Tying-up is a generic term (also referred to as RER [recurring or intermittent exertional rhabdomyolysis], azoturia, and Monday morning sickness) commonly used to describe muscle disease in performance horses. 

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Classically, tying-up occurs after 15 to 30 minutes of aerobic exercise. Horses lose impulsion, develop a stiff, stilted gait, and may pause and stretch out as if to urinate or paw the ground. They may sweat excessively, develop a high respiratory rate, and then become locked in place, unable to walk forward due to painful muscle contractions, generally involving the back and hindquarters. 

When horses have repeated episodes of tying-up, the disease is considered chronic. Sporadic tying-up is more than just muscle soreness from pre-existing lameness or fatigue. The most frequent causes of sporadic tying-up are exercise that exceeds a horse’s level of training, electrolyte imbalances, hyperthermia, and strenuous exercise while suffering from a viral respiratory disease. A darkening of the urine may indicate severe muscle damage.

Skeletal muscle shows remarkable ability to regenerate following injury, and complete repair of muscle tissue is possible within four to eight weeks after an episode.

Sporadic tying-up should be considered a veterinary emergency if horses are sweating profusely, reluctant to move or have dark urine. Source: KER 2011 - Here is an informational link: