June 13, 2023

Signs your horse may be dealing with a muscle disorder

Image of a horse's loss in topline

Has your horse’s performance declined while you seem to be exerting more and more effort to get and keep your horse going? Read on as this month’s Equine Blog Post delves into muscle disorders, specifically Polysaccharide Storage Myopathy(PSSM) and Myofibrillar Myopathy (MFM).

Muscle disorder symptoms can be difficult to identify and even confused with other conditions such as Gastric ulcers, EPM, training and/or behavioral issues.

Here are the most common signs:

  • Loss of energy and enthusiasm for exercise
  • Loss of muscle over the topline/back    
  • Difficulty engaging and collecting
  • Harder to pick up the canter, to keep leads or to perform flying changes
  • Asymmetry in their gaits or even a slight lameness that seems to move around
  • Sensitive to grooming and/or saddling
  • Starts out ok under saddle, but after a short time becomes reluctant to move forward and perform
  • Back and/or hind end soreness/tightness


What are these muscle disorders?

Polysaccharide StorageMyopathy (PSSM) - Polysaccharide Storage Myopathy is an equine muscle condition in which there is a buildup of excessive glycogen in the muscle fibers, making them more prone to injury and fatigue. There are two types of PolysaccharideStorage Myopathy, PSSM 1 and PSSM 2. Both have abnormal muscle biopsies, but PSSM1 has a mutated GYS1 gene. GYS1 handles the storage of glycogen in muscle fibers for fuel later. PSSM 1 is predominantly seen in Quarter Horses, Paints,Appaloosas, and their crosses and rare to non-existent in rare to nonexistent in Thoroughbreds, Standardbreds, and Arabians.

PSSM 2 has no genetic mutation and is still unclear as to its underlying cause.

Draft horses, Morgans, Haflingers, warmbloods, and gaited breeds liked the Tennessee Walking Horses have been diagnosed with both Type 1 and Type 2 PSSM.

Myofibrillar Myopathy(MFM) - Myofibrillar myopathy or MFM is a new disease in horses, particularlyArabian and Warmblood horses, identified by the College of Veterinary Medicine at Michigan State University. It is characterized by the degeneration of the myofibrils, which are the structural components of muscle fibers. This leads to muscle weakness, stiffness, and difficulty moving. MFM up until recently was also confused with PSSM 2.

Muscle Biopsy locations and closeup of an incision


Diagnosing PSSM and MFM typically involves the following steps:

1.   Clinical Examination: Your veterinarian will conduct a thorough physical examination to assess the horse’s muscle condition and look for any signs of pain or weakness. They will also rule out much more common issues, such as poor saddle fit, lameness from hock, stifle of SI degeneration, soft tissue injuries, inflammation of the neck or back vertebrae, etc.

2.   Blood tests: Blood tests can be used to rule out other potential causes of muscle disease and check for elevated levels of certain enzymes that can indicate muscle damage.

3.   Biopsy: A muscle biopsy is often performed to obtain a sample of muscle tissue for examination under a microscope. This can help confirm the presence of PSSM or MFM and determine the severity of the disease.

4.   Genetic testing: Genetic testing can be used to determine if the horse is carrying the specific genetic mutation associated with PSSM 1. It is not recommended for PSSM 2 and MFM horses.

Treating horses with these muscle conditions

There are no cures for PSSM and MFM, but there are ways to manage these conditions and ensure optimal performance for your horse. It is important to work with a veterinarian experienced in diagnosing and treating muscle disorders in horses to obtain an accurate diagnosis and develop an appropriate treatment plan.

Some suggestions may include: 


Adding quality protein and specific amino acids to their diet, so your horse can make the proteins necessary to rebuild muscle and energy. It is also believed that oxidative stress aids the degenerative process, thus antioxidants are important to add to your horse’s diet to support cellular mitochondria.



Consistent exercise helps the proteins in the muscle to strengthen them and build the enzymes needed to burn energy as fuel. Long and low warm-ups and stretching are key as is their frame. It’s important to have relaxation at the base of the neck and stretching over the topline to activate and build up their core muscles. You may even want to look into an Equiband system, Vienna reins, a neck stretcher or a Pessoa-like system. These are recommended over side reins to get that long and low stretch. Vary your routine when possible. Some days get out and do some hill work, a trail ride, pole or cavaletti are all excellent exercises for PSSM and MFM horses. Also, play around with your program. Does your horse go better with 2 days on, 1 day off? Or 3 days on 2 days off.