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" (בחוּרים bachûriym, Greek ἐκλεκτοὶ eklektoi), 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. The meaning is, that the most chosen or select of the human family - the most vigorous and manly, must be worn down by fatigue, or paralyzed by sickness or death; but that the powers of God never grow weary, and that those who trust in him should never become faint.

40:30 The youths - The youngest and strongest men, left to themselves.