Yet those who wait for the Lord will gain new strength; they will mount up with wings like eagles, they will run and not get tired, they will walk and not become weary. (Isaiah 40:11)

The point being made through the repetition is clear: God is not contingent; even the most powerful of earth are contingent; God graciously makes his vitality available to the failing of earth. But does the receiving depend on any particular condition? Only one, and it is specified here: waiting on the Lord. This expression implies two things: complete dependence on God and a willingness to allow him to decide the terms. To wait on him is to admit that we have no other help, either in ourselves or in another. Therefore we are helpless until he acts. By the same token, to wait on him is to declare our confidence in his eventual action on our behalf. Thus waiting (qāwâ) in Hebrew is not merely killing time but a life of confident expectation (see 8:17; 25:9; 33:2; 49:23; 64:2 [Eng. 4]). Those who give up their own frantic efforts to save themselves and turn expectantly to God will be able to replace or exchange (yaḥalîpû) their worn-out strength for new strength. How like God: he takes the useless and gives back the good (53:11). (Oswalt, John N. The Book of Isaiah, Chapters 40–66 (the New International Commentary on the Old Testament). Eerdmans, 1998, 74.)