strenuous
adj.needing a lot of physical or mental effort. You use this to describe tasks that are very tiring or difficult.
adj.requiring or using great exertion or high intensity. Often describes physical activities or mental efforts that test the limits of one's stamina.
“The doctor told him to avoid strenuous exercise for a week.”
“After a strenuous day of hiking up the mountain, the group was exhausted but happy.”
“The legal team made a strenuous effort to overturn the verdict, citing several procedural errors that occurred during the initial trial.”
Commonly modifies nouns like 'exercise', 'effort', 'activity', or 'objection'.
“The result is a comedy without gags, a comedy which, for all the strenuous high-jinks, is afraid to be flippant or mean or, indeed, funny in any way.”
“It was against the league champions, whose pre-season has involved far less strenuous travel than their own.”
From Latin strenuus (“quick”).
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.