WordNet sense 1 (taxing to the utmost; testing powers of endurance):

arduous, strenuous

WordNet sense 2 (requiring great physical effort):

effortful

WordNet sense 3 (the act of distorting something so it seems to mean something it was not intended to mean):

distortion, torture, overrefinement, twisting

WordNet sense 4 (an intense or violent exertion):

strain