In this verse the prophet emphasizes that Yahweh will strengthen any weary person who relies on him.

But they who wait for the LORD shall renew their strength: God’s people complained that he had forgotten them (verse 27), so here the prophet tells them to keep trusting in God. But renders the common Hebrew conjunction. Here it introduces a contrast with the previous verse: young people become weary, but not those who trust in the LORD. The Hebrew verb rendered wait for basically has the sense of hoping or longing for something (see 8.17, where it is translated “hope”). In this context it means “to depend on.” Good News Translation says “trust in … for help” (similarly New Jewish Publication Society’s Tanakh). New International Version, New Jerusalem Bible, and New American Bible use “hope in,” while Revised English Bible has “look to.” Another possible rendering is “have patient trust in.” Such patient trust in God will renew their strength. Strength renders the same Hebrew word translated “power” in verses 26 and 29. Here it refers to physical and psychological strength for the demoralized exiles. Bible en français courant provides a helpful model for this whole line with “But those who count on the Lord receive new strength.”

They shall mount up with wings like eagles: This simile means those who receive new strength will soar like eagles. However, the Hebrew is literally “they will go up wings like eagles.” The Hebrew verb rendered mount up can have the sense of sprouting or growing something. For this reason several versions and a number of commentators believe this simile is saying the people will grow new wing feathers; for example, New Jerusalem Bible has “they will sprout wings like eagles,” and New Jewish Publication Society’s Tanakh says “As eagles grow new plumes.” This sense for the simile provides a better parallel with the previous line that speaks of renewal. The eagle is thought to regain its youth after it molts and grows new feathers. But most translations opt for the same interpretation as in Revised Standard Version and Good News Translation. Translators may follow either interpretation here. The “eagle” is a large bird of prey, known for its strength and powerful flight. If such a bird does not exist in the receptor culture, a general expression such as “a large bird [of prey]” may be used.

They shall run and not be weary, they shall walk and not faint: These two parallel lines complete the picture of renewed strength for those who depend on Yahweh. The verbs run and walk represent activity of any kind. Whatever the activity, God will strengthen those who trust in him. The key verbs be weary and faint occur again. Most versions keep the parallel lines here, even Good News Translation. Revised English Bible translates “they will run and not feel faint, march on and not grow weary” (similarly Bible en français courant).

For the translation of this verse consider the following examples:

• But those who trust in Yahweh will gain new strength,
they will sprout new wing feathers like an eagle.
When they run, they will not get tired;
when they walk, they will not faint.

• But the ones who depend on the LORD will gain new vigor,
like eagles they will soar [or, fly high] [on wings].
They will run but not grow weary,
they will walk but not become tired.

Quoted with permission from Ogden, Graham S. and Sterk, Jan. A Handbook on Isaiah. (UBS Helps for Translators). New York: UBS, 2011. For this and other handbooks for translators see here .