quadriceps femoris vs extensor

quadriceps femoris and extensor both are nouns.

Word	Noun	Adjective	Verb	Adverb
quadriceps femoris	Yes	No	No	No
extensor	Yes	No	No	No

As nouns, extensor is a hypernym of quadriceps femoris; that is, extensor is a word with a broader meaning than quadriceps femoris:

quadriceps femoris: a muscle of the thigh that extends the leg
extensor: a skeletal muscle whose contraction extends or stretches a body part

Other hypernyms of quadriceps femoris include extensor muscle.

quadriceps femoris (noun)	extensor (noun)
a muscle of the thigh that extends the leg	a skeletal muscle whose contraction extends or stretches a body part