Making our way up the body from the feet and ankles, the next major joints we come to are the knees. This knobby pair of joints are often an enthusiastic topic of conversation among yogis. It seems everyone knows somebody who’s done “something” to injure their knee. But, those injuries are a result of the inherent vulnerability in the knee joint. So, in this article we’ll focus on the bigger picture, the anatomy of the knee joint.
Structures of the knee joint
The anatomy of the knee is unique among the joints in our body. It’s those unique structures of the knee joint that make it possible for the knee to do its important functions. All joints have at least two bones that come together to create the joint. But, as you’ll see below, our knee anatomy includes some additional structures not found in most other joints, which support its functions.
Bones of the knee
You’re probably familiar with the femur, or thigh bone. Where it meets the tibia, the larger of the two lower leg bones, is the main joint that we consider to be the knee joint. It’s called the femorotibial joint, or sometimes the tibiofemoral joint. It is primarily a hinge joint.
But, we also have a third bone at the knee joint. And that is the patella, or knee cap. Where it meets the femur, we have a gliding joint called the patellofemoral joint. The patella assists the function of the knee by helping to keep the tibia in a more or less straight line as the knee flexes and extends.
Primary muscles that move the knee
The anatomy of the knee doesn’t just include bones of course. So, let’s talk soft tissues (muscles). One of the largest groups of muscles in the body are the thigh muscles, known as the quadriceps. Feel free to blow the dust off that old anatomy book and follow along with their pictures. The quadriceps literally means four heads. They are vastus lateralis, vastus medialis, vastus intermedius, and rectus femoris.
The first three of those muscles only cross the knee joint. The fourth of that muscle group crosses two joints, the knee and hip joint above it. Remember that the rectus femoris originates on the AIIS (anterior inferior iliac spine) of the pelvis, while the three vastus muscles originate at points along the femur. Those four muscles converge around then and attach to the patella first. Finally, they combine to form a common tendon, called the patellar tendon, which attaches to the top of the tibia.
As we said above, the three vastus muscles only cross the knee joint. Therefore they are dedicated to doing one action, and that is, to extend (straighten) the knee. This simple action is essential for walking and running. Since the rectus femoris crosses two joints, the knee and the hip joint, it does two actions. Like the other quadriceps, it extends the knee joint. But, it also contributes to flexion of the hip joint, like when we lift our leg forward from a standing position.
In addition to moving the knee joint, the quadriceps muscles are also the last line of defense in dealing with loss of stability of the knee when we have a ligament tear. Often, minor tears of the anterior cruciate ligament (ACL) are left alone (depending on factors of age and activity level). So, the quadriceps primarily become responsible for taking up the slack, quite literally, from the torn ACL. You can see how our specific knee anatomy is important for knee function.
The hamstrings in the back of the thigh create the balance for the quadriceps. There are three muscles that make up the hamstring group, biceps femoris, semitendinosus, and semimembranosus. All three hamstrings originate on the sit bone, technically called the ischial tuberosity. At their distal end, the semitendinosus and semimembranosus attach the top of the tibia. The biceps femoris attaches to the top of the fibula, the smaller of the two leg bones.
All three of the hamstring muscles cross both the knee and the hip joint. They do the opposite actions of the quadriceps. The hamstrings flex the knee and extend the hip joint. Extension of the hip, if you need an example, happens when we take our leg backward from a standing position. The other action the hamstrings do is rotation of a flexed knee. Semimembranosus even has an attachment to the medial meniscus and helps it move backward as the knee flexes.
Another function of the hamstrings at the knee is to be the last line of defense in a hyperextended knee. The first line of defense is the posterior cruciate ligament (PCL), which you can read more about below. The hamstrings merely play back up to that important ligament. They almost act like a sling, and cradle the back of the knee. To keep it simple, the balance between the length and strength of the quadriceps and hamstrings is lost in a hyperextended knee. In the case of a hyperextended knee, the quadriceps is winning the battle. There is also a genetic component to hyperextended knees which may play more of a role than simple tension in tissues.
Other muscles that move the knee
In addition to the large muscles that do the bulk of the heavy lifting at the knee joint, we also have several smaller muscles that are important parts of the anatomy of the knee. These muscles can assist in those larger actions or they can fine tune movements, especially rotations, at the knee. Additional muscles that cross in the knee joint include: gracilis, sartorius, popliteus, and gastrocnemius. All four of those muscles assist the hamstrings with flexion of the knee joint. Gracilis, sartorius, and popliteus also assist the medial hamstrings, semimembranosus and semitendinosus, with medial rotation at the knee when it’s in a flexed position.
Ligaments of the knee
Like every other joint in the body, the anatomy of the knee includes ligaments. Ligaments at the knee allow for and restrict movement of that joint. Ligaments are another key part of knee anatomy that supports knee function. There are four main ligaments in and around the knee.
The collateral ligaments
Two of those are called the collateral ligaments. Both of these ligaments connect the femur above to the tibia below. One is on the medial (inner) side of the two bones. The other is exactly opposite, on the lateral (outside) surface of the joint. These two strap-like ligaments primarily prevent the tibia from slipping sideways under the femur ends.
In addition, these two ligaments help control rotation of a bent knee along with the cruciate ligaments that we’ll talk about in a moment. For the most part the ligaments slacken when the knee bends. But, if rotation happens at that bent knee, they will tighten again during that movement. So, they can be injured when doing movements like lotus and leg behind head.
