Then the king’s countenance was changed - The word rendered “countenance” is, in the margin, as in Daniel 5:9, “brightnesses.” The Chaldee word means “brightness, splendor” (זיו zı̂yv), and the meaning here is bright looks, cheerfulness, hilarity. The word rendered was changed, is in the margin changed it; and the meaning is, that it changed itself: probably from a jocund, cheerful, and happy expression, it assumed suddenly a deadly paleness.
And his thoughts troubled him - Whether from the recollection of guilt, or the dread of wrath, is not said. He would, doubtless, regard this as some supernatural intimation, and his soul would be troubled.
So that the joints of his loins were loosed - Margin, “bindings,” or “knots,” or “girdles.” The Chaldee word rendered “joints” (קטר qeṭar) means, properly, “knots;” then joints of the bones, as resembling knots, or apparently answering the purposes of knots in the human frame, as binding it together. The word “loins” in the Scriptures refers to the part of the body around which the girdle was passed, the lower part of the back; and Gesenius supposes that the meaning here is, that the joints of his back, that is, the vertebral are referred to. This part of the body is spoken of as the seat of strength. When this is weak the body has no power to stand, to walk, to labor. The simple idea is, that he was greatly terrified, and that under the influence of fear his strength departed.
And his knees smote one against another - A common effect of fear Nahum 2:10. So Horace, “Et corde et genibus tremit.” And so Virgil, “Tarda trementi genua labant.” “Belshazzar had as much of power, and of drink withal to lead him to bid defiance to God as any ruffian under heaven; and yet when God, as it were, lifted but up his finger against him, how poorly did he crouch and shiver. How did his joints loose, and his knees knock together!” - South’s Sermons, vol. iv. p. 60.