In part 1 of this series we looked at the passive structures in human anatomy that are relevant to the knee joint (Anatomy of the knee – part 1: passive structures). In this post we will investigate the active stabilizing structures surrounding the knee. On the front of your body this would be the quadriceps and on the back it’s the hamstrings, gastrocnemius, as well as the popliteus. Let’s get acquainted with the quads first.
The Quadriceps
Rectus Femoris Muscle
The Quads are a large muscle group on the front of your thigh. They are divided into several heads: the rectus femoris, the vastus lateralis, the vastus medialis and the vastus intermedius. The quadriceps muscles all produce knee extension (straightening the knee, as opposed to knee flexion, which would mean bringing the heel closer to buttocks), but the rectus femoris is a bit more special.
The rectus femoris
The rectus femoris is the only quad muscle that crosses two joints: the hip and the knee. Therefore, aside
from knee extension the rectus femoris can also produce hip flexion. Because of how the rectus femoris attaches to the hip and the patella you can shorten it by flexing your hip (bringing your thigh in front of you to decrease the angle between your thigh and hip) and by extending your knee. This causes the rectus femoris to be a weaker hip flexor when the knee is extended, since it is already shortened, just as it will be weaker in knee extension when the hip is already flexed. In both cases other muscles are more important for the respective movements.
If we extend at the hip and flex at the knee we can stretch the rectus femoris. If you want a demonstration of what that looks like and some additional information, just watch Kelly Starrett’s video on the couch stretch (the stretch is at 3:20).
The Vastus Lateralis
The vastus lateralis
The vastus lateralis lies on the outer side of the thigh and attaches to the femur on one end and to the patella on the other end. This is a very simplified description of its origin and insertion, but it’s enough for our purposes.
The function of the vastus lateralis is to extend the knee. If you try to extend your knee while your hip is flexed (e.g. while sitting) the action will be primarily performed by the vastus lateralis, vastus medialis and vastus intermedius. The rectus femoris cannot contribute as much to the action since it is already shortened due to hip flexion.
The vastus medialis
The vastus medialis is on the inner side of 
the thigh. Because of its interesting shape it is sometimes called “teardrop muscle”. The vastus medialis also attaches to the femur and it connects to the medial part of the quadriceps femoris tendon, as well as the medial border of the patella.
The existence of a separate aspect of the vastus medialis Is subject to controversy. As is the ability of the vastus medialis to pull the patella towards the midline of the body, thereby opposing the forces exerted on the patella by the vastus lateralis. Be this as it may, to achieve optimum function you have to use a holistic approach to dealing with weak or misfiring muscles, as well as tissue quality. With that in mind the importance of a healthy vastus medialis cannot be denied.
The knee joint with patella and quadriceps tendon
The Hamstrings
The hamstrings are the group of muscles on the back of the thigh. These posterior thigh muscles are: biceps femoris, semitendinosus and the semimembranosus. The hamstrings produce knee flexion and hip extension.
The hamstring muscle group is important for knee health for several reasons. They are active stabilizers for the knee joint, because they prevent forward slide of the tibia and thereby prevent undue tension on the ACL (see Anatomy of the knee – part 1: passive structures). If your hamstrings are weak or don’t fire properly, you’re more at risk for an ACL-injury.
What’s more is that the hamstrings work in concert with the gluteal muscle group (a strong group of muscles on the posterior of your hip, aka your buttocks) to produce hip extension. If the gluteals don’t function properly the hamstrings have to work overtime, which can lead to hamstring pulls, low back pain and many other problems.
The hamstring muscles also have an impact on internal and external rotation of the knee. So there are enough reason to pay close attention to the health of your hammies.
Gastrocnemius, Popliteus and Sartorius
Gastrocnemius ... source: wikipedia
Before we end this brief foray into the anatomy of muscles around the knee let us take a look at the gastrocnemius, popliteus and sartorious.
Gastrocnemius
The gastrocnemius attaches to the femur just above the knee and the Achilles tendon on the back of your foot. It can flex the knee, although its main function is to plantar flex the foot (plantar flexing the foot means pointing your foot away from you or, if you’re standing, going from standing on your whole foot to standing on your toes).
The gastrocnemius is a very strong muscle and it also has a lot of endurance. If the gastrocnemius gets too tight it can cause posterior knee pain (pain in the back of your knee).
Popliteus
The popliteus originates to side of the femur right above the knee and inserts on the back of the tibia. The main actions of the popliteus are lateral rotation of the femur and flexion of the knee. This inconspicuous muscle also helps the lateral meniscus to move out of the way when you’re bending your knee. This way we happen not to crush our lateral meniscus when we squat.
Sartorius
The Sartorius originates on the hip and inserts into the medial part of the tibia close to the knee. Actually, before it inserts into the tibia, the Sartorius ends in a tendon right behind the medial part of the femur close to the knee. That tendon is then joined by two other tendons (tendon party! Not to be confused with tendinopathy) and those three tendons form what’s called the pes anserinus. The pes anserinus then attaches to the tibia right above the MCL.
Bottom Line
So there you have it: the anatomy of the active structures around the knee in very basic terms. Ok maybe not very basic, but hey, terminology serves to improve accuracy in communication and when dealing with issues such as knee health you just can’t be accurate enough.
I realize that the importance of the information on this page in relation to knee pain might not be obvious, but I can also promise you that knowledge about anatomy will come in handy in the future.
Oh and there will be a test!