The knee was a compound joint linking the thigh and the leg, specifically the femur and tibia and the femur and patella.
Going to one's knee or bending one's knee, also called kneeling, was in many cultures considered an act of submission or respect. (TOS: "Who Mourns for Adonais?", "Return to Tomorrow") Marriage proposals were often done on one or both knees. (TOS: "Is There in Truth No Beauty?")
The symptoms of kaylo poisoning included a sharp pain in the knees working its way up the body. (VOY: "State of Flux")
While extracting a core sample of eisillium on Archer's Comet in 2151, Travis Mayweather injured his knee when he fell down a crater. (ENT: "Breaking the Ice")
An alien prisoner on Rura Penthe in 2293 had his genitals in the same spot where other humanoid species had their knees. (Star Trek VI: The Undiscovered Country)
The soccer player Golanga hurt his knee in 2366. Though it was replaced by a bio-implant, he never fully recovered from that injury. (DS9: "Paradise")
During the 77th Emperor's Cup, Takashi's knee broke the sand, leading to the match being awarded to Kar-pek as recalled by Chakotay and Tuvok. (VOY: "Latent Image")
In 2371, Kasidy Yates' brother tore some ligaments in his knee during a baseball game. (DS9: "Family Business")
In 2372, a plasma charge destroyed the knee joint of Meso'Clan, so he could no longer stand. (DS9: "Hippocratic Oath")
Ekoria planned to kiss the knee of her unborn son when he scraped it in a fall. (DS9: "The Quickening")