"strenuous" in the adjective sense
1. strenuous
characterized by or performed with much energy or force
"strenuous exercise"
2. 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
Source: WordNet® (An amazing lexical database of English)
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