bending vs flexion

bending and flexion both are nouns.

Word	Noun	Adjective	Verb	Adverb
bending	Yes	No	No	No
flexion	Yes	No	No	No

As nouns, flexion is a hyponym of bending; that is, flexion is a word with a more specific, narrower meaning than bending:

bending: the act of bending something
flexion: act of bending a joint; especially a joint between the bones of a limb so that the angle between them is decreased

Other hyponyms of bending include flexure, crouch, hunch, incurvation.

bending (noun)	flexion (noun)
the act of bending something	act of bending a joint; especially a joint between the bones of a limb so that the angle between them is decreased
the property of being bent or deflected	deviation from a straight or normal course
movement that causes the formation of a curve	the state of being flexed (as of a joint)