Knee valgus, commonly known as “knock-knees” or dynamic knee collapse, is a biomechanical pattern where the knee joint angles inward toward the midline during movement. This misalignment results primarily from muscular imbalances acting upon the lower body joints. The knee is part of a kinetic chain, dictated by muscles above and below it. This collapse involves the femur rotating internally and adducting. Understanding the muscle groups that fail to resist this motion or actively contribute to it is key to addressing the issue.
Weakness in Hip Stabilizing Muscles
The primary driver of dynamic knee valgus originates in the hip, due to insufficient strength or activation of the muscles controlling the femur’s position. These muscles normally abduct and externally rotate the thigh, keeping the knee tracking outward and in line with the foot. When weak, the femur falls into an internally rotated and adducted position, defining the knee collapse.
The Gluteus Medius is a primary stabilizer, and its weakness significantly contributes to this faulty movement pattern. During single-leg stance, such as walking or running, a weak Gluteus Medius fails to adequately stabilize the pelvis. This failure can result in the dropping of the pelvis on the non-weight-bearing side (Trendelenburg sign).
This lack of control translates down the leg, causing the femur of the weight-bearing leg to rotate internally and adduct toward the opposite leg. The Gluteus Maximus is also important, as its posterior fibers are powerful external rotators of the hip. When underactive, it cannot counteract internal rotation forces, allowing the inward movement of the thigh unchecked.
Furthermore, smaller, deep external rotators, including the Piriformis and Gemelli, assist in maintaining hip external rotation. Their dysfunction can contribute to overall rotational instability. The collective weakness of these hip-abducting and external-rotating muscles creates the fundamental deficit that permits the dynamic valgus posture during functional movements like squatting or landing.
Overactivity of Internal Rotators and Adductors
While some muscles fail to prevent the inward collapse, other muscle groups actively contribute to the valgus position by becoming tight or overly dominant. These muscles perform hip adduction and internal rotation, pulling the thigh bone toward the body’s midline. The inward pull exacerbates the forces created by the weakness in the hip stabilizers.
The Adductor muscle group, particularly the Adductor Magnus and Adductor Longus, are powerful hip adductors that draw the femur inward. When chronically dominant, they can overpower the weaker stabilizing muscles, pulling the thigh into the adducted component of the valgus position. This is often seen in individuals who spend prolonged periods sitting, leading to muscle shortening and stiffness.
The Tensor Fasciae Latae (TFL) is a muscle that contributes to internal rotation of the femur. While it assists with hip abduction, its tendency to become overactive as a synergist to a weak Gluteus Medius means it can pull the femur into internal rotation. The TFL’s overactivity and subsequent tightness can create tension on the iliotibial band (ITB), which may indirectly influence knee joint alignment.
The medial hamstrings (Semimembranosus and Semitendinosus) also internally rotate the tibia relative to the femur. While these muscles serve a stabilizing function against valgus forces, their tightness can contribute to the rotational component of dynamic valgus. This combined dominance of the TFL and the adductor group creates an imbalance where inward-pulling forces consistently overcome outward-stabilizing forces.
The Role of Lower Leg Muscles in Dynamic Valgus
The muscular chain causing knee valgus does not end at the knee; the position of the foot and ankle significantly influences alignment from below. The foot’s arch plays a considerable role in shock absorption and directly affects the rotation of the leg above it. A loss of the medial arch, known as excessive foot pronation, initiates a chain reaction that travels up the leg.
Excessive foot pronation causes the tibia to internally rotate to compensate for the arch collapse. This internal rotation stresses the knee joint and reinforces the inward collapse initiated by the hip muscles. The Tibialis Posterior muscle is the primary muscle responsible for supporting the arch and resisting this pronation.
Weakness or fatigue in the Tibialis Posterior allows the arch to flatten, leading to internal tibial rotation that contributes to dynamic valgus alignment. Conversely, tight calf muscles, specifically the Gastrocnemius and Soleus, can indirectly contribute to the issue. Tightness limits the ankle’s ability to dorsiflex, forcing compensation higher up the kinetic chain. This compensation often manifests as increased knee valgus during movements like squatting.