what i make out of it although not too sure is that when muscle contract and relax they need minerals and affect the osmotic balance and hence a signal to the brain is sent asking for more salt. is this hypothesis true
The salt concentration overall should be constant under normal conditions in the body. When you excercise you sweat, thats when the loss of Na comes in. Try to taste your sweat to prove this. Your craving of salt is to replenish that.