There is a chassidic saying on the passage: "But those who put their hope in the L-rd shall renew their vigor" (Isaiah 40:31). The word used to convey "renew" (yachalifu) [i.e., their vigor,] can also mean "to exchange" (l'hit'chaleif). The Jewish people make an exchange with G-d; they give G-d their thoughts, speech and action, and in turn G-d gives them His strengths. And because G-d's strengths are not limited or mitigated by the aging process - since aging does not apply to Him - the Jews, too, can forever be a youthful people.

Toras Menachem 5716, vol. 1, p. 108