Objection 1. It would seem that violence does not cause involuntariness. For we speak of voluntariness and involuntariness in respect of the will. But violence cannot be done to the will, as shown above (Article 4). Therefore violence cannot cause involuntariness.
Objection 2. Further, that which is done involuntarily is done with grief, as Damascene (De Fide Orth. ii, 24) and the Philosopher (Ethic. iii, 5) say. But sometimes a man suffers compulsion without being grieved thereby. Therefore violence does not cause involuntariness.
Objection 3. Further, what is from the will cannot be involuntary. But some violent actions proceed from the will: for instance, when a man with a heavy body goes upwards; or when a man contorts his limbs in a way contrary to their natural flexibility. Therefore violence does not cause involuntariness.
On the contrary, The Philosopher (Ethic. iii, 1) and Damascene (De Fide Orth. ii, 24) say that "things done under compulsion are involuntary."
I answer that, Violence is directly opposed to the voluntary, as likewise to the natural. For the voluntary and the natural have this in common, that both are from an intrinsic principle; whereas violence is from an extrinsic principle. And for this reason, just as in things devoid of knowledge, violence effects something against nature: so in things endowed with knowledge, it effects something against the will. Now that which is against nature is said to be "unnatural"; and in like manner that which is against the will is said to be "involuntary." Therefore violence causes involuntariness.
Reply to Objection 1. The involuntary is opposed to the voluntary. Now it has been said (04) that not only the act, which proceeds immediately from the will, is called voluntary, but also the act commanded by the will. Consequently, as to the act which proceeds immediately from the will, violence cannot be done to the will, as stated above (Article 4): wherefore violence cannot make that act involuntary. But as to the commanded act, the will can suffer violence: and consequently in this respect violence causes involuntariness.
Reply to Objection 2. As that is said to be natural, which is according to the inclination of nature; so that is said to be voluntary, which is according to the inclination of the will. Now a thing is said to be natural in two ways. First, because it is from nature as from an active principle: thus it is natural for fire to produce heat. Secondly, according to a passive principle; because, to wit, there is in nature an inclination to receive an action from an extrinsic principle: thus the movement of the heavens is said to be natural, by reason of the natural aptitude in a heavenly body to receive such movement; although the cause of that movement is a voluntary agent. In like manner an act is said to be voluntary in two ways. First, in regard to action, for instance, when one wishes to be passive to another. Hence when action is brought to bear on something, by an extrinsic agent, as long as the will to suffer that action remains in the passive subject, there is not violence simply: for although the patient does nothing by way of action, he does something by being willing to suffer. Consequently this cannot be called involuntary.
Reply to Objection 3. As the Philosopher says (Phys. viii, 4) the movement of an animal, whereby at times an animal is moved against the natural inclination of the body, although it is not natural to the body, is nevertheless somewhat natural to the animal, to which it is natural to be moved according to its appetite. Accordingly this is violent, not simply but in a certain respect. The same remark applies in the case of one who contorts his limbs in a way that is contrary to their natural disposition. For this is violent in a certain respect, i.e. as to that particular limb; but not simply, i.e. as to the man himself.
Source: Summa Theologica (New Advent)