First, the elephant in the room – Yes, your knees should be tracking toward your toes. Second, telling your athletes to push the knees-out isn’t probably either the best option or the best cue. Here’s why.
Let’s assess the problem before prescribing a solution – before prescribing a cue in this scenario. The doctor isn’t giving you medication before evaluating you, it’s the same with the squats, you won’t scream “push your knees out” to an athlete without knowing WHY he’s squatting this way that. Those are some quick and not-so-quick fixes “Out of My Head” to help someone who’s been struggling with the valgus of the knee - This is a non-exhaustive list.
Hip Int/Ext Rotation Assessment to adjust squatting position. If the athlete has a predominant hip ext. rotation, I would suggest him to squat with the toes-out. If the athlete has a predominant hip int. rotation, I would suggest him to squat with the toes facing forward. This is not white/black, it's a spectrum.
Dorsiflexion/Lateral Tibial Glide Assessment. When an athlete can’t physically track his knees correctly during an Air Squat, I’m always asking myself if this person's Ankle Dorsiflexion or Lateral Tibial Glide (they’re often related) isn’t the problem. If the knees can’t go over the toes and if the tibia can’t rotate laterally on the foot-joint... where’s the knee going to go? There’s only 1 answer: Inside. Ensure that the athlete or yourself has at least a 5inches Ankle Dorsiflexion and that his foot shouldn’t start supinated (lifting off-the-floor) before 15-20 degrees of lateral tibial glide – when comparing to the front-foot pointing forward.
Barefoot Assessment. Who has heard that: While squatting, the weight should be on your heels! I mean.. I’ve said it to people when first started coaching because I’ve heard it from somebody else and just repeated it. Also, I might have learned that during my CF L1 in 2015, not sure if they still teach this “knees out” and “heel” cues, but they shouldn’t. If the tripod foot position isn’t maintained – heel, 1st and 5th toes – the hip external torque needed to create tension and lift heavier load isn’t possible and therefore, the glute muscles won’t engage and the valgus will happen. Lot of PT will automatically prescribed glute strengthening exercises to correct this disorder, but they should be starting from the ground-up and assess the actual problem. I’d say this problem is the most frequent one I've seen over the years, especially for those that only starts experimenting the issue when the weight gets heavy.
Technique/Knowledge Assessment. If the athlete is new to this sport or new to squatting and he’s unaware that he’s supposed to have a knee-tracking-over-toes and you see him starting his decent with a valgus and probably no hip-hinge… cueing this athlete to push-out the knees is probably acceptable. If every time the athlete’s fatigue, he goes back to his lazy habits of letting the knees track inside, I would prescribe RNT (Reactive Neuromuscular Training) Lunges or Air Squats prior to the training session to ensure a proper and stronger muscle memory. This can be as simple as 10/10 Split Squat w/ knee pushing out against a light/mod resistance band.