strenuous

Strenuous means needing great effort. The kind of work that leaves you unable to speak for a minute afterwards.

Exercise is the usual home for it, and doctors warn people off strenuous activity after an operation. The word also describes effort of other kinds, and strenuous efforts, strenuous objections and a strenuous denial are all standard phrases where nobody is lifting anything. Those uses mean forceful and sustained rather than physically hard. The adverb strenuously nearly always attaches to denied, which is a habit of court reporting.