What's do you think is the best movie ever made? Citizen Cane? The Godfather? Casablanca? Now what do you think is the worst movie ever made? Showgirls? Battlefield Earth? The Room? Finally, what is your favorite movie of all time? Star Wars? Love Actually? Gross Point Blank? The reason I am asking is because this is how I think about exercise form. People ask me all the time what the "correct" way to squat is, and my answer is I'll tell them as soon as someone tells me the "correct diet" or the "correct religion." Yes, it's not an answer, but it is. As Rita Rudner said, "humor is the truth but faster."
When coaches watch someone squat, we are not holding in our head the perfect squat and comparing yours to it like a machining jig. We are experienced, educated critics watching a movie. We know what good movies are, we know what bad movies are, and we know what our favorites are based on our own biases. But no one would argue there's a "correct" movie and I'm sure we could agree that both Citizen Kane and Casablanca are awesome.
But What About Science?!
"Valgus and interior rotation and varus and glute mediuses and IT bands and clamydialises!" Yes, those are all words. Some of them even mean something. The same way that "establishing shot" and "character development" and "denouement" are all excellent words with which to critique movies. And there are lots of studies that indicate some things (knees out a little) are better than others (knees in a little) the same way that Citizen Cane shows up on a lot of top 100 lists and that there are lots of film studies departments that will tell you establishing shots help people understand where proceeding action is about to take place. But kinesiology has only been around since 1967 (the year In the Heat of the Night won the Academy Award for best picture) and there just hasn't been enough studies on enough populations through all the variables of the squat in all the different study designs to determine what the "correct" way to squat is. It might take decades for a zeitgeist of scientific evidence to recommend a single squat pattern, if there even is one.
So what are we left with? Movies. I'm damn comfortable