Emily Rose ( Anneliese Michel )

Father Amorth

  According to Ed Warren, the late noted demonologist and assistant to exorcists of the Roman Catholic Church, in the book The Demonologist: The Extraordinary Career of Ed and Lorraine Warren, by Gerald Daniel Brittle, iUniverse Press, 2002 softcover reprint edition.

[...] As for changes to the body, the strength of the possessed is completely overwhelming. I have seen a possessed child throw grown adults around the room like a sumo wrestler. And a possessed adult is completely unmanageable. I know – I have been attacked by the possessed on more than one occasion, and big as I am, at two-hundred-and-twenty pounds, I can report to you that no human being could ever single-handedly fight off a possessed individual. You're dealing with something that has the strength of six men.

In the work An Exorcist Tells His Story by Father Gabriele Amorth, the former Chief Exorcist for the Vatican, Ignatius Press of San Francisco, 1994 softcover Twelfth Edition.

Father Amorth goes into detail as well about the enhanced physical strength of someone afflicted with possession: 

One begins with the physical symptoms. The two areas most commonly affected by evil influences are the head and the stomach. In addition to headaches that are severe and unresponsive to prescriptions, symptoms include a sudden in­ ability to learn. In particular, young people who never had trouble at school all of a sudden cannot study anymore, and their ability to concentrate disappears. The signs listed in the Ritual are limited to the most spectacular manifestations of possession, such as the ability to speak or understand unknown languages perfectly, to know the hidden and the remote or to demonstrate a superhuman physical strength. As I have already mentioned, I was able to detect these signs only during the blessings (this is how I always refer to exorcisms), never before. Often I am told of strange or violent behavior. A typical symptom of evil influence is an aversion to the sacred. It happens then that prayerful people suddenly stop praying. Others stop going to church and become full of rage; some suddenly blaspheme often and act violently against sacred images. Almost always we also encounter asocial behavior, such as anger against relatives or acquaintances, and many types of bizarre behavior.

In the book Hostage to the Devil: The Possession and Exorcism of Five Contemporary Americans by Father Malachi Martin, HarperOne Press, 1992 softcover reprint edition goes into a little bit of information on the phenomena of superhuman strength briefly, mainly by telling you how the actual exorcist and his assistants deal with this on page 16, paragraph 2:

  An exorcist comes to know from experience what he can expect by way of violent behavior; and, for their own sakes, possessed people must usually be physically restrained during parts of the exorcism. The assistants therefore must be physically strong. In addition, there may be a straitjacket on hand, though leather straps or rope are more commonly used.