But those who wait upon the LORD will renew their strength; they will mount up with wings like eagles; they will run and not grow weary, they will walk and not faint.
31. But they that wait for Jehovah. Hebrew writers employ the phrase, “exchanging strength,” “The phrase translated ‘they shall gain new strength,’ properly means ‘they shall exchange strength;’ but the usage of the verb determines its specific meaning to be that of changing for the better, or improving. The sense is therefore correctly given in the English Version (‘they shall renew their strength’).” — Alexander.
Here, I. The prophet reproves the people of God, who are now supposed to be captives in Babylon for their unbelief and distrust of God, and the dejections and despondencies of their spirit under their affliction (Isa 40:27): "Why sayest thou, O Jacob! to thyself and to those about thee, My way is hidden from the Lord?
Commenting on Isaiah 40:27-31
mount up-- (Sa2 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).