The inward collapse of the knees during a squat is a common movement pattern often called dynamic knee valgus. This is not just a matter of poor form; it is a biomechanical fault that signals an imbalance or limitation somewhere in the lower kinetic chain. When the knees cave toward the centerline of the body, it indicates that the joints are not tracking over the feet as intended. This article will explore the specific mechanical breakdowns, from the hips down to the ankles, that contribute to this inward knee movement.
Understanding Knee Valgus and Injury Concerns
Dynamic knee valgus is a complex chain reaction involving multiple joints. The movement is characterized by the femur (thigh bone) internally rotating, the tibia (shin bone) rotating externally, and the foot often simultaneously rolling inward in a motion called pronation. This combination of rotational forces causes the knee to move medially. It is essentially an expression of instability in the frontal and transverse planes of motion.
Allowing the knees to collapse inward places undue stress on the structures of the knee joint. Specifically, the medial collateral ligament (MCL), which runs along the inside of the knee, can be stretched or strained by the excessive lateral force. This pattern also disrupts the alignment of the kneecap, leading to increased pressure on the patellofemoral joint. Over time, this poor alignment can contribute to chronic issues such as patellofemoral pain syndrome (PFP).
Primary Cause: Gluteal Weakness and Hip Instability
The most frequently cited cause of dynamic knee valgus originates at the hip. The gluteus medius and gluteus maximus are the primary muscles responsible for stabilizing the femur and preventing its internal rotation during a squat. These muscles work to externally rotate and abduct the thigh, which keeps the knee tracking outward and aligned over the foot.
When these gluteal muscles are weak or underactive, they fail to exert the necessary pull to stabilize the hip joint against the internal rotation forces of the squat. This functional weakness is sometimes referred to as “gluteal amnesia” or “sleepy glutes,” a common consequence of prolonged sitting in a sedentary lifestyle. The hip adductor muscles on the inner thigh may then overpower the weak hip abductors, pulling the femur into adduction and internal rotation, which results in the knee caving inward.
The inability of the glutes to stabilize the hip creates a cascade of faulty movement down the leg. The body, seeking to maintain its center of gravity and complete the movement, compensates for the lack of hip control by allowing the knee to drift. Strengthening the gluteal complex is therefore a direct way to restore the necessary external rotation torque to the femur.
Secondary Causes: Ankle Mobility and Foot Mechanics
While hip control is paramount, limitations in the ankle joint can also force the knees to cave as a compensatory strategy. Restricted ankle dorsiflexion prevents the body from achieving sufficient squat depth while maintaining an upright posture. When the calves or Achilles tendon are tight, the body searches for an alternate way to move the hips closer to the ground.
To compensate for the lack of forward ankle movement, the body may allow the foot to excessively pronate, or flatten, which causes a chain reaction up the leg. This foot pronation is linked to the internal rotation of the tibia and subsequent valgus collapse at the knee.
Individuals with excessive foot pronation, or flat feet, possess a lack of rigid stability in the arch that can translate rotational forces upward. This instability at the base of the kinetic chain can contribute to the inward knee movement observed during the squat. Additionally, tightness in the adductor muscles of the inner thigh can actively contribute to the internal rotation of the femur, even if gluteal weakness is not the sole issue.
Actionable Steps for Correcting Knee Caving
Correcting dynamic knee valgus requires a two-pronged approach focusing on both muscle activation and joint mobility. A foundational step is to prime the glutes before squatting with specific activation exercises. Exercises like banded hip thrusts, clamshells, and lateral band walks target the gluteus medius and maximus, teaching them to fire effectively and stabilize the hip. These movements should be performed with a focus on quality contraction rather than heavy load.
Addressing ankle mobility requires consistent effort to increase dorsiflexion range of motion. Specific stretches like the weight-bearing lunge stretch, where the knee is driven forward over the foot while the heel remains on the ground, can improve the flexibility of the calf muscles. Elevating the heels slightly with small plates or using weightlifting shoes can also temporarily reduce the demand for ankle dorsiflexion, allowing the individual to practice the correct knee alignment cue.
During the squat itself, motor control cues can help retrain the movement pattern. It is important to reduce the external load until the correct movement pattern can be performed consistently without the knees caving inward.
Motor Control Cues
Coaches often use specific phrases and techniques to encourage external hip rotation:
- Push your knees out
- Spread the floor with your feet
- Track your knees over your pinky toes
- Practicing with a resistance band looped just above the knees provides a tactile cue, forcing the glutes to work harder to push against the inward pull of the band