Source : Webster's Revised Unabridged Dictionary (1913)

Strenuous \Stren"u*ous\, a. [L. strenuus; cf. Gr. ? strong,
   hard, rough, harsh.]
   Eagerly pressing or urgent; zealous; ardent; earnest; bold;
   valiant; intrepid; as, a strenuous advocate for national
   rights; a strenuous reformer; a strenuous defender of his
   country.

         And spirit-stirring wine, that strenuous makes.
                                                  --Chapman.

         Strenuous, continuous labor is pain.     --I. Taylor.
   -- {Stren"u*ous*ly}, adv. -- {Stren"u*ous*ness}, n.

Source : WordNet®

strenuousness
     n : extreme effortfulness [syn: {arduousness}]