The Collaborative International Dictionary of English v.0.48:

effortful \effortful\ adj.
   1. requiring great physical effort. Opposite of effortless.
      [Narrower terms: arduous, backbreaking, back-breaking,
      grueling, gruelling, hard, heavy, laborious, labourious,
      punishing, slavish, strenuous, toilsome; exhausting,
      tiring, wearing, wearying] Also See: difficult, hard.
      [WordNet 1.5]

WordNet (r) 3.0 (2006):

effortful
    adj 1: requiring great physical effort [ant: effortless]