Human effort is seen here both in its limits and its possibilities. To be sure, even strong young people eventually grow tired--not to mention those who have been victimized and oppressed--but God gives power to the powerless enabling them to renew their strength and take off like eagles.
In Old Testament thought, the eagle is an unclean animal (Leviticus 11:13) and cannot be eaten; this text is the only positive eagle reference in the prophetic books. Still, "unclean" does not mean that the eagle is not pronounced "very good" along with the rest of creation (Genesis 1:31)--indeed, good enough that both God and Moses can compare God to the eagle (Exodus 19:4; Deuteronomy 32:11-12). Just as God bore Israel on eagles' wings at the first exodus (Exodus 19:4), now, in the second exodus--the return from exile--God's people are given power to mount up themselves with wings like eagles.
29 He gives power to the faint,
and strengthens the powerless.
30 Even youths will faint and be weary,
and the young will fall exhausted;
31 but those who wait for the Lord shall renew their strength,
they shall mount up with wings like eagles,
they shall run and not be weary,
they shall walk and not faint.
New Revised Standard Version Bible: Anglicized Edition, copyright © 1989, 1995 National Council of the Churches of Christ in the United States of America. Used by permission. All rights reserved worldwide. http://nrsvbibles.org