Unlocking of knee is caused by

Locking is a mechanism that allows knee to remain in the position of full extension as in standing without much muscular effos. Locking occurs as a result of medial rotation of the femur during the last stage of extension. The AP diameter of lateral femoral condyle is less than that of medial condyle . as a result ,when the lateral condyle aicular surface is fully used up by extension ,pa of medial condyle remains unused , at this stage the lateral condyle serve as an axis around which medial condyle rotate backwards , I.e medial rotation on femur occurs ,so that the remaining pa of the medial condylar surface is also taken up. this movement locks knee joint. * locking by action of quadriceps femoris , especially vastus medialis pa * locked knee joint can be flexed only after it is unlocked by reversal of medial rotation, I.e by lateral rotation of femur. It is brought about by action of popliteus muscle. Ref: BD chaurasia, volume 2, page no: 156