Equine muscle disorders, or myopathies, can cause stiffness, pain, reduced performance, and recurring “locking up.” Proper nutrition, together with appropriate exercise, is a central part of both treatment and prevention.
What are muscle disorders?
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PSSM1 and PSSM2 (Polysaccharide Storage Myopathy):
Muscles store sugar in an abnormal way. As a result, the horse may become stiff, reluctant to move, or show symptoms particularly after rest.
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ER (Exertional Rhabdomyolysis, “Monday morning disease”):
During exercise, muscle cells break down. Symptoms include stiffness, pain, sweating, and in some cases even lying down. The condition may occur sporadically or recur frequently.
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MFM (Myofibrillar Myopathy):
There is a defect in the muscle’s structural proteins, which causes rapid fatigue and muscle stiffness.
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MFM-ER in Arabians:
Common in Arabian horses, with symptoms such as post-exercise stiffness and slow recovery.
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MFM in warmbloods:
Occurs in sport horses; symptoms are similar to those described above, but nutritional emphasis differs slightly.
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MFM-ER in Arabians:
The role of nutrition in managing these conditions
The correct diet for a horse with a muscle disorder is selected based on the diagnosis. There is no single solution that fits all cases.
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PSSM1 and PSSM2:
➝ Low sugar and starch (low NSC), more fat as an energy source.
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Recurrent ER:
➝ Moderately reduced carbohydrate intake, additional energy from fat.
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MFM-ER (Arabians):
➝ Moderate carbohydrate levels, added fat, amino acids, and antioxidants.
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MFM (warmbloods):
➝ Adequate protein, moderate carbohydrate intake, less fat than in Arabians.
| Muscle disorder | Recommended feeding |
| PSSM1 & PSSM2 | Low sugar and starch, added fat |
| Recurrent ER | Moderately reduced carbohydrates, some energy from fat |
| MFM-ER (Arabians) | Moderate NSC, fat, amino acids, antioxidants (NAC + CoQ10) |
| MFM (warmbloods) | Adequate protein, moderate NSC, less fat |
Diagnosis
Equine muscle disorders are most often diagnosed using a combination of several methods, because symptoms can resemble one another and underlying causes vary.
Common diagnostic methods:
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Clinical signs and history
- Stiffness, pain, sweating, reluctance to move, reduced performance, dark urine (rhabdomyolysis).
- The veterinarian evaluates symptom patterns, workload, and diet.
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Blood tests (muscle enzymes)
- Creatine kinase (CK) and aspartate aminotransferase (AST) increase when muscle breakdown occurs.
- Samples are often taken immediately after exercise and again a few hours later.
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Genetic testing
- PSSM1 can be confirmed with a DNA test (known GYS1 gene mutation).
- Genetic tests related to PSSM2 and MFM are under development but are less precise.
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Muscle biopsy
- A small muscle sample is examined under a microscope.
- This allows detection of abnormal glycogen accumulation (PSSM) or structural myofibril issues (MFM).
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Exercise and stress testing
- A controlled exercise test may be performed, monitoring symptoms, muscle enzymes, and recovery.
- A controlled exercise test may be performed, monitoring symptoms, muscle enzymes, and recovery.
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Differential diagnosis
- Other conditions (e.g. lameness, respiratory disease) are ruled out to ensure that muscle disease is the true cause of symptoms.
Summary
Correct nutrition supports muscle function and can make a significant difference to a horse’s wellbeing. Because different muscle disorders require different nutritional strategies, a veterinary diagnosis is always the starting point. High-quality forage (hay) is always the foundation of feeding. Energy needs are then met according to individual requirements—some horses require reduced sugar and starch, while others need more emphasis on protein and amino acids. In PSSM horses, high starch and sugar (grains, molasses, etc.) are avoided, and energy is supplied through fat (oils, fat-rich feeds). Horses with MFM benefit from adequate protein and key amino acids (e.g. lysine, threonine), which support muscle repair and structure.
Vitamin E is an important antioxidant in managing muscle disorders. It protects muscle cells from oxidative stress and supports recovery.
Selenium works together with vitamin E as an antioxidant and supports muscle and immune function. Magnesium may help muscle tension in some cases, although scientific evidence is more limited. For horses with muscle disorders, a regular feeding schedule is important. Adequate water and electrolyte intake—especially around exercise—is also essential.
