This installment in the Squat Form Series will focus on mobility at the knee joint. Squatting with good form requires mobility into both flexion (bending) and extension (straightening) at the knee joint. In this article, I will describe the anatomy of the knee joint, the biomechanics as they relate to squat form, common limitations to knee mobility, and some strategies to improve knee mobility for squatting.
Bony Anatomy of the Knee Joint
The knee joint is a hinge-type joint involving three main bones. These are the femur (thigh bone), tibia (shin bone), and the patella (knee cap). The articulation of the femur and tibia forms the main part of the knee joint, while the patella actually sits within the tendon connecting your quadriceps muscles to the lower leg. Bones like the patella that develop within a tendon are called sesamoid bones, and typically function as a pulley to increase the lever arm of the associated muscle. The patella increases the force production of the quadriceps by as much as 33%-50%.
Muscular Anatomy of the knee Joint
There are two main muscle groups that produce movement at the knee joint. These are the quadriceps femoris and the hamstrings. The quadriceps produce extension (straightening) of the knee, while the hamstrings produce flexion (bending). Both muscle groups also contribute to movement at the hip joint. While these two muscle groups are the main movers at the knee joint, most of the side-to-side stability of the knee actually comes from above and below – relying on muscles at the hip and ankle. More on this in the next article on knee stability.
The Quadriceps Femoris
The largest muscle group in the human body, the quadriceps are one of the main force producers in a squat. Their primary action is to extend the knee. As indicated by the name, the quadriceps consists of four separate muscles – the vastus lateralis, vastus intermedius, vastus lateralis, and rectus femoris. The three vastus muscles originate on the femur, while the rectus femoris actually attaches above the hip joint on the front of the pelvis. This allows the rectus femoris to act as a hip flexor as well as a knee extensor. All four muscles of the quadriceps coalesce into the quadriceps tendon that inserts into the top of the patella.
The Hamstrings
The hamstrings are a large muscle group located on the back of the thigh. Their primary action is knee flexion. Three muscles make up the hamstrings – the biceps femoris, semitendinosus, and semimembranosus. All three hamstring muscles cross both the hip and knee joints, functioning in both knee flexion and hip extension. They originate on the back of the pelvis and insert on the back of the lower leg. In addition to their functions of knee flexion and hip extension, the hamstrings also produce and control rotation at the knee joint.
Squat Biomechanics – the Knee
As mentioned above, the tibio-femoral joint is a hinge-type joint. This means that it mainly moves in one plane (you can think about the hinges on a door). However, the knee joint also has a small amount of rotation available. Although small, this rotation is essential to healthy knee function and full range of motion. Motion of the knee joint also requires movement of the patella. As the knee flexes, the patella must be able to slide up toward the hip (superiorly). As the knee extends, the patella must be able to slide down (inferiorly). Check out this video for an animation of the knee joint in motion.
While squatting, the knee flexes as you lower your hips toward the ground. The quadriceps group is active eccentrically (contracting while lengthening) to control the rate of knee flexion. As you lift yourself back up from the bottom of the squat, your quadriceps act concentrically (contracting while shortening) to extend your knee and return your body to standing.
In the squat, the hamstring group primarily produces motion at the hip, which I will discuss in a future article about the hip during the squat. The hamstrings also counter the pull of the quadriceps to stabilize the knee during the squat, which I will discuss further in the next installment on knee stability.
The rotational component of knee motion that I mentioned earlier comes into play mostly at the end ranges of flexion and extension. Full knee extension requires some external rotation of the tibia relative to the femur. Contrastingly, full knee flexion requires some internal rotation of the tibia relative to the femur.
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Common Knee Mobility Limitations in the Squat
Squat mobility limitations from the knee are less common than at the hip or ankle, but they do happen. If you find that you have difficulty achieving full knee flexion, there could be a limitation in your tibial internal rotation. Another common limitation that can affect squat form is poor patellar mobility. There also must be adequate mobility in the quadriceps, gastrocnemius, and hamstrings.
Knee Mobility – Addressing Limited Tibial Internal Rotation
Tibial internal rotation can be addressed with some simple mobility drills. You’ll start in half-kneeling (down on one knee) with the limited knee in front. Grasp your lower leg firmly with both hands. Let’s assume that you’re working on your right knee. The heel of your right hand should be pressed into the back of the outer side of your right calf, while the fingers of your left hand wrap firmly around the front of your right shin. Lunge your knee forward into flexion as you twist your lower leg inward with both hands. Check out this video to see it in action.
Knee Mobility – Addressing Limited Patellar Mobility
Limited patellar mobility can affect movement of your knee into both flexion and extension. It can also impact the ability of your quadriceps to effectively generate force. When your quadriceps are relaxed and not under tension, the patella should move relatively easily in all directions. If you find that this movement is restricted, you could benefit from working on it. You can do this by simply grasping your patella with the fingers of both hands and gently pushing it in all directions. Again, make sure that your quadriceps are completely relaxed and that your knee is close to fully extended to place the quad tendon on slack. Check out this video for an example.
Knee Mobility in the Squat – Conclusion
That gives you a solid background on the anatomical structure of the knee, the biomechanics of the knee in relation to a squat, as well as the common knee mobility limitations that affect the squat. This does not cover the full range of possible knee issues that could limit your ability to squat. These are just some common issues I have seen in my practice. Give these a shot and seek professional guidance if you find your mobility is not improving.
I want to emphasize that there should be NO PAIN at all with any of the movements described in this article. If you have pain currently or notice pain during any of the exercises, get yourself evaluated by a Doctor of Physical Therapy at Stoke.
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Health Advice Disclaimer
This article provides examples that are applicable to many, but not all people. They are based on typical presentations seen in my personal clinical practice. This information represents common findings in the population discussed, but can in no way take the place of professional evaluation and treatment by a licensed medical practitioner. It is impossible to provide 100% accurate diagnosis or prognosis without a thorough physical examination and likewise the advice given for management or prevention of any injury cannot be deemed fully accurate in the absence of this examination.
If you are currently experiencing any pain or injury, seek professional evaluation before undertaking this or any exercise program. Ensure that you are medically cleared for exercise before undertaking any exercise program. Significant injury risk may occur if you do not seek proper evaluation. No guarantees of specific outcome are expressly made or implied in this article.