It says in Shemot 17:12:
"But Moses’ hands grew heavy; so they took a stone and put it under him and he sat on it, while Aaron and Hur, one on each side, supported his hands; thus his hands remained steady until the sun set."
Yet at the end of the Torah, in Devarim 34:7 it says:
"Moses was a hundred and twenty years old when he died; his eyes were undimmed and his vigor unabated."
In one place it says that his arms grew heavy, but in another place it says that in all his 120 years, he didn't lose his vigor/strength. Don't these two contradict each other? Is there an answer?