Antonyms for the phrase "arm twisted" would be terms that suggest a voluntary, willing, or unpressured action. Perhaps the most apt antonym is "acted freely," which connotes that the person chose to take a certain course of action and wasn't pressured or forcibly coerced. Other possible antonyms are "done willingly," "decided independently," or "performed enthusiastically." These phrases suggest that the person acted out of their own free will and that no external forces or persuasion were involved. In essence, any term that implies that the person acted willingly and without any prodding or persuasion could be considered an antonym for "arm twisted.