noun (n)

• use of physical or mental energy; hard work(noun.act)

  Example:
  They managed only with great exertion.
  

  Synonym:
  effort, elbow grease, sweat, travail
  

  source: wordnet30
• 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.(noun)

  source: webster1913