Tying-up is the most common muscle problem in horses. This syndrome is also called azoturia, set fast, paralytic myoglobinuria, and chronic exertional rhabdomyolysis (ER). HYPP (hyperkalemic periodic paralysis) in Quarter Horses is a different problem. Tying-up is not a single disease, but a collection of clinical signs with various causes. Several of these causes for tying-up have already been identified, and more are on the research horizon.

Classic tying-up symptoms include sweating, stiffness, and reluctance to move forward. Some horses with chronic tying-up might resent exercise. These symptoms are all manifestations of pain due to muscle damage that is most severe in the rear legs.

Some horses suddenly exhibit signs of tying-up with no history of a problem. These horses usually recover with rest and treatment and go on to perform successfully. Causes can include exercise in excess of training; exhaustive exercise, especially on hot, humid days; and respiratory infections (tying-up seems more common after viral infection has swept through a barn). Some horses have chronic tying-up from a young age, even when exercised lightly.

Diet appears to influence some forms of ER. In some athletic horses, a normal diet of oats and hay doesn't provide sufficient salt and minerals to fulfill demands on the muscle. Supplementing with a balanced vitamin/mineral mixture and access to salt might alleviate symptoms. Unfortunately, the majority of horses with ER don't respond to mineral supplementation alone, and a more thorough investigation into their cause of ER is necessary.

Polysaccharide Storage Myopathy

One cause of chronic ER in Quarter Horse-related breeds, draft horses, and warmbloods is a metabolic defect called polysaccharide storage myopathy (PSSM). Horses with PSSM store excess glycogen (sugar) in their muscles--the excess of sugar and its byproducts disrupts the balance of energy metabolism and the muscles of these horses cramp and become stiff.

Treat PSSM horses by feeding them to maintain low blood sugar and insulin concentrations. High-starch, high-sugar feeds such as grains and sweet feed should be avoided. High-fat feeds, such as processed rice bran (20% fat), are good choices to provide energy without increasing blood glucose and insulin.

However, without changes in daily exercise, PSSM horses will have problems despite fat supplementation. In order to prevent muscle stiffness, these horses should be turned out as much as possible and exercised regularly. If they have been laid up for more than a few days, return them to work very gradually. Over 90% of horses will return to full athletic performance by following these changes in diet and exercise.

Recurrent Exertional
Rhabdomyolysis (RER)

Some horses tie-up because of an abnormality in the way the muscle cells regulate intracellular (within the cell) calcium during a muscle contraction. This abnormality is not related to dietary calcium intake. This form of tying-up is called recurrent exertional rhabdomyolysis (RER).

RER occurs most often in Arabians, Standardbreds, and Thoroughbreds. It occurs in 5% of racing Thoroughbreds, especially young, nervous fillies. Episodes of muscle stiffness usually occur when they are faced with exercise and excitement. In Standardbreds, this form of tying-up often occurs after 15 minutes of jogging. Manage these horses by minimizing excitement and stress and de-conditioning them to excitable stimuli. Provide daily turnout and exercise with little time spent in a stall.

One challenge of feeding horses in heavy training which are predisposed to RER is maintaining enough calories without feeding so much grain that they become high-strung. There are many commercial dietary fat supplements available that are good energy sources for horses in moderate to heavy training, including newly developed feeds designed for horses with RER.


In recent years, we have seen dramatic advances in the characterization of tying-up. Research continues to develop better diagnostic tests and treatments for the various of forms of tying-up. Investigators hope that this research will help owners and trainers better deal with these frustrating, painful equine muscle conditions.

About the Author

Stephanie Valberg, DVM, PhD, Dipl. ACVIM, ACVSMR

Stephanie Valberg, DVM, PhD, Dipl. ACVIM, ACVSMR, is the Mary Anne McPhail Dressage Chair in Equine Sports Medicine and a Professor of Large Animal Clinical Sciences at Michigan State University. She is a leading researcher on the subject of tying-up and the genetic basis for equine neuromuscular disorders.

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