The cruciate ligaments
The other two ligaments at the knee are the cruciate ligaments. The word cruciate means crossed. And, when you look at these two ligaments, you see that they cross one another. These two ligaments, instead of being on the outsides of the knee, are actually on the inner part of the knee. They keep the femur and tibia held closely together. They are named the anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL). The ACL attaches to the front part (anterior) of the tibia and the PCL attaches to the back part (posterior) of the tibia, hence their names.
These are very strong ligaments which prevent and allow for movements at the knee joint. The ACL becomes more taught during two actions. The first action is an anterior displacement of the tibia under the femur. In other words, the ACL keeps the tibia from sliding forward under the femur. The other main action this ligament prevents is too much medial or inward rotation of the flexed knee. As the tibia rotates medially the ACL becomes more taught as it lengthens. This is why an extreme medial rotation is often the main cause of ACL tears.
The PCL is basically the opposite of the ACL. Its main purpose is to restrict the tibia from sliding backwards under the femur. In an extended knee it allows almost no room for give in the tibia. So, it’s the main force holding together those hyper-extended knees. To some degree, the PCL also restricts lateral rotation. It seems to play a minor role in this, but a role nonetheless. Injuring this ligament is not common, although it is of course possible.
The menisci (plural) are unique to the anatomy of the knee. They consist of two semi-circular pieces of additional cartilage attached to the very top, almost flat area of the tibia. There is one on either side. The medial meniscus is on the inside and the lateral meniscus is on the outside of the knee. In addition to their semi-circular shape, they have what are known as “horns”. The horn is the rounded end that is facing the middle of the tibia. For each meniscus there is both an anterior (front) and posterior (back) horn.
These two additional pieces of cartilage have a few different purposes in the knee. First, because of their shape they create a deeper cup for the knobby ends of the femur to meet the relatively flat plateau of the tibia. This creates additional stability at the knee. Because they are also flexible, they play a role in shock absorption and transfer of force coming through the knee joint. Finally, the meniscus helps with the functional movements that happen at the knee joint, like flexion and extension.
Instead of being completely fixed in place, the menisci actually move and distort in shape based on the movements being performed by the knee joint. Every time the knee flexes the meniscus slides back on top of the tibia to help keep those knobby ends on the tibia. In extension, the opposite happens. Rotations of the knee also force the menisci to move and shift according to the direction of rotation.
Functions of the knee joint
The knee is normally classified as a hinge joint. This means that it simply opens (extension, straightening the knee) and closes (flexion, bending the knee). However, you may remember from other articles that the particular anatomy of the knee also allows it to rotate. This makes it different than the other hinge joint in the body, the elbow. Rotation can take place when the knee is flexed ten degrees or more. Technically it is the tibia itself that does the rotation against the femur, which stays in one place.
Rotation is a very natural and important action that is required for walking, running and a variety of other movements. It happens in two possible directions. The knee can rotate medially (tibia rotating inward) or laterally (tibia rotating outward) rotation. If the knee is flexed between 30 and 90 degrees, then about 45-50 degrees of lateral rotation is possible if assisted with your hands, and about 10-25 degrees of medial rotation is possible.
Biomechanical function of the knee
Understanding potential problems with the knee requires that you look at knee anatomy and function in our most common activity. No, I don’t mean sitting, walking! Imagine your body without a knee. Frankenstein comes to my mind with a sideways waddle and the swing of one big heavy leg to get it in front of the other. We would not get around very well, or very quickly for that matter, without knees.
The knee is the middle joint of the leg. That is, it sits between the ankle and the hip joint. It is the connection between the tibia below and the femur above. At the other end of the tibia is the ankle joint. And, at the other end of the femur is the hip joint where the femur connects to the pelvis. Because the knee is located between these two joints it can regulate their movement. While standing, the three joints of the leg become a functional chain where movement at one joint influences movement of the others. The knee in particular cannot move while the feet are on the ground without the ankle and hip joint also moving.
The levers in the leg
The tibia and femur are the two longest bones in the body. Since all joints could be considered to be levers, this makes the tibia and femur long lever arms. And, when a lever has a long arm, it means there is potential to create more force with that lever. This is a good thing because the knee has a lot to do with regulating how we move and run which requires a lot of force.
But, the fact that the tibia and femur are such long lever arms makes for more work in the knee. So, the knee has to be strong enough to handle the force created by the two longest bones in the body acting on one another. At the same time it has to be flexible enough to handle the variety of movements that occur below at the ankle joint as well as above at the hip joint. This makes its functions somewhat contradictory in nature, and is one reason why we need that special knee anatomy.
What happens, anatomically, when we bend our knees?
Bending the knees in standing does a few things. Because the hips have to lower, bending your knees lowers your center of gravity and can bring you back into balance if you have lost it. This is particularly useful when transitioning between poses. Although the pose itself may require you to have straight knees, transitioning in and out of poses with a slightly bent knee can give you a very strong sense of your feet and legs, which are the foundation of standing poses.
Bending your knees also changes the length of the hamstrings, which attach to your ischial tuberosities (sit bones) on the bottom of your pelvis. Because of their attachment to the pelvis, the hamstrings can influence the ability of the pelvis to tilt anteriorly (pubic bone going down) and posteriorly (pubic bone going up). When the knees are bent the pelvis has the ability to tilt anteriorly more freely which can make all the difference in the world in a forward bend or a downward facing dog.
In transitions between standing poses, take advantage of how your knees function and bend them. Bending your knees can help prevent strain in your lower back and the sides of your back. This is because bending the knees makes the large muscles of the leg do more of the work of maintaining body weight that would otherwise be handled by muscles in your back. Keep in mind the legs are all about foundation and connection to earth energy. Even in your seated poses, active and strong legs lend themselves to giving the asanas a foundation from which the upper half of the body can grow and move out of.