For, criminal liability, the actus should be reus, i.e. the act should be prohibited by law. Act as per Austin is a movement of body which is in consequence of the determination of the will. Hence, only voluntary actions will become criminal acts.
The word Actus Reus includes not only Act but omission as well. The Actus reus of omission is well illustrated in English case of ‘Gibbins and Proctar (1918 13 Cr. App. Rep. 134) where a man and the woman with whom he was living were held guilty of murder of the man’s child when the woman with the man’s concurrence with held food from the child, intending its death or grievous bodily harm.