Knee valgus describes the knee moving toward the midline of the body relative to the hip and foot. In a static sense it refers to a knock-kneed alignment. In movement analysis the term usually means dynamic knee valgus: the knee drifting inward during a loaded task such as a squat, a single-leg squat, a landing, or a change of direction.
Dynamic knee valgus is rarely a knee-only motion. It typically combines several joint actions, including hip adduction and internal rotation, knee abduction, tibial rotation, and foot pronation. Because these happen together, the visible inward position of the knee is a summary of the whole lower limb rather than one isolated joint.
Why It Is Observed
Research has linked larger or repeated dynamic valgus to higher knee loading in some tasks, and it is a common focus in landing and cutting screens. The relationship to injury is not simple or deterministic, and a single observation does not predict injury or indicate a diagnosis.
Filming and Measuring It
- Use a front view with the camera at knee height, centered and level
- Keep the athlete facing the camera squarely; turning the body makes the knee appear to move in or out
- Common 2D measure is the frontal plane projection angle, drawn from hip to knee to ankle
- Mark landmarks consistently, such as the middle of the thigh, the center of the kneecap, and the middle of the ankle
- Landings and cuts are fast, so 120 fps or higher makes the deepest position easier to find
Common Pitfalls
- Mistaking knee flexion combined with hip rotation for true frontal-plane collapse
- Comparing angles from different camera heights or distances
- Judging from the fastest phase at low frame rates, where the peak may fall between frames
- Treating a 2D projection angle as a precise three-dimensional joint angle
A front-view clip reviewed frame by frame, with lines drawn through the hip, knee, and ankle, is a practical way to document dynamic valgus and compare it between sides or over time. Interpreting causes, such as strength, mobility, or technique, usually requires additional assessment.