- strenuous - characterized by or performed with much energy or force; "strenuous exercise"
Antonyms: lethargic, unergetic (indirect, via energetic)
- arduous, straining, strenuous - taxing to the utmost; testing powers of endurance; "his final, straining burst of speed"; "a strenuous task"; "your willingness after these six arduous days to remain here"- F.D.Roosevelt
Antonym: effortless (indirect, via effortful)