A reminiscence of older psalms. In addition to the marginal references, see Psalms 26:3.
Unite my heart — i.e., unite all my powers and concentrate them on Thy service. No doubt with recollection of Deuteronomy 6:5; Deuteronomy 10:12. Comp. also Jeremiah 32:39, on which apparently the expression is directly based. An undivided will is in morals and religion equally essential.