Gypsy, the Equine Encyclopedia, has something to say about Rhabdomyolysis. Like all shetlands, she is always watching, learning and listening.
Rhabdomyolysis, also known as tying up, Monday Morning Sickness or azoturia, is the destruction of muscle tissue which leads to a stiff gait, mainly in the hindquarters.
It is genetic, with dominant gene inheritance. Equine Polysaccharide Storage Myopathy (EPSM), is a form of Rhabdomyolysis and is genetic.
Tying Up also can be caused by abrupt changes in exercise or failure to walk a horse out after hard work and earned the name “Monday Morning Sickness” because when horses were worked hard all week and given a day off, on Monday morning they would become stiff and “tie up”.
If a horse has tied up, it is best not to move him and to call the vet immediately.