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 alternate name for Doppelflöte. Most tibias are made from wood, as by Wurlitzer etc., although examples of metal tibias may be found made by the John Compton Organ Company. The Tibia Clausa, or Tibia, is generally found at 16′, 8′, 4′ and 2′ pitches as a unified rank. The mutation ranks Tibia Quint 5⅓′, Twelfth 2⅔′ and Tierce 1′ are also drawn from this unified rank of 97 pipes. In some larger organs, a second Tibia rank may be present, extended to 1′ instead of 16′, allowing a 1⅓′ Nineteenth mutation and a 1′ Piccolo to be drawn from this