rigorousness vs stiffness

rigorousness and stiffness both are nouns.

Word	Noun	Adjective	Verb	Adverb
rigorousness	Yes	No	No	No
stiffness	Yes	No	No	No

As nouns, rigorousness and stiffness are synonyms defined as:

Other synonyms of rigorousness include hardness, harshness, inclemency, rigor, rigour, rigourousness, severeness, severity.

rigorousness (noun)	stiffness (noun)
something hard to endure	the physical property of being inflexible and hard to bend
excessive sternness	the property of moving with pain or difficulty
	the inelegance of someone stiff and unrelaxed (as by embarrassment)
	excessive sternness
	firm resoluteness in purpose or opinion or action