We know that once someone has pain, they tend to have weakness in their glutes and hip external rotators. It isn’t predictive of pain, but it is likely once pain has begun. 

It is true that a knee valgus force combined with 30 degree of knee flexion (small knee bend) may rupture the ACL. It is true that some studies have linked landing with a knee valgus with a higher risk of ACL injury (and other knee injuries/pains). That said, the research is inconclusive, (some studies have and some studies haven’t linked it) and the “why” may not be what we think it is.

Some research has found that ACL injury prevention programs are VERY effective at reducing risk of ACL injury, but don’t necessarily change landing mechanics. So the person lands the same way, perhaps in a valgus, but is less likely to damage their knee.

Why? Could it be that muscle strength increased enough to mitigate forces going through the knee, something which we can’t actually see with our naked eyes? Is it then effective to categorise athletes into low or high risk of ACL injury based on how they land when how they land may not visually change, but their injury risk has?

I really think that a LOT of what goes on with the human body is unable to be visualised because there are internal forces going on that we can’t SEE with our eyes alone. We try to make sense of what we can SEE , not recognising the body is way more complicated than that! But also way smarter than us, in a good way.

The ‘why’ matters, because if you just blame VALGUS, and direct all treatment at trying to stop that, when in many cases it is not possible (I am case in point), then you may be missing the point.A dynamic knee valgus occurs when the knee flexes. The femur internally rotates and adducts relative to tibial external rotation and abduction. This will look like the knee “going in”. A knee valgus with movement isn’t necessarily bad, at all. It depends on context and person. 

My advice? The best way to reduce risk of ACL injury is to do strength training that also includes jumping and landing based exercises, in multiple directions. That way, you increase your quads' ability to absorb force, which may mean you land in a greater degree of flexion, and absorb more force in the frontal plane, avoiding the shift of force to the staggital plane, thus causing less of an adduction and internal rotation of the femur. Training in this way also increases the muscle's ability to absorb force, so will take more force to exceed the capacity of the knee if the muscles are strong. 

Reference: 

Cronström, A., Creaby, M. W., & Ageberg, E. (2020). Do knee abduction kinematics and kinetics predict future anterior cruciate ligament injury risk? A systematic review and meta-analysis of prospective studies. BMC Musculoskeletal Disorders, 21(1), 563. https://doi.org/10.1186/s12891-020-03552-3