Ahhh, the knee cave, my old friend. This, by far, is the most common strength and movement pattern deficit I see in developing athletes. More officially known as a valgus position of the knee, it signifies not only a severe lack of specific and general strength, but also may be an indicator of poor body control overall (due to other common muscular strength deficits that generally come as part of the "package").
The valgus position, in my experience, is an oversized red flag waving high in the air. This red flag is warning of a looming knee ligament injury.
This is a very important topic, as most coaches, parents, and athletes have no idea how to correct the problem or even identify that it is a very big - and potentially dangerous - problem.
Check out the video where I break down film of an athlete in for training and discuss what I've found and how we're going to fix the problems: