Albert Barnes Commentary Psalms 77:15

"Thou hast with thine arm redeemed thy people, The sons of Jacob and Joseph. Selah" — Psalms 77:15 (ASV)

Thou hast with thine arm - That is, with strength or power, the arm being a symbol of strength (Exodus 6:6; Exodus 15:16; Psalms 10:15).

Redeemed thy people - You did rescue or deliver them from Egyptian bondage. See the notes at Isaiah 43:3.

The sons of Jacob and Joseph - The descendants of Jacob and Joseph. Jacob is mentioned because he was the ancestor of the twelve tribes; Joseph, because he was conspicuous or eminent among the sons of Jacob, and particularly because he acted so important a part in the affairs of Egypt, from whose dominion they were redeemed.