Charles Ellicott Commentary Isaiah 40:31

"but they that wait for Jehovah shall renew their strength; they shall mount up with wings as eagles; they shall run, and not be weary; they shall walk, and not faint." — Isaiah 40:31 (ASV)

They that wait upon the Lord. —The waiting implies, of course, the expectant attitude of faith.

Shall mount up with wings. —Better, shall lift up their wings, or, shall put forth wings' feathers, the last, like Psalms 103:5, implying the belief that the eagle renewed its plumage in extreme old age. For the faithful there is no failure, and faith knows no weariness.