The condition of tying up (muscle breakdown) is the painful muscle disorder most horse people have probably heard about. It is muscle breakdown that results in cramping.  However, tying up is more complicated than you might think. Dr. Erica McKenzie indicates in Equine Disease Quarterly that tying up is actually divided into three distinct disorders that are heritable and therefore affect certain breeds. 

Currently, the most clearly defined muscular disorders of athletic breeds are polysaccharide storage myopathy, hyperkalemic periodic paralysis, and glycogen branching enzyme deficiency because the genetic mutations causing these diseases have been found.  Polysaccharide storage myopathy is common in quarter horses and related breeds and typically causes signs of tying up.  Standardbreds and thoroughbreds also have a disorder causing tying up with clinical signs that usually occurs during training rather than racing.  The disorder is called recurrent exertional rhabdomyolysis and it appears to be more common in faster horses and the genetics of the disease in thoroughbreds and standardbreds is still unknown despite many years of work.  Warmblood and Arabian horses might also share a disorder called myofibrillar myopathy, which causes tying up in Arabians as well as poorly defined movement abnormalities in warmbloods.  The genetic cause of the syndromes in Arabians, warmbloods and thoroughbreds is unknown and no genetic tests are available for the condition.  However, there is a genetic test that looks for polysaccharide storage myopathy in at least one type of tying up in quarter horses and related breeds.  So, if you have a horse that ties up frequently, depending on the breed, there may be some genetic testing available to help diagnose the condition.