Tying-up is the most common muscle problem in horses. This syndrome is also called azoturia, set fast, paralytic myoglobinuria, and chronic exertional rhabdomyolysis (ER). HYPP (hyperkalemic periodic paralysis) in Quarter Horses is a different problem. Tying-up is not a single disease, but a collection of clinical signs with various causes. Several of these causes for tying-up have already been identified, and more are on the research horizon.

Classic tying-up symptoms include sweating, stiffness, and reluctance to move forward. Some horses with chronic tying-up might resent exercise. These symptoms are all manifestations of pain due to muscle damage that is most severe in the rear legs.

Some horses suddenly exhibit signs of tying-up with no history of a problem. These horses usually recover with rest and treatment and go on to perform successfully. Causes can include exercise in excess of training; exhaustive exercise, especially on hot, humid days; and respiratory infections (tying-up seems more common after viral infection has swept through a barn). Some horses have chronic tying-up from a young age, even when exercised lightly.

Diet appears to influence some forms of ER. In some athletic horses, a normal diet of oats and hay doesn’t provide sufficient salt and minerals to fulfill demands on the muscle. Supplementing with a balanced vitamin/mineral mixture and access to salt might alleviate symptoms. Unfortunately, the majority of horses with ER don’t respond to mineral supplementation alone, and a more thorough investigation into their cause of ER is necessary