The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

         And spirit-stirring wine, that strenuous makes.
                                                  --Chapman.
   [1913 Webster]

         Strenuous, continuous labor is pain.     --I. Taylor.
   [1913 Webster] -- Stren"u*ous*ly, adv. --
   Stren"u*ous*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

strenuous
    adj 1: characterized by or performed with much energy or force;
           "strenuous exercise"
    2: taxing to the utmost; testing powers of endurance; "his
       final, straining burst of speed"; "a strenuous task"; "your
       willingness after these six arduous days to remain here"-
       F.D.Roosevelt [syn: arduous, straining, strenuous]

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "strenuous":
   Herculean, active, arduous, breathless, burdensome, demanding,
   determined, difficult, dogged, dynamic, eager, earnest, effortful,
   energetic, enthusiastic, exhausting, hard, indefatigable, intense,
   laborious, lusty, mean, operose, persistent, pertinacious,
   red-blooded, resolute, sincere, taxing, tenacious, tireless,
   tiring, toilsome, tough, uphill, vigorous, vital, wicked,
   zealous