Now that you’re an expert on how knee mobility affects squat form (you did read this already, right?), let’s dig into stability at the knee joint. Since the muscles that cross the knee joint primarily exert force in only one plane of motion (the sagittal plane), the side-to-side (a.k.a. medial/lateral) stability must come from the joints above and below the knee. Thus, in this discussion of knee stability, I will be focusing heavily on muscles at the hip and ankle. I already covered the muscular and bony anatomy of the knee joint in the last article. Here, we’ll discuss muscular anatomy of the important stabilizers at the hip and ankle, their role in knee stability, and suggestions for training.
Muscular Stabilizers of the Knee
We covered the detailed anatomy of the quadriceps and hamstrings in the last article linked above. At the ankle, I will cover the tibialis posterior and the peroneal group. These muscles on the inside and outside of the ankle are responsible for controlling motion at the arch of the foot. At the hip, I will focus on the gluteus medius, gluteus minimus, and the adductor group. These hip muscles on the outer and inner side of the joint help to keep the knee centered during movement.
Knee Stability from the Ankle
The tibialis posterior muscle originates on the back of the tibia and fibula, runs down along the inside of the ankle, and inserts at the arch of the foot. Its main action is to raise and stabilize the arch of the foot when the foot is on the ground. If the foot is in the air, its action will point the foot down (into plantarflexion) and in (into inversion).
The peroneal group consists of the peroneus longus, peroneus brevis, and peroneus tertius. These muscles originate primarily on the fibula and pass along the outside of the lower leg and ankle. Peroneus brevis and tertius insert into the outer part of the foot, while the peroneus longus tendon actually passes underneath the foot and attaches to the inside of the foot near the base of the big toe. The insertion locations are important as they determine the ultimate movement/action that each muscle produces with contraction. These muscles work together with the tibialis posterior to control and stabilize the arch of the foot when the foot is on the ground. With the foot in the air they point the foot down (into plantarflexion) and out (into eversion). More on this later during the discussion of biomechanics.
Knee Stability from the Hip
Gluteus Medius and Minimus
The gluteus medius and minimus are the smaller muscles of the gluteal group and lie beneath the larger gluteus maximus muscle. All three gluteal muscles originate on the pelvis and insert on the femur. While the maximus serves primarily to extend the hip, the medius and minimus mainly function as as hip abductors (pulling the leg away from midline). This makes them incredibly important for stabilizing the position of your knee and controlling knee valgus. Valgus is when the knee drops inward toward midline with respect to the hip and ankle. This is a normal part of human motion, but when uncontrolled it is associated with injury of various important structures in the knee such as the ACL, meniscus, and MCL.
The Adductor Group
On the other side of the hip joint, in the inner thigh, are the adductor muscles. This group of five muscles functions primarily to bring the lower limb toward midline (into adduction). The adductors include the pectineus and gracilis along with the adductors magnus, brevis, and longus. All five muscles originate on the pelvis. The gracilis inserts below the knee on the tibia, while the other four insert into the femur.
BIomechanics of Knee Stability
Knee Stability at the Ankle
The ankle influences knee stability from below, providing a foundation for movement. Position of the ankle determines the position of the lower leg, in turn influencing stability of the knee. When the arch of the foot falls toward the floor (moves into pronation), there is an associated movement into tibial internal rotation and a tendency toward knee valgus. As the arch of the foot raises up (moves into supination), the tibia rotates externally and the knee moves more centrally. Some pronation is normal as the knee flexes forward over the foot. However, if this motion is uncontrolled the body is left without a stable base from which to move. This can lead to uncontrolled knee valgus, creating stress and increased potential for injury.
As mentioned earlier, the tibialis posterior and peroneal group work together to control the arch of the foot and position of the ankle when the foot is on the ground. Supination of the foot requires co-contraction of the tibialis posterior and the peroneal group. This simultaneous contraction of muscles on opposite sides of the ankle essentially twists the bones of the foot in a way that creates rigidity and stability. Thus, strength and coordination in these muscles is essential to squat well.
Knee Stability at the Knee
Stability for the knee joint at the level of the knee is mainly a question of muscle balance. In order for the knee joint to remain stable, there must be adequate strength and activation of the muscles on both sides of the joint. This means that the difference in strength and activation between the hamstrings and the quadriceps must not be too large.
Likewise, the activation and strength of the different muscles within each group must be balanced. In the hamstrings, this influences tibial rotation. The quads, on the other hand, play a role in the movement of the patella. Imbalances between the muscles of the quadriceps can contribute to abnormal patellar movement, frequently creating a painful condition called Patellofemoral Pain Syndrome a.k.a. “Runners Knee”.
Knee Stability at the Hip
Knee stability from the hip is again all about balance. There must be contraction of both the gluteal muscles and the adductors simultaneously to stabilize the knee. If the gluteal muscles are weak, there is again a dangerous tendency toward uncontrolled knee valgus. With weakness of the adductors, the squat movement will likely be weak and there may be compensatory wobble side-to-side at the knee as the gluteus medius and minimus attempt to create stability with no counterbalancing force.
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How to Improve Your Knee Stability in the Squat
I’m going to go through two different exercises to improve knee stability for squatting. The first is one of my personal favorites, the Standing Clam. I love this exercise because it addresses the peroneal group, tibialis posterior, gluteus medius, and gluteus minimus all in one fell swoop. To target the adductors, we’ll use the Copenhagen Plank. This one is a bit of a love/hate relationship because it’s both brutally hard and insanely effective. In addition to these two, I recommend that everyone include some targeted hamstring strengthening in the their training to make sure they have the strength to effectively counterbalance the quadriceps.
As you go through this exercise, make sure you maintain solid ground contact at the ball of the foot, outer sole, and heel. As you push your knee outward, drive down through the ball of the foot squeezing the ground between the ball of your foot and the heel to raise the arch. While the knee comes in, control the arch as it flattens while keeping the outer sole of the foot firmly planted.
The Copenhagen Plank is a great way to work on adductor muscle strength to increase stability throughout the lower body, including the knee. Try out this progression, starting with Phase 1 and working your way up to Phase 5.
Conclusions on Knee Stability for Squatting
Creating a solidly stable knee for squatting involves balanced strength throughout the lower body. Again, most of the side-to-side stability for the knee (which is where most people run into issues) actually relies on activity of muscles at the hip and the ankle. If the hip and ankle are weak or imbalanced, the knee is likely to lack stability and wind up with pain. As always, if you have any pain or a currently active injury please get yourself evaluated by a Doctor of Physical Therapy. This article is in no way intended as a guide to self-treat any pain or injury.
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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.