Horses with exertional myopathies can experience muscle fatigue, pain, cramping, and damage. Special modifications to diet and exercise can help these horses. Stephanie Valberg, DVM, PhD, Dipl. ACVIM, ACVSMR, of the Michigan State University College of Veterinary Medicine’s Department of Large Animal Clinical Sciences described how to maintain these horses in her presentation at the 2016 American Association of Equine Practitioners Convention, held Dec. 3-7 in Orlando, Florida. She stressed that for horses to improve, they need both regular exercise and dietary management.


Valberg said recurrent exertional rhabdomyolysis (RER), also known as tying-up, is most likely caused by abnormal calcium cycling, a process that controls muscle contraction and relaxation within the muscle cell. Horses suffering from an acute episode of RER have very stiff, hard, painful muscles and are reluctant to move. Thoroughbreds, Standardbreds, and Arabians appear to be predisposed. The condition often occurs in fit animals, and is frequently triggered by stress and exercise.

Polysaccharide storage myopathy (PSSM) comes in two forms. Type 1 PSSM is caused by a mutation in the gene that regulates the body’s synthesis of the enzyme glycogen synthase, causing a buildup of glycogen (the storage form of glucose) in the muscle cell. Type 2 PSSM has no known cause, but is diagnosed via muscle biopsy revealing abnormal glycogen “clumping,” in cells. These horses do not have the genetic mutation seen in Type 1 PSSM.

Diet and Exercise: Special Care Required

The general rule of thumb with these horses is to keep nonstructural carbohydrate (NSC, starch and sugar) levels low and provide extra calories with fat. In horses with RER, Valberg recommended restricting calories provided by NSCs to less than 20% of the diet to help minimize excitability, and providing up to 20% of calories with fat. Body condition of the horse matters, as do caloric needs. Horses receiving 5 kilograms (11 pounds) or more of grain tend to be at higher risk for an episode of RER than horses receiving 2.5 kilograms (5.5 pounds) or less.

For Type 1 PSSM horses, keep NSCs to 10-15% of the total diet. A diet lower in NSCs will lessen the secretion of insulin in response to a meal. Since insulin stimulates the body’s production of glycogen synthase, reducing the amount of circulating insulin in PSSM horses is important to help manage the amount of glycogen in the muscles. The amount of fat needed depends on the horse’s body condition and exercise demands. Traditionally, horses diagnosed with PSSM have been fed similarly, but researchers have recently shown that Type 2 PSSM horses do not have the same high muscle glycogen concentrations and, so, might not need strict restriction of NSCs. For horses with Type 1 or Type 2 PSSM that experience decreased muscle mass, an amino acid or protein supplement may be warranted to help support muscle development.

“The easiest thing to change when a horse is diagnosed with PSSM is the diet, but I always emphasize that the diet itself is a small piece of the solution and for muscle to properly utilize the appropriate diet horses need to be exercised regularly in a training program,” Valberg said.

“For Quarter Horses with Type 1 and Type 2 PSSM regular daily exercise in the form of riding or round pen work for at least 15 minutes really helps to decrease episodes of muscle damage,” she said. “For Type 2 PSSM in Warmbloods and English riding, long and low warmup exercise to relax muscles in the base of the neck, lift the back, and engage abdominal and core muscles is really essential to decrease muscle soreness and enhance willingness to go forward and work under saddle.”

She cautioned that changes to exercise regimens need to be introduced gradually and walk breaks should be interspersed with intervals of work to build proper strength.

Take-Home Message

Exertional myopathies in horses come in several forms, and good management starts with a proper diagnosis. A trained equine nutritionist can help formulate a diet with proper amounts of carbohydrates, fat, and nutrients to best support proper nourishment of an affected animal. In addition to diet, owners of these horses should follow a carefully prescribed exercise regimen to gradually enhance muscle metabolism and promote optimal fitness.