unbend vs change posture

unbend and change posture both are verbs.

Word	Noun	Adjective	Verb	Adverb
unbend	No	No	Yes	No
change posture	No	No	Yes	No

As verbs, change posture is a hypernym of unbend; that is, change posture is a word with a broader meaning than unbend:

unbend (verb)	change posture (verb)
release from mental strain, tension, or formality	undergo a change in bodily posture
become less tense, rest, or take one's ease
make less taut
free from flexure
unfasten, as a sail, from a spar or a stay
straighten up or out; make straight