Traditional descriptions of the knee suggest that the function of the patella is to
facilitate knee extension by increasing the moment arm of the quadriceps muscles.
Through modelling and evidence from the literature it is shown in this paper that the
presence of the patella makes the ability of the quadriceps to rotate the thigh
greater than their ability to rotate the tibia. Furthermore, this difference increases as
the knee is flexed, thus demonstrating a pattern that is consistent with many human
movements. This paper also shows that the anterior cruciate ligament plays a
previously unheralded role in extending the shank and that translation at the
tibiofemoral and patellofemoral joints is important in improving the capacity for thigh
rotation when the knee is flexed. This study provides new insights as to how the
structure of the knee is adapted to its purpose and illustrates how the functional
anatomy of the knee contributes to its extension function.