So I was asked by a dance student about the different kinds of subtle movements in the knee joint. There is the obvious flexion and extension, but there are also a couple less obvious things. So what happens to the patella? Does it move with the tibia, or does it move with the femur? And what about that weird thing when I lock my knees? What is that all about? Both ideas are about relative motion: one part of the body is in motion relative to the other, and what we see as being in motion depends on our view point.
So first the patella during flexion. When we flex our knees, the patella maintains a constant relationship with the tibia: it does not move relative to the tibia. This is because the patelar ligament, when fully extended is of a fixed length. As our knee bends, the front face of the femoral condyles start in contact with the posterior surface of the patella, but as the flexion continues, then the distal ends of the femur have contact.
So you could say the femoral condyles glide under the patella, or the patella glides over the femoral condyles. It depends which end of the leg is fixed. If we sit in a chair and extend and flex our knees, the tibia and patella move together over the condyles of the femur. If we plant our foot on a step and step up, the end of the femur glides across the top of the tibia and back of the patella together. In this video, the patelar ligament extends to full length as it is placed under tension (I did not talk about this extension in class, because it is not important for our purposes). Then because the femur is fixed and the tibia is in action, the patella glides over the condyles of the femur. But relative to the tibia, the patella does not really move.
Additionally, the patella does a little side-to-side dance as it glides over the femur. It starts medial, and moves more laterally. This is because the posterior surface of the patella is forced the by tension on the quadruceps muscle and patelar tendon into the ‘intercondylar notch’ which is the indentation at the distal end of the femur. The patella moves laterally at the very beginning of the knee flexion.
The Screw-home Mechanism
Standing for a long time can be energy consuming, so your body likes to find biomechanically efficient solutions. One solution your body uses is to ‘lock’ your knees into full extension. In this way neither your quads nor your hamstrings really has to work to maintain stance. Your friends in grade 5 knew about the screw-home mechanism, and would run up behind you when you were waiting in line, put their knees just behind yours, and pop their knees into the back of your legs at the knee joints. Suddenly your knees were ‘unlocked’ and you would have to recover. Sadly, the teacher always looked just as you were getting revenge…
The screw-home mechanism works because in the last 20° of extension, the end of the femur glides relative to the tibia and moves into a position of relative stability. This rotation occurs because the lateral condyle of the tibia is smaller than the medial condyle, so the lateral side acts as a pivot point, and as the knee approaches full extension, medial side experiences a larger shift of the femur and tibia relative to one another.
What we say is rotating depends on which end is fixed, and whether we are moving INTO the locked position, or OUT OF the locked position.
As we are moving into full extension, that is ‘locking’ the knee into the screwed-home position, then the femur rotates inward if the tibia is fixed, for example when we coming to the top of a plié (that is bending the knees with the feet on the ground for you mortals). If the tibia is free, for example at the top of a jump on full leg extension, the tibia will rotate outward relative to the femur.
As we are moving out of full extension, that is ‘unlocking’ the knee from the screwed-home position, then the femur rotates outward if the tibia is fixed, such as when we begin our plie. If the tibia is free, for example if you were for some reason going directly from tendu dégagé devant to a retiré (for you non-dance types, standing on one leg, with your gesture leg fully extended in front with your toe barely off the ground,then moving to your toe at the side of the standing leg), then the tibia would rotate inward relative to the femur. This outward rotation of the femur/inward rotation of the tibia is cause by the popliteus muscle. In this video, the femur is fixed, so the tibia rotates laterally/outwardly as the knee reaches full extension.
There is one more small knee movement that I will mention briefly. It is the action of a small muscle called the ‘articularis genu’. Its job is to move the suprapatellar bursa out of the way so you don’t pinch it behind your patella when you extend your leg fully. The suprapatellar bursa prevents friction of the cojoined tendon of the four muscles of the quadruceps. And I am very glad for this action, because pinching a bursa can be very painful.
There are other subtle movements of the knee, since the knee is an enormously complicated joint, but these three are big enough that I can kind of ‘get’ their significance on a visceral level. The others will have to wait for another question, or a day when my mountain bike is not calling me!
Happy knee movements everyone!