"I saw a fellow in his mid-80s who was finding it much more difficult to walk and to stand," Tobin said, "and I think he had a great anxiety about falling, which is not unusual for people of his age."
Kathleen Sprows Cummings, director of the Cushwa Center for the Study of American Catholicism at the University of Notre Dame in South Bend, said Benedict had said more than once that the pope has the right, and perhaps even the obligation, to resign if he feels he cannot carry out the duties of his office.
"For decades the world watched the long, slow decline of Pope John Paul II, who characteristically shared his suffering in a very public and poignant way. His successor, a scholar who has never been very comfortable in the limelight, has chosen to impart to the faithful a different lesson: how and when to make a courageous exit," Cummings said.
Cardinals elected Benedict to succeed John Paul II in 2005.
"I trust that in prayer he knew that he didn't have the strength to do this," said Fort Wayne-South Bend Bishop Kevin Rhoades.
Lafayette Bishop Timothy Doherty said Benedict has had to endure the ramifications of the abuse of children by priests over the decades.
"Certainly, coming to know the widespread abuse of children by priests has been an awful experience for the pope, because he realizes the damage to the victims," Doherty said. "Just as painful for him is trying to find a way to deal with those who abused power by not listening to victims or removing abusers from ministry."
Evansville Bishop Charles Thompson said Benedict, who speaks multiple languages and is widely read, kept a strong focus on the intellectual role of the church.
"He is probably one of the most intellectual popes we have ever had," Thompson said. "He has produced some major documents."