erectly vs straight-backed

erectly is not an adjective while straight-backed is an adjective.

Word	Noun	Adjective	Verb	Adverb
erectly	No	No	No	Yes
straight-backed	No	Yes	No	Yes

As adverbs, erectly and straight-backed are synonyms defined as:

erectly (adjective)	straight-backed (adjective)
	having a straight back

erectly (adverb)	straight-backed (adverb)
in a straight-backed manner	in a straight-backed manner