Once Loki is chained in the cave with a serpent dripping venom over him, he is destined to remain there trough the ages until the day of Ragnarok.
But in other parts of Norse myth, it's made clear that the gods must eat Idun's apples from time to time or they grow old and weak.
How does Loki survive through the ages without eating those apples? And when Ragnarok finally comes, how is he fit enough to go into battle and kill other gods like Heimdall?