Figuratively, a child said to be ‘born upon the knees’ of a person other than the mother, and thus enjoying that one’s favor and care, was acknowledged as that person’s child, or descendant, just as Bilhah’s child was counted as Rachel’s.​—Ge 30:3-6; compare Ge 50:23.

Jehovah promised restoration for his people and likened them to children of Zion, or Jerusalem, who would be ‘fondled upon the knees,’ that is, well cared for, brought back into a favored state.​—Isa 66:12, 13.

This joint in the leg is important for support of the body. Hence, wobbling or enfeebled knees portray weakness; and knocking knees, fear.​—Job 4:4; Ps 109:24; Isa 35:3; Da 5:6; Heb 12:12.