Actus Reus & Voluntariness
According to the criminal law, a crime requires voluntariness or, in other words, a muscular contraction that is willed. Only fully willed actions can incur criminal liability, and without voluntariness no actus reus - and so also no crime - can be said to occur.
For example, if Al hits Ed after being insulted, then the action is clearly voluntary and willed, and Ed could bring aggravated assault charges against Al. However, if the two men are in a vehicle which swerves suddenly, involuntarily causing Al's outstretched arm to strike Ed's nose, then no actus reus is present and Ed will probably have to take care of his own hospital bill. Likewise, if Sue takes Al's arm and throws it in Ed's face, it will probably be ruled that Al committed no actus reus (the guilty action came from Sue, it would seem).
Actus reus still exists if the defendant is threatened or in some other way coerced into performing a criminal action. If Dan intimidates Chris into robbing convenience store with the threat of death, Chris could argue that criminal intent (mens rea) was absent and the act was performed under duress; the action was voluntary, however, and so actus reus is present. On the other hand, if Chris was sleepwalking when he robbed the convenience store, it might be argued, with the automatism defense, that his actions were completely involuntary and that no actus reus occurred.
Sometimes criminal offenses that occur while the defendant is suffering an epileptic fit (a decidedly involuntary affliction) can merit the automatism defense. A person suffering from an epileptic fit is not acting voluntarily, and cannot, strictly speaking, incur criminal liability for what his body does during the fit. However, if the defendant places himself in a potentially dangerous situation - for example, behind the wheel - while reasonably suspecting he might suffer a fit he can still be found liable for criminal offenses under the precept of "should have known better." He voluntarily placed himself in a situation dangerous to himself and others. Similarly, if the defendant voluntarily allowed herself to be hypnotized before committing a crime, she could still be held liable for her actions.
It is absolutely essential for the actus reus requirement that the defendant's actions were voluntary and willed, including voluntarily placing himself in a situation where he might involuntarily cause harm. No matter how strongly an injured party may object to it, it is absolutely against the principles of the United States criminal law system to punish people for actions that they did not intend and could not have foreseen.