strenuous
     adj 1: characterized by or performed with much energy or force;
            "strenuous exercise"
     2: 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 [syn: {arduous}, {straining}]
Source: WordNet® 2.0