When your horse just doesn’t feel “right” under saddle, it could be more than laziness or a training issue—chronic muscle pain may be the hidden culprit.

Chronic muscle pain can significantly impair performance, and in many horses, it’s overlooked or mistaken for behavioral problems, poor fitness, or lameness. According to Dr. Stephanie Valberg, a leading expert in equine muscle disorders, recognizing and diagnosing this type of pain is critical to returning a horse to comfortable, effective work.

Understanding Muscle Pain in Horses

Horses suffering from chronic muscle pain may show subtle signs: lack of impulsion, reluctance to go forward, stiffness, behavioral resistance, or difficulty maintaining rhythm. These signs often overlap with orthopedic issues or poor conditioning, making diagnosis tricky.

Muscle soreness may stem from:

• Exercise intolerance

• Myopathies like PSSM (Polysaccharide Storage Myopathy) or MFM (Myofibrillar Myopathy)

• Exertional rhabdomyolysis (tying-up)

• Inappropriate conditioning or diet

• Pain secondary to poor saddle fit or rider asymmetry

Key Diagnostic Clues: What Vets Look For

To distinguish muscle pain from other causes of poor performance, veterinarians will:

• Assess muscle symmetry and tone

• Palpate for tight, painful areas

• Observe performance under saddle

• Measure muscle enzyme activity—particularly creatine kinase (CK) and aspartate aminotransferase (AST)

CK and AST: Key Enzymes in Muscle Health

These enzymes are released into the bloodstream when muscle cells are damaged:

• Creatine kinase (CK) rises rapidly after muscle injury, usually peaking within 4–6 hours of exercise. It’s a highly sensitive and specific indicator of acute muscle damage. Persistently elevated CK levels may suggest ongoing muscle stress or an underlying myopathy, even if clinical signs are mild.

• Aspartate aminotransferase (AST) is less specific to muscle tissue and can be elevated due to liver damage as well. However, when CK is high and AST is also elevated, this combination strongly supports muscle injury. AST rises more slowly than CK and stays elevated longer, often for several days, which can help track the progression or resolution of muscle damage.

Together, CK and AST give a clearer picture of muscle health and help differentiate between:

We often recommend exercise testing: a simple test involving some exercise combined with a blood test to check muscle enzymes. This test can be performed in a single day on your property and can often provide invaluable information. Contact us for more information.

Common Diagnoses

• PSSM Type 1: A glycogen storage disorder caused by a genetic mutation (GYS1). Common in Quarter Horses, it leads to abnormal carbohydrate storage in muscle cells. Diagnosed via a hair or blood genetic test.

• PSSM Type 2: More variable and seen in Warmbloods and other breeds. Muscle biopsy is typically needed to confirm.

• MFM (Myofibrillar Myopathy): Characterized by clumping of proteins within muscle fibers. Common in Arabians and Warmbloods. Diagnosis requires special histopathology stains.

• RER (Recurrent Exertional Rhabdomyolysis): Linked to temperament and stress. Often seen in Thoroughbreds and Arabs. Triggered by excitement, high-sugar diets, or rest days before exercise.

Treatment & Management

Management depends on the specific diagnosis, but general principles include:

• Dietary modifications: Low-starch, high-fat diets to reduce reliance on glycogen and improve muscle metabolism.

• Controlled exercise routines: Consistent, daily light exercise to prevent flare-ups and maintain muscle tone.

• Supplement support: Antioxidants such as vitamin E and selenium can be helpful, especially in MFM cases where oxidative stress may play a role.

It’s also important to note that rest is not always beneficial. For horses with PSSM, daily exercise helps manage muscle energy metabolism. In contrast, horses with MFM often benefit from rest days to recover between sessions.

Why Diagnosis Matters

Without a proper diagnosis, many horses are mislabelled as “naughty,” “lazy,” or “resistant,” when in fact they are trying to communicate discomfort. Chronic muscle disorders are common in certain breeds and may worsen over time without appropriate intervention.

If your horse consistently struggles under saddle or has elevated CK/AST, it’s worth pursuing a deeper muscle workup, including bloodwork, genetic testing, or muscle biopsy where appropriate.

Want to know if muscle pain could be the reason your horse isn’t performing?
Book a consultation with us at Avon Ridge Equine Veterinary Services—we can help with diagnosis and create a targeted management plan tailored to your horse’s condition.