It is not clear, but the story suggests that the Singer who performs the incantations in the annual ritual of "The Ruin" does so against his will. Such is the case of the little girl named Jo who is held prisoner in the Council Edifice, the same building where Kira and Thomas live. Although just a child, she is trained for hours on end to learn by heart and sing the same lines, as she has been appointed to be the next Singer. Kira hears her sobbing at night and tries to comfort her in her distress.
Another possibility - the Singer's shackled ankles could possibly be part of the ceremony itself since that would be an adequate symbol of the village's distress after "the Ruin." Other ceremonial objects are an elaborately embroidered robe the Singer wears and a staff he leans upon during the day-long incantations (which recount the story of the world before and after the global holocaust). Once Kira gets a glimpse of the Singer's ankles and discovers to her horror that they are swollen and bleeding, and that he his using the sacramental staff as a crutch.