Exertional rhabdomyolysis syndrome (ERS), also known as "tying up", is a term used to describe a disease in which horses develop abnormal muscle pain and cramping during or immediately after exercise. ERS can develop as a one-time event, sporadic episodes, or chronic recurring events.

• Sporadic or One-time episodes: Sporadic or one-off episodes are usually caused by a dietary imbalance (electrolyte imbalance, vitamin E and/or selenium deficiency, excessive grain in diet), overexertion (increase in exercise intensity beyond current level or heat exhaustion which causes a muscle strain), or from a viral infection.
• Recurring (repeated) episodes: Recurrent tying up episodes are more likely to be related to horses with an underlying myopathy, such as Polysaccharide Storage Myopathy, Immune-Mediated Myositis, and Recurrent exertional rhabdomyolysis. The recurrent form is usually found after the horse reaches a level of fitness, during slower pace exercise, or before an event.

Typing episodes typically occur within 15 to 30 minutes of exercise. All of a sudden, the horse becomes stiff and develops a stilted, short-strided gait and may even refuse to move. Other clinical signs may include muscle fasciculations or trembling, excessive sweating, increased heart rate, firm and painful muscles, lameness, or sudden onset of weakness.