the knee: the leg joint that carries a person's weight, and that gives way when they kneel, bow, or fall before someone
The word means simply the knee, the joint of the leg. Just once in the New Testament it is plain anatomy: Hebrews 12:12 tells worn-out believers to straighten their limp hands and buckling knees, borrowing an image from Isaiah of a body too tired to stand. Everywhere else the knee is in motion, and what it is doing is the whole point.
Three fixed expressions account for nearly every use:
- putting the knees down, the ordinary way of saying that someone knelt to pray
- bending the knee, the gesture a subject makes before a king or a god
- falling at another person's knees, the move of someone overwhelmed and pleading
Standing, not kneeling, was the usual posture for prayer in this world, so going down on the knees marks a prayer as urgent or grave. That is why the writer of Luke and Acts keeps noting it: Jesus in the garden, Stephen under a hail of stones, Peter beside a dead woman's body, Paul saying goodbye on a beach. Paul presses the same picture into a letter when he says he bows his knees to the Father (Ephesians 3:14).
The bowing sense is where the weight of the word lies. Isaiah's oath, that to God every knee will bow, is quoted twice in the New Testament (Romans 14:11 and Philippians 2:10), and in Philippians it is at the name of Jesus that the knee bends. The gesture can also be counterfeited: the soldiers who knelt before Jesus in Mark 15:19 use the body language of worship to mock him. And the seven thousand in Israel who never bent the knee to Baal (Romans 11:4) show that a bent knee was a declaration of who owned you. Greek can also fold the whole gesture into a single verb (γονυπετέω), and it is more often the feet (πούς) than the knees that a suppliant falls at, which makes Peter's fall at Jesus' knees in Luke 5:8 stand out.