膝 (xī) is the Chinese word for "knee," which is a crucial joint in the human body. It connects the thigh bone (股骨) to the shin bone (胫骨) and allows for movement such as bending and straightening the leg. The knee is essential for activities like walking, running, and jumping. The knee joint is made up of bones, cartilage, ligaments, and tendons. It is surrounded by a capsule filled with synovial fluid, which helps reduce friction during movement. Injuries to the knee, such as tears in the 韧带 or cartilage, can significantly impact mobility and require medical attention.