10 Commending this to the ears of God, let him say, “Lord, Lord.” Thou Lord-Lord, that is, most truly Lord, not like the lords-men, not like the lords who buy with money-bags, but the Lord who buys with His Blood. “Lord, Lord, Thou strength of my health”, that is, who givest strength to my health. What is the meaning of “strength of my health”? He complained of the stumbling-blocks and snares of sinners, of wicked men, vessels of the devil, that barked around him and laid snares around him, of the proud that envy the righteous.
But He immediately added a comfort, “He that shall endure unto the end, the same shall be saved.” This he observed and feared, and, distressed at the abundance of iniquities, turned himself to hope. Verily I shall be saved, if I endure unto the end: but endurance, so as to win salvation, pertains unto strength; You are “the strength of my salvation;” You make me to endure, that I may attain salvation....Toiling then in this warfare, he looked back to the grace of God; and because already he had begun to be heated and parched, he found, as it were, a shade, whereunder to live. “You have overshadowed my head in the day of battle:” that is, in the heat, lest I be heated, lest I be parched.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)