Isaiah 40:30

Even the youths shall faint -The most vigorous young men, those in whom we expect manly strength, and who are best suited to endure hardy toil. They become weary by labor. Their powers are soon exhausted. The design here is, to contrast the most vigorous of the human race with God, and to show that while all their powers fail, the power of God is unexhausted and inexhaustible. And the young men -The word used here denotes properly “those who are chosen or selected” (בחוּריםbachûriym, Greekἐκλεκτοὶeklektoi), and may be applied to those who were selected or chosen for any hazardous enterprise, or dangerous achievement in war; those who would be selected for vigor or activity.