"Knees out!" One of the most widely used cues in lifting. It is especially utilized during the squat.
But why knees out?... Injury prevention. Of course! But let's take a deeper look.
The knee is the largest joint in our body; it is a complex system composed of ligaments, tendons, bones, and muscles. Our knees enable movement, and in order to move well throughout our life, the system needs to remain healthy.
When squatting, many people have a tendency to let their knees collapse. (See the picture surrounded by red.) When our knees fall inward, they are compromised. In this position, called valgus, we're more prone to both acute and long-term damage -- ACL tears and patellofemoral pain syndrome (runner's knee or jumper's knee, to be less specific) are a few commonly seen valgus-related injuries.
If a large enough force were placed on an internally rotated knee, acute damage may occur. Think: a sudden change of direction after moving at a high speed while playing soccer; or landing from an elevated position. If a number of small forces were placed on an internally rotated knee, chronic damage may occur. Think: a runner or walker who puts on mile after mile of poorly executed steps; or a businessperson who regularly climbs several flights of stairs.
As for the squat: we definitely don't want to cause harm to the knee (or any bodypart for that matter) during the exercise. So there's one reason for the "knees out" cue. As for the other reason, well, we want to properly build strength around the joint so that when every day activities ensue, we're better able to withstand the stresses placed upon the knee. (This is what we call "functional fitness.")
(I can hear the "squatting-is-bad-for-your-knees" crowd right now. This unfortunately ill-informed and ignorant number of people are doing themselves a disservice. A biomechanically efficient, properly executed squat performed on healthy knees is not a bad thing. It's a good thing! In fact, it can easily be argued that it is foolish to not squat, and that when done correctly, the squat improves the knee and helps with longevity! Ok, rant over.)
With stronger hips, legs, and muscles surrounding the knee, we're able to not only lift better, but also handle life better. Walking improves; running improves; climbing stairs, getting off the toilet, standing up from a chair, getting out of bed, remaining upright (the list goes on and on) ... everything improves! And as for our athletes: by appropriately executing a strength-building tool, such as the squat, (1) motor engrams reinforce the right movement pattern when running, jumping, etc., and (2) the knee is more prepared for and better capable of withstanding a sudden change in direction or position -- added stress.
So, just remember, "knees out!" *They should mirror the feet.* (Knees that are too wide is a different issue meant for another story on a separate day). And keep improving on that squat. Your future self will thank you.