leg extensor vs leg exercise

leg extensor and leg exercise both are nouns.

Word	Noun	Adjective	Verb	Adverb
leg extensor	Yes	No	No	No
leg exercise	Yes	No	No	No

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

leg extensor: an exercise designed to strengthen the extensor muscles of the leg
leg exercise: exercise designed to strengthen the leg muscles

leg extensor (noun)	leg exercise (noun)
an exercise designed to strengthen the extensor muscles of the leg	exercise designed to strengthen the leg muscles