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
Arthropod leg with saltatorial legs because the typical leaping mechanism is to straighten the joint between the femur and the tibia, and the femur contains the necessary massive bipennate musculature. The tibia is the fourth section of the typical insect leg. As a rule the tibia of an insect is slender in comparison to the femur, but it generally is at least as long and often longer. Near the distal end there is generally a tibial spur, often two or more. In the Apocrita the tibia of the foreleg bears a large apical spur that fits over a semicircular gap in the