Ergonomically is the adverb: in an ergonomic way. It tells you how something is designed or arranged — so the body can work with less strain. A desk may be ergonomically designed; a handle may sit ergonomically in the hand. You will hear it in product talk and in workplace advice. It is a long, technical-sounding word; in speech people sometimes just say “it’s better for your back.” Use it when you need to modify a verb or a whole design claim. It does not name the chair itself (that is ergonomic) and it does not name the field (ergonomics).