Your body uses sodium to contract the muscles, cells transport sodium in and out to regulate expressions.
if you eat too much sodium, it becomes harder to transport it out of the cells, so they will keep more of it in, which will trigger muscle contraction (as it is supposed to).
this is one of the reasons why eating too much salt is bad for you (and even eating 'normal' food, without adding salt yourself puts you at risk of getting too much)