Pope Francis says he can no longer travel like he used to because of his strained knee ligaments.

He says his week-long Canadian pilgrimage was “a bit of a test” that showed he has to slow down and one day possibly retire.

Speaking to reporters while traveling home to the Vatican, the 85-year-old Francis stressed  that he hadn’t thought about resigning but said that “the door is open” and there’s nothing wrong with a pope stepping down.

He said: “I think at my age and with these limitations I have to save (my energy) to be able to serve the church, or on the contrary think about the possibility of stepping aside.”