Tying Up or Monday Morning Disease

Muscle Disease of Performance Horses

Muscle disease in performance horses commonly is referred to as tying-up, exertional rhabdomyolysis, azoturia, or Monday morning disease. Horses experiencing tying-up show a stiff gait; reluctance to move; firm, painful muscle cramps; profuse sweating; increased heart rate; and increased respiratory rate. Horses that tie up also have moderate to marked elevations of certain muscle proteins in their blood, indicating muscle cell damage.

For years, all horses that tied up following exercise were thought to suffer from the same disease. However, treatment and prevention protocols that worked on some horses did not help others. As a result, confusion and controversy developed regarding the cause and treatment of tying-up. Researchers have only recently begun to classify the different diseases that result in the common signs of tying-up.

Initial classification of tying-up is now based on the frequency of mus-cle cramps and damage following exercise. Horses which tie up only a few times in their life are classified as “sporadic,” while horses that tie up on a repeated basis are termed “chronic.”

Sporadic Tying-Up

Many horses experience some muscle soreness and strain associated with exercise. Muscle strains are much more severe and are typically accompanied by elevated muscle proteins (aspartate aminotransferase and creatine kinase) in serum and myoglobinuria (coffee-colored urine). This darkening of the urine is a result of the kidneys filtering myoglobin (a muscle protein) from blood, which is an indication of severe muscle damage.

The most frequent causes of sporadic tying-up are exercise that exceeds a horse’s underlying state of training, electrolyte imbalances, hyperthermia (overheating), and strenuous exercise while suffering from a respiratory disease.

Sporadic tying-up should be considered a veterinary emergency if horses are sweating profusely, are reluctant to move, or have dark urine. Veterinarians might administer medications to relieve anxiety and muscle pain and correct dehydration. Further management includes stall rest followed by hand walking and turnout once initial muscle stiffness is gone. Grain intake is drastically reduced or eliminated, since these horses are on a reduced exercise program. The amount of time the horse must remain out of training has not been established by science. Following an episode of tying-up, training should be resumed gradually and consistently to prevent further muscle damage.

Prevention of further tying-up episodes is often a result of common sense: Don’t overexert unfit horses; do fortify the diet with salt on a daily basis; use electrolytes prior to heavy sweat loss; and don’t overfeed carbohydrates (grain). Feed grain concentrates fortified with fat and necessary antioxidant vitamins and minerals to provide energy while supplying the building blocks to protect muscle tissue. The combination of these strategies can prevent another bout of tying-up. In areas where the soil is deficient in selenium, adding that and vitamin E can also improve muscle health.

Chronic Tying-Up

Many breeds of horses have been reported to have chronic bouts of tying-up, including Quarter Horses, Thoroughbreds, Standardbreds, Paints, Morgans, Arabians, and various breeds of draft and warmblood horses. It is the study of these chronically tied-up animals that has advanced our knowledge of the causes, treatments, and preventions of the syndrome.

There have been many proposed causes for tying-up that are unsubstantiated. Recently, muscle biopsy techniques have identified two causes of chronic tying-up–a disorder in carbohydrate storage (polysaccharide storage myopathy, PSSM) in Quarter Horses, warmbloods, and drafts; and a muscle contraction disorder (recurrent exertional rhabdomyolysis, RER) in Thoroughbreds.

Preliminary genetic research and breeding trials point to these separate conditions as being inherited. Both of these causes of tying-up appear to be at least partially responsive to treatment with decreased starch and increased fat in the diet, gradually increasing daily training, and daily turnout. Commercial diets have also been developed for horses with these diseases.



Tying-up is a syndrome or description of a horse with muscle damage that has many different causes. It probably is one of the most misunderstood and controversial syndromes in the athletic horse. Since there are several causes, some of which appear to be inherited, there is no single cure. Typical signs of tying-up include a horse which becomes stiff, sweats, and is reluctant to move. Researchers have learned a great deal about tying-up—or exertional rhabdomyolysis—in recent years. Unfortunately, the information has shown that some of the most common beliefs about tying-up have been proven wrong by scientific study. Thus, what was considered by some early researchers to be a problem that had one basic cause, e.g. lactic acid, is actually a broad-scale syndrome that will require continued research on a variety of fronts before every aspect is understood.

