There are many forms of tying up but there are two main ones that will respond to nutritional management.
Recurrent Exertional Rhabdomyolysis (RER) - this is when muscles contract, calcium is released from muscle cells and then is taken back up to allow relaxation. It is thought that this mechanism is 'faulty' in horses with RER.
Polysaccharide Storage Myopathy (PSSM) - Sub-clinical PSSM is common and may simply result in a horse feeling stiff or lazy when ridden. These horses often have persistently abnormal blood CK levels.