Sforno on Exodus 12:11/ספורנו על שמות י״ב:י״א

1מתניכם חגורים, ready to commence marching. We find a similar expression in connection with the prophet Elijah in Kings I 18,46 where he is described as וישנס מתניו, “he bound up his loins.” The expression reflects absolute confidence in G’d’s assistance of the undertaking one is about to embark on. This is displayed while the prisoner is still inside the walls of his jail, the door not having opened yet to let him out.