"Involuntarily" is an adverb that describes how an action or response is done without a person's intention or control. As an alternative, there are several similar words that can be used such as "unwillingly," "unintentionally," "instinctively," "reflexively," and "spontaneously." "Unwillingly" implies doing something resentfully or with reluctance. "Unintentionally" describes doing something without any premeditated thought. "Instinctively" suggests acting based on natural or reflexive response. "Reflexively" means acting without conscious thought. "Spontaneously" implies acting on impulse or sudden decision. All of these words convey a similar sense of action being prompted involuntarily or without intention.