Popliteus muscle assists in flexing the leg upon the thigh; when the leg is flexed, it will rotate the tibia inward. It is especially called into action at the beginning of the act of bending the knee, inasmuch as it produces the slight inward rotation of the tibia, which is essential in the early stage of this movement. When the knee is in full extension, the femur slightly medially rotates on the tibia to lock the knee joint in place. Popliteus is often referred to as the "Key" to unlocking the knee since it begins knee flexion by laterally rotating the femur
Tibia Clausa rank. A few of the largest theatre organs, and some church organs, may have a separate 32′ Tibia Clausa rank of 12 pipes. In smaller organs, a "Bourdon" or "Stopped Diapason" may be substituted for a Tibia Clausa at 16′ pitch. The Tibia may be voiced on wind pressures from 10″ to 25″. The Tibia is generally used as a chorus stop, with or without tremulant; it is not normally used as a solo stop due to its relatively dull tone. Other variants of the Tibia Clausa include: Tibia Bass, Tibia Flute, Tibia Major, Tibia Plena (open tibia) and Tibia