Based on our current knowledge, horses which experience episodes of tying-up can be placed in one of two broad categories:
1) Sporadic exertional rhabdomyolysis--this applies to horses which, on rare occasion, experience an episode of generalized tying-up;
2) Chronic exertional rhabdomyolysis--when a horse experiences repeated episodes of ER, with the first episode usually occurring when they are young.

A common cause of sporadic tying-up is exercise that exceeds the horse's underlying state of training, e.g., asking a horse to undertake competition exercise after a lay-off and with only minimal training before the event. Electrolyte imbalances (particularly low sodium) and deficiencies of vitamin E and/or selenium might play a role in some cases. Sporadic tying-up can occur in horses which have concurrent illness, particularly respiratory viral infections.

The second common cause of chronic tying-up, polysaccharide storage myopathy, is a condition characterized by accumulation of glycogen (a form of carbohydrate) and an abnormal form of polysaccharide (a complex carbohydrate) in muscle. PSSM has been identified in Quarter Horses and related breeds (Appaloosas, Paints), warmbloods, draft horses, and a few Thoroughbreds. In the Quarter Horses and related breeds, there appears to be a hereditary basis for PSSM. Horses with PSSM have increased ability to clear glucose from the blood, resulting in an increase in the rate of glycogen synthesis in muscle; muscle glycogen concentrations are 1.5 to four times those of normal horses.