Recently it has been suggested that walking may be harmful but many mild cases have improved by being walked. Bute should be given for pain. If the stiffness is severe current recommendations are to not move the horse. Many severe cases are associated with intense exercise. If dehydration or electrolyte imbalances are possible, IV fluids should be used to correct these problems.
Corticosteroids make sense for early treatment to help prevent further cellular damage.
Muscle relaxers seem to be rational therapy but questions of efficacy and potential toxicity do not make them a clear choice. Check with your vet for the latest on these two. Routine use of electrolyte additions to the feed are not neccassarily good. However electrolyte supplementation during times of maximal stress is recommended. As a horse sweats more and his electrolyte concentrations deplete his sweat becomes less salty thereby conserving needed electrolytes.