adj. 25B6; adjective
a strenuous climb : ARDUOUS, difficult, hard, tough, taxing, demanding, exacting, exhausting, tiring, gruelling, back-breaking; informal killing; Brit. informal knackering; archaic toilsome.
strenuous efforts : VIGOROUS, energetic, zealous, forceful, strong, spirited, intense, determined, resolute, tenacious, tireless, indefatigable, dogged; formal pertinacious.
easy, half-hearted.