09/22/2025
💡 When Stiffness Isn’t Simple: Making Sense of Equine Muscle Disorders
When your horse shows muscle stiffness, reluctance to move, or poor performance, it can be overwhelming trying to figure out the cause. Muscle disorders like PSSM, RER, and MFM can present with similar outward signs, but each requires a different management approach.
This flow chart, created in collaboration by Dr. Michelle DeBoer from Equine, PhD and Dr. Katie Young at Kentucky Equine Research, both past guests on the Scoop and Scale podcast, is an excellent tool to help horse owners sort through the complexities of equine muscle disorders. It’s not a substitute for veterinary diagnostics, but it’s a very helpful guide to better understand where your horse might fit and what next steps to consider.
🎧 Want to dive deeper? Check out our podcast episodes covering these conditions:
👉 Ep. 44: Understanding PSSM1 & Nutritional Management
https://scoopandscale.com/ep-44-understanding-pssm1-nutritional-management-for-your-horse/
👉 Ep. 47: Decoding Muscle Myopathies, Part 1 (RER, MFM, PSSM2-ER)
https://scoopandscale.com/ep-47-decoding-muscle-myopathies-part-1-understanding-rer-mfm-mfm-er-mfm-wb-pssm2-er/
👉 Ep. 48: Decoding Muscle Myopathies, Part 2 (Management Strategies)
https://scoopandscale.com/ep-48-decoding-muscle-myopathies-part-2-managing-rer-mfm-mfm-er-mfm-wb-pssm2-er/
📌 Save this flow chart for quick reference, and always consult your veterinarian for an accurate diagnosis and individualized plan.
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🔒 𝐖𝐡𝐞𝐧 𝐌𝐮𝐬𝐜𝐥𝐞𝐬 𝐋𝐨𝐜𝐤 𝐔𝐩: 𝐄𝐱𝐩𝐥𝐨𝐫𝐢𝐧𝐠 𝐓𝐲𝐢𝐧𝐠 𝐔𝐩 𝐢𝐧 𝐇𝐨𝐫𝐬𝐞𝐬 🐴
When your horse ties up, stiffens after work, or seems off in their muscle performance, it can feel like a mystery with too many possible answers. Is it PSSM? RER? Or something else entirely? Myopathies in horses often look alike on the surface, but the underlying causes, management, and outcomes can be very different. Knowing how to recognize the clues can make all the difference in keeping your horse comfortable, safe, and performing at their best.
I was able to work with Dr. Katie Young on this post, inspired by her guest appearance on the Scoop and Scale podcast hosted by Dr. Clair Thunes and Jill Jackson, as well as Dr. Stephanie Valberg, an international leader in diagnosing and treating equine neuromuscular disorders. For this post, I wanted to dive into different forms of exertional myopathies often related to “tying up” in horses.
Similar to colic, tying up is a broad term used to describe several different underlying conditions that present with similar clinical signs, all involving muscle damage. These disorders may stem from factors such as diet, overexertion, or genetic mutations. Some are classified as exertional myopathies (RER, PSSM, etc.), where episodes are triggered during or after exercise, while others fall under nonexertional myopathies (MYHM, IMM, etc.), which can develop independent of activity and are often linked to nutrition, toxins, or immune causes. To make things even more complex, certain myopathies show the classic markers of exertional rhabdomyolysis (tying up), such as high CK and AST on bloodwork, while others cause weakness, stiffness, or muscle loss without those obvious changes in serum CK activity. In this post, we’ll focus specifically on exertional myopathies.
Each form of tying up has its own distinct trigger and management strategy, what works for one horse could make things worse for another. That’s why pinpointing the exact type is so important. While this post will walk you through key differences to help you spot the signs, only a veterinarian can give you a definitive answer through diagnostics including a thorough clinical history, blood tests, genetic testing, and potentially muscle biopsies. When it comes to muscle disorders, guessing isn't good enough, successfully managing your horse’s condition depends on getting it right.
𝐒𝐩𝐨𝐫𝐚𝐝𝐢𝐜 𝐄𝐱𝐞𝐫𝐭𝐢𝐨𝐧𝐚𝐥 𝐑𝐡𝐚𝐛𝐝𝐨𝐦𝐲𝐨𝐥𝐲𝐬𝐢𝐬 (𝐄𝐑)
Often called azoturia, Monday morning disease, or simply the “classic” form of tying up, this sporadic type can affect any horse. It’s most common in horses that aren’t properly conditioned for the level of work they’re asked to perform. Triggers can include overexertion, electrolyte imbalances, heat, or an inconsistent training schedule, making it a frustrating but preventable challenge for many horse owners. Horses with this condition should have free access to or be supplemented with salt or electrolytes to meet requirements, and selenium and vitamin E status should be evaluated and the diet balanced for nutrients.
𝐑𝐞𝐜𝐮𝐫𝐫𝐞𝐧𝐭 𝐄𝐱𝐞𝐫𝐭𝐢𝐨𝐧𝐚𝐥 𝐑𝐡𝐚𝐛𝐝𝐨𝐦𝐲𝐨𝐥𝐲𝐬𝐢𝐬 (𝐑𝐄𝐑)
This form of tying up is most commonly seen in highly excitable horses, particularly young, fit Thoroughbreds, Standardbreds, and Arabians, especially those in race or endurance training with high energy demands. This condition is thought to stem from abnormal calcium regulation within muscle cells. Since calcium plays a key role in muscle contraction, this disruption can lead to painful muscle damage during or after exercise. Clinical signs include unwillingness to move, firm muscles, heavy sweating, labored breathing, muscle tremors, dark brown urine, and elevated serum creatine kinase (CK) and aspartate aminotransferase (also known as aspartate transaminase; AST) levels. If these signs are observed, a vet should be contacted immediately.
While a specific gene has not been directly linked to RER, research suggests it’s influenced by both genetics and environment. Risk factors include a nervous temperament, being female, limited turnout, inconsistent exercise, and high-stress environments (Valberg et al., 1999). While these horses aren’t as sensitive to high starch and sugar in the diet, a low to moderate nonstructural carbohydrate and moderate to high fat diet can help manage this condition, along with regular turnout and consistent work. Veterinarians may prescribe a drug called dantrolene to help manage the disease.
𝐏𝐨𝐥𝐲𝐬𝐚𝐜𝐜𝐡𝐚𝐫𝐢𝐝𝐞 𝐒𝐭𝐨𝐫𝐚𝐠𝐞 𝐌𝐲𝐨𝐩𝐚𝐭𝐡𝐲 𝟏 (𝐏𝐒𝐒𝐌𝟏)
This condition is linked to a mutation in the glycogen synthase 1 (𝘎𝘠𝘚1) gene, which results in excessive glycogen storage in the muscle. Unfortunately, horses with PSSM1 are not efficient at converting stored glycogen to energy, especially when consuming a high-sugar or starch diet. This may result in lethargy or episodes of tying up. Clinical signs are similar to RER but may also include lack of energy, gait abnormalities, or mild colic and are not related to stress and excitement.
This form is commonly found in draft and stock-type horses but has been identified in many breeds to date, excluding Thoroughbreds, Standardbreds, and Arabians. Thankfully, PSSM1 is well understood and can be identified with genetic testing. Management includes forage with
