Jamieson, Fausset and Brown on Isaiah 40:31

31. mount up--(2Sa 1:23). Rather, "They shall put forth fresh feathers as eagles" are said to renovate themselves; the parallel clause, "renew their strength," confirms this. The eagle was thought to moult and renew his feathers, and with them his strength, in old age (so the Septuagint, Vulgate, Psa 103:5). However, English Version is favored by the descending climax, mount up--run--walk; in every attitude the praying, waiting child of God is "strong in the Lord" (Psa 84:7; Mi 4:5; He 12:1).