Psalms 57:7 - Coke's Commentary on the Holy Bible

My heart is fixed— Or, prepared, as in the Margin of our Bibles. He says, in the conclusion of the foregoing verse, that his enemies had fallen into the pit which they had prepared for him; as Saul, who sought his life, had fallen in the cave into David's hands, and put his life in his power. Therefore, in the triumph of his joy, he cries out, "My heart is fixed, and prepared to celebrate God's goodness:" and his repeating the words adds great emphasis to them.