The Collaborative International Dictionary of English v.0.48:

Exertion \Ex*er"tion\, n.
   The act of exerting, or putting into motion or action; the
   active exercise of any power or faculty; an effort, esp. a
   laborious or perceptible effort; as, an exertion of strength
   or power; an exertion of the limbs or of the mind; it is an
   exertion for him to move, to-day.

   Syn: Attempt; endeavor; effort; essay; trial. See Attempt.
        [1913 Webster]

WordNet (r) 3.0 (2006):

exertion
    n 1: use of physical or mental energy; hard work; "he got an A
         for effort"; "they managed only with great exertion" [syn:
         effort, elbow grease, exertion, travail, sweat]

Moby Thesaurus II by Grady Ward, 1.0:

43 Moby Thesaurus words for "exertion":
   action, active use, activity, appliance, application, assiduity,
   assiduousness, consumption, determination, diligence, drive,
   effort, elbow grease, employ, employment, endeavor, exercise,
   exercising, good use, hard pull, hard usage, hard use, ill use,
   industry, misuse, operation, pains, push, resolution, rough usage,
   sedulity, sedulousness, strain, striving, struggle, toil, trouble,
   usage, use, using up, while, work, wrong use