Isaiah 40:31

they that wait upon the Lord(shall) renew(lit. "exchange" cf. ch. Isaiah 9:10) their strength.

mount up with wings although an excellent sense, is doubtful grammatically. The authorities are divided between the Targ. on the one hand, and LXX. and Vulg. on the other. The former has "lift up (their) wings"; the latter "put forth (lit. "cause to grow") pinions" (LXX. πτεροφυήσουσιν). The second is by far the best. An allusion to the popular notion that the eagle renews his feathers in his old age (Cheyne) is not probable; it is even doubtful if the idea of renewalis in the metaphor at all. It is rather a description (and a very fine one) of the new kind of life which comes to him who waits on the Lord; he is borne aloft on wings of faith and hope.