knee jerk vs reflex action

knee jerk and reflex action both are nouns.

Word	Noun	Adjective	Verb	Adverb
knee jerk	Yes	No	No	No
reflex action	Yes	No	No	No

As nouns, reflex action is a hypernym of knee jerk; that is, reflex action is a word with a broader meaning than knee jerk:

knee jerk: a reflex extension of the leg resulting from a sharp tap on the patellar tendon
reflex action: an automatic instinctive unlearned reaction to a stimulus

knee jerk (noun)	reflex action (noun)
a reflex extension of the leg resulting from a sharp tap on the patellar tendon	an automatic instinctive unlearned reaction to a stimulus