Arduous

1. adjective

   characterized by effort to the point of exhaustion; especially physical effort

   “worked their arduous way up the mining valley”

   Similar: backbreaking, grueling, gruelling, hard, heavy, laborious

2. adjective

   difficult to accomplish; demanding considerable mental effort and skill

   “the arduous work of preparing a dictionary”

3. adjective

   taxing to the utmost; testing powers of endurance; ; ; - F.D.Roosevelt

   “his final, straining burst of speed”

   Similar: straining, strenuous