Albert Barnes Commentary Daniel 5:6

"Then the king`s countenance was changed in him, and his thoughts troubled him; and the joints of his loins were loosed, and his knees smote one against another." — Daniel 5:6 (ASV)

Then the king’s countenance was changed - The word translated “countenance” is, in the margin, as in (Daniel 5:9), “brightnesses.” The Chaldee word (זיו zı̂yv) means “brightness, splendor,” and the meaning here is bright looks, cheerfulness, hilarity. The word translated “was changed” is in the margin “changed it”; and the meaning is, that it changed itself: probably from a joyful, cheerful, and happy expression, it suddenly assumed 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 translated “joints” (קטר q^eṭar) means, properly, “knots”; then joints of the bones, resembling knots or apparently functioning as knots in the human frame, 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. Gesenius supposes that the meaning here is that the joints of his back, that is, the vertebrae, 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, or 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 power, and of drink as well, to lead him to bid defiance to God as any ruffian under heaven; and yet when God, as it were, did but lift his finger against him, how poorly did he crouch and shiver! How did his joints loosen, and his knees knock together!” - South’s Sermons, vol. iv. p. 60.