And as thy days, so shall thy strength be.--The word for "strength" does not occur elsewhere in the Old Testament, but the Targums and the LXX., and other authorities, seem to agree in its interpretation, and the form of the word points to this meaning, "strength," so that there is little doubt as to its correctness. But the meaning of the clause is variously given by Jewish authorities. "Thy strength in old ago shall be as the strength of thy youth;" or, "As thou spendest thy days (in doing the will of the Holy One or not), so shall thy strength be."
and as thy days, so shall thy strength be; the same in old age as in youth; which is the sense of the Latin Vulgate version, and all the Targums: such were the vigour and strength of. Moses himself, Deuteronomy 34:7; and so may denote a renewal of youth, like that of eagles; and, in a spiritual sense, a revival of the graces of the Spirit of God, as to the exercise of them, and an increase of spiritual strength, so that the inward man is renewed day by day; and may also denote such a measure of strength given, as is proportioned to the events that daily befall, or to the services and sufferings men are called unto; see 1 Corinthians 10:13.
(g) Laert. in Vit. Empedocl. l. 8. p. 613. Aelian. Var. Hist. l. 12. c. 32. (i) Odyss. 15. l. 424.