(NewsNation) —  Pope Francis has been canceling some events because of health concerns, causing some to speculate that he may be ready to retire.

Recently, Francis had to postpone indefinitely a trip to Africa because of “walking problems,” according to Reuters. The 85-year-old pontiff’s knee ailment has forced him to use a wheelchair for more than a month.

While the pope himself has never mentioned anything about resigning, there have been rumors circulating about it, with publications from the National Review to TMZ writing about the possibility.

Rumors of retirement really gained steam in May, when Francis announced a consistory to create 21 new cardinals scheduled for Aug. 27. Sixteen of those cardinals are under age 80 and eligible to vote in a conclave to elect Francis’ successor. While there is no guarantee for how the cardinals will vote, there’s a good chance they will tap a successor who shares Francis’ pastoral priorities.

Francis has been open about his health issues, with the Vatican saying that the decision to postpone the Africa trip was made at the request of his doctors. And last month, he even made some jokes about his knee pain with reporters.

His knee isn’t the only issue Francis has been dealing with. He also has sciatica, with pain in his hip, legs and back. Those flare-ups have caused him to miss some major events recently: Francis sat out Ash Wednesday in March, and last July, he was hospitalized for 10 days after a surgery to remove part of his colon.

Should Francis end up retiring, he would be only the second pope to do so since the Middle Ages, after Pope Benedict XVI.

Francis has praised Benedict’s decision to retire as “opening the door” for future popes to do the same, and he had originally predicted a short papacy for himself of two to five years.

Still, Francis has shown no signs he wants to step down, and he has major projects on the horizon, such as a 2023 meeting of the world’s bishops to debate the increasing decentralization of the Catholic Church.

Joseph Capizzi, a professor of moral theology and ethics at Catholic University in Washington D.C., said this kind of speculation about someone’s health is natural for someone of Francis’ age.

“There’s no reason to think that he’s thinking about resigning,” Capizzi said on NewsNation’s “Morning in America.” “(We) really don’t have any evidence of that— he certainly has an agenda in front of him that he seems animated about, he seems quite vigorous.”

The Associated Press and Reuters contributed to this report.