As researchers have learned more about muscle disorders, it has become clear that the term "tying-up" consists of several muscle disorders. Recurrent Exertional Rhabdomyolysis (RER) is one of these.

RER is a painful muscular disorder brought on by exercise. It is more common in Thoroughbreds and some other light breeds. In this condition, muscles groups along the back, pelvis and rump become hard and sore during exercise. Muscle cells are damaged, and their inner contents leaked into the circulation. Muscle pigments (myoglobin) are released from this damaged muscle into the blood and filtered through the kidneys, often resulting in brownish coffee-colored urine, and possibly causing kidney damage.

Horses with RER can show a wide variety of signs. The key is that the signs are shown during or immediately after exercise. Along with stopping under saddle, seeming back pain, spasm, and swelling, other classic signs are reluctance to move and an appearance of stiffness and soreness. Severely affected horses can also show signs similar to colic: pawing, sweating, stretching and other signs.

Prompt treatment is required to prevent life-threatening kidney damage. It is important to determine the nature of any tying up episode early in the process, so the proper preventative steps and management programs can be instituted right away.

RER is caused by defective calcium metabolism in the muscle cell. The precise genetic origin is still not fully known. RER can also occur as a consequence of nutritional deficiencies and certain metabolic diseases. Regardless of the cause, a horse that develops RER must have a history of having been recently exercised.

While the signs of Tying Up may seem obvious, confirmation of the diagnosis of the condition requires blood testing to look for elevated muscle enzymes. For horses that have repeated problems with ER, muscle biopsy and microscopic examination (by a pathologist with a special interest in these disorders) is required to understand the problem and institute the proper treatment and management and to know the prognosis.