WordNet sense 1 (taxing to the utmost; testing powers of endurance):

strenuous, straining

WordNet sense 2 (characterized by toilsome effort to the point of exhaustion; especially physical effort):

backbreaking, grueling, gruelling, heavy, laborious, punishing, toilsome, operose

WordNet sense 3 (not easy; requiring great physical or mental effort to accomplish or comprehend or endure):

difficult, hard