Horses suffer from a variety of muscle disorders that can result in tying-up. It goes by several other names such as exertional rhabdomyolysis or Monday morning disease.

Clinical signs of tying-up include reluctance to exercise, mild stiffness, exercise intolerance, poor performance, and muscle weakness or contractures.

What Is Tying-Up?

It is a complex syndrome of events and imbalances that has many causes. The most common form is acute myositis (inflammation of muscle tissues). Exercising a horse beyond its level of physical fitness can cause tying-up.

In some horses this causes a build-up of metabolic wastes that the body is not able to remove rapidly enough. Eventually, this causes changes in acid-base balance, energy metabolism, lactic acid levels, muscle pH, electrolyte balance, contractility mechanism, and blood flow. That results in muscle fatigue, abnormal contracture, pressure, and pain.

Other Contributors

Concurrent respiratory infections (EIV/EHV) and dietary imbalances (high carbohydrate diets, electrolytes) have implicated in some acute, sporadic episodes of tying-up.

Chronic or recurrent cases of tying-up can be frustrating to owners, trainers, and veterinarians. These horses might have an underlying abnormality of muscle function that is hereditary in nature.

A variety of factors can cause Chronic exertional rhabdomyolysis. Electrolyte depletion due to dietary inadequacies or due to sweat losses during exercise can be a basis for developing a chronic condition.

High dietary starch can stimulate repeated muscle problems. This might be due to excitability levels or could be related to interaction with glycogen storage within the muscle tissue.

Reproductive hormones are implicated in some cases of chronic exertional rhabdomyolysis.

PSSM and Tying-Up

Polysaccharide storage myopathy (PSSM) have an inherited abnormality associated with glycogen and polysaccharide storage in muscle. This can cause tying-up PSSM horses.

Altered hexokinase binding to mitochondria can lead to insufficient ATP production and tying-up. Most PSSM horses have a relatively calm demeanor.

PSSM occurs in many horse breeds. Warmbloods, Quarter Horses, Draft breeds, Paint horses, and Appaloosas are among the most common breeds that suffer from tying-up. PSSM is characterized by the accumulation of glycogen and an abnormal polysaccharide in skeletal muscle.

Recurrent Tying-Up

A genetic autosomal dominant trait causes recurrent exertional rhabdomyolysis. It is an intermittent defect in the regulation of muscle contraction and manifests as periods of tying-up of varied severity.

The condition tends to be more frequent in horses with a nervous disposition. Dietary starch, season of year, exercise duration, exercise intensity, and lameness can trigger tying-up.

Managing Tying-Up

Prevention of tying-up involves a variety of approaches that include nutrition, exercise management, and recognition of the factors that might lead to myositis in your particular horse.

Working with your veterinarian can help to develop programs that will minimize the occurrence of tying-up.

Editor’s note: This information about tying-up in horses is courtesy of the equine veterinary experts at Hagyard Equine Medical Institute in Kentucky.

Further Reading