straining vs arduous

straining is a noun but arduous is not a noun.

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

As adjectives, straining and arduous are synonyms defined as:

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

Other synonyms of straining include strenuous.

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

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