In other words, tying-up is not one disease, but several different diseases that have similar signs and different causes. Therefore, the management of a Thoroughbred that suffers from tying-up would differ from the management of a Quarter Horse that is tying-up, would differ from the management of a backyard pleasure horse that has the same symptoms.

Some horses are healthy athletes that tie-up sporadically likely due to exercise in excess of their training level, electrolyte depletion, or dietary imbalances. They respond well to rest, a gradual return to a graduated training regime, and balancing the diet. Other horses will suffer from chronic episodes of tying-up that can be debilitating. Our research suggests that there might be several inherited reasons for chronic tying-up.

Certain lines of Thoroughbreds seem more susceptible to one form of tying-up that has a basis in abnormal regulation of muscle contraction. Muscle contractions are initiated by propagation of electrical impulses along the outer cell membrane and along membranous connections extending into the cell, which then stimulate the release of calcium from intracellular membranous storage sites. Muscle relaxation requires energy-dependent pumping of calcium back into storage sites.

Our research indicates that a common cause of tying-up in Thoroughbreds is an inherited abnormality in the way calcium is regulated by membrane systems in the skeletal muscle. The narrow genetic origin of Thoroughbreds and the common lineage of the pedigrees of horses with tying-up would support the possibility of an inherited trait. The disease might lie dormant unless specific factors trigger the calcium regulatory system to malfunction. Triggering events include stress, excitement, lameness, high grain diets, and exercise at submaximal speeds.

Young fillies are most commonly affected and usually are the most nervous and high strung. Prevention of further episodes of tying-up in susceptible horses should include standardized daily routines and an environment that minimizes stress and excitement. The diet should be adjusted to include a balanced vitamin and mineral supplement, high-quality hay, and a minimum of carbohydrates (such as grain and sweet feed). Feeding less than five pounds of sweet feed and adding additional calories in the form of fat decrease muscle damage with exercise. Daily exercise is essential, either in the form of turnout, longing, or riding.

Another form of tying-up that affects Quarter Horses and related breeds, as well as warmbloods and draft breeds, is polysaccharide storage myopathy (PSSM). With this disorder, horses have an increased sensitivity to insulin resulting in increased storage of sugar (glycogen) in the muscle. This imbalance in sugar storage and accumulated precursors apparently disturb energy metabolism, and affected horses develop muscle cramping with light exercise. Affected horses are often calm, sedate horses that tie-up after a lay-up, especially when fed grain.

Treatment of polysaccharide storage myopathy involves supplying them with an alternative source of energy rather than sugar. Eliminating grain and sweet feed completely and feeding fats such as rice bran stabilizes blood sugar and decreases sugar storage in muscle. It is essential that horses with PSSM be turned out as much as possible and exercised often, even if only for 10 minutes every day. Horses with mild to moderate clinical signs might be able to return to full athletic performance with careful dietary and management changes, which include regular daily exercise without extended periods of inactivity.

Breeding Quarter Horses with polysaccharide storage myopathy has resulted in offspring with the same disorder. More research is needed in this area.

If your horse ties up, here are suggestions of what to do: 1) Stop exercising the horse and move it to a box stall. Do not force the horse to walk. 2) Call your veterinarian. 3) Blanket the horse if the weather is cool. 4) Determine if the horse is dehydrated due to excessive sweating. 5) Provide fluids with small, frequent sips of water to hot horses, and provide free access to water once the horse has cooled out. 6) Relieve anxiety and pain. Drugs such as acepromazine might be prescribed by your veterinarian. 7) Remove grain and feed; provide only hay until signs subside. 8) Small paddock turnout is good once the horse walks freely, usually in 12-24 hours. If the problem recurs, have the horse evaluated for a specific cause of recurrent exertional rhabdomyolysis.



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.