arduous vs straining

arduous is not a noun while straining is a noun.

Word	Noun	Adjective	Verb	Adverb
arduous	No	Yes	No	No
straining	Yes	Yes	No	No

As adjectives, arduous and straining are synonyms defined as:

arduous and straining: taxing to the utmost; testing powers of endurance

Other synonyms of arduous include strenuous.

arduous (noun)	straining (noun)
	the act of distorting something so it seems to mean something it was not intended to mean
	an intense or violent exertion

arduous (adjective)	straining (adjective)
difficult to accomplish; demanding considerable mental effort and skill	taxing to the utmost; testing powers of endurance
characterized by effort to the point of exhaustion; especially physical effort
taxing to the utmost; testing powers of endurance