Learn how the knee muscles can both power movement by contracting, and also allow the opposite movement by paying out slack, so that opposing muscle groups act in concert with one another to ensure that movement is controlled and smooth.
Transcript of the video
There are two separate things to consider when we talk of knee flexion. The one includes the muscles and tendons that power flexion, and the other includes those structures that allow flexion.
Let’s bend the knee and take a look firstly at the bones. Because the articular surface of the femur bone is oval, rather than round, while the top of the tibia is largely flat, if the knee worked purely as a hinge, then in full flexion the femur would simply fall off the back of the tibia.
What actually happens is that the femur rolls into flexion but the cruciate ligament restraints force it to glide forward and maintain stability. This 'roll-and-glide' results in a wide arc of excursion of the patellar tendon during bending and straightening. Note how widely the anterior interval opens up during the range of motion.
Although the hamstrings muscles at the back of the knee are powering this flexion. it is the quads that is allowing it. You see, as the knee bends, and the roll-and-glide begins, the top quads muscle - the rectus femoris - has to pay out its length to accommodate the movement. Because the quads muscle head just underneath it - the vastus intermedius - is fixed to the shaft of the femur, this means that the two quads heads effectively slide over one another. If there are adhesions between them, then this cannot happen and attempts at flexion are difficult and painful.
I don’t really want to go into anatomy in any more depth, because you should by now have enough medical words and sufficient understanding to follow the rest of the course. I will put a series of anatomy images into the ‘resources’ folder of each one of these anatomy lectures for you to refer to at any time.