sit back vs loosen up

sit back and loosen up both are verbs.

Word	Noun	Adjective	Verb	Adverb
sit back	No	No	Yes	No
loosen up	No	No	Yes	No

As verbs, loosen up is a hypernym of sit back; that is, loosen up is a word with a broader meaning than sit back:

Other hypernyms of sit back include decompress, relax, slow down, unbend, unwind.

sit back (verb)	loosen up (verb)
settle into a comfortable sitting position	become less tense, rest, or take one's ease
be inactive or indifferent while something is happening	cause to feel relaxed
	make one's body limber or suppler by stretching, as if to prepare for strenuous physical activity
	cause to become unblocked
	become less tense, less formal, or less restrained, and assume a friendlier manner