔋 Elevated muscle enzymes in blood tests

It may occur after exercise or even during mild work, especially if the horse is predisposed. The science now shows that several distinct disorders may be behind these symptoms. 🧬🔍

🧬 Three Known Genetic Muscle Disorders

Dr. Erica McKenzie and other researchers have identified multiple muscular conditions that can lead to tying up, and many are linked to genetic mutations. Here are the most clearly defined disorders: 👇

1️⃣ Polysaccharide Storage Myopathy (PSSM)

• 📍 Common in: Quarter Horses, Paints, Appaloosas, and related breeds
• 🧪 Cause: Abnormal accumulation of glycogen in muscle tissue
• 🧬 Genetic test available: Yes (Type 1 PSSM)
• ⚠️ Symptoms: Muscle stiffness, reluctance to move, pain after light exercise

Horses with PSSM benefit from diet changes and specific exercise regimens. Genetic testing can confirm the diagnosis. 🧾🍽️🏃

2️⃣ Hyperkalemic Periodic Paralysis (HYPP)

• 📍 Common in: Quarter Horses (especially Impressive bloodlines)
• 🧪 Cause: Defective sodium channels in muscle cells
• 🧬 Genetic test available: Yes
• ⚠️ Symptoms: Muscle twitching, weakness, collapse in severe cases

HYPP is manageable with low-potassium diets and medication. Genetic testing helps breeders prevent passing it on. 🧬🚫

3️⃣ Glycogen Branching Enzyme Deficiency (GBED)

• 📍 Found in: Quarter Horses
• 🧬 Genetic test available: Yes
• ⚠️ Symptoms: Lethargy, weakness, sudden death in foals

This fatal disorder affects energy storage and is usually diagnosed in young horses. Carriers can be identified and avoided in breeding programs. 🚨🐣

🔁 Recurrent Exertional Rhabdomyolysis (RER)

RER is a form of tying up commonly seen in:

• 🏇 Thoroughbreds
• 🏁 Standardbreds

Symptoms tend to appear during training rather than racing and often affect highly strung, fit horses. Unfortunately, no specific gene has been linked to RER yet. 🧩

RER management focuses on:

• 🍽️ High-fat, low-starch diets
• 📈 Gradual conditioning schedules
• 😌 Stress reduction and consistent routines

🧬 Myofibrillar Myopathy (MFM)

This disorder is found in:

• 🧬 Warmbloods
• 🧬 Arabians

MFM can lead to subtle tying-up signs in Arabians and vague movement problems in warmbloods, such as:

• ❓ Stiffness without clear muscle pain
• 🔄 Inconsistent gaits
• 📉 Declining performance without lameness

There is no current genetic test for MFM, but it’s suspected to be inherited. Diagnosis often requires a muscle biopsy. 🧫🔬

📋 Diagnosing Muscle Disorders in 2025

If your athletic horse “ties up” frequently, the evaluation may include:

• 🧪 Blood tests for CK and AST (muscle enzymes)
• 💉 Genetic testing (especially for PSSM, HYPP, GBED)
• 🧫 Muscle biopsy (for MFM or unclear cases)
• 🧠 Evaluation of diet, exercise routine, and stress levels

Diagnosis is key for appropriate treatment—guessing or using a one-size-fits-all approach won’t work. 🛠️

🩺 Managing Muscular Disorders

Treatment and management depend on the underlying cause, but general principles include:

• 🥗 High-fat, low-carb diets to stabilize muscle energy use
• 🏃 Regular, controlled exercise to prevent muscle stiffness
• 📊 Monitoring electrolytes, hydration, and potassium levels
• 🧠 Minimizing stress and sticking to routines

For horses with genetic conditions, avoiding long stall rest and ensuring daily turnout or exercise is critical. 🐎💡

📲 Genetic Testing & Breeding Decisions

Breeders and trainers should consider genetic testing before breeding horses known to be prone to tying up. Many breed registries now encourage or require screening for HYPP and PSSM. 🧬📑

Testing helps prevent future generations from suffering and guides better management for affected horses. 🧠❤️

📞 AskAVet.com Can Help

Not sure if your horse is tying up—or confused about which disorder might be involved? Reach out to AskAVet.com or download the Ask A Vet App. 🩺📱

Dr Duncan Houston and the Ask A Vet team can help interpret genetic test results, suggest management plans, or guide you through a suspected tying-up episode. 🐎💬

🏁 Final Thoughts

In 2025, muscular disorders in athletic horses are better understood than ever before. With genetic testing, modern diet strategies, and personalized training plans, horses that once struggled with tying up can now lead healthy, high-performing lives. 🧠💪

Act early, seek expert guidance, and take advantage of the tools science gives us today for healthier horses tomorrow. 🐴🌟

Visit AskAVet.com for expert help with tying up and other performance horse challenges 🐎🧬