strenuous vs straining

strenuous is not a noun while straining is a noun.

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

As adjectives, strenuous and straining are synonyms defined as:

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

Other synonyms of strenuous include arduous.

strenuous (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

strenuous (adjective)	straining (adjective)
taxing to the utmost; testing powers of endurance	taxing to the utmost; testing powers of endurance
characterized by or performed with much energy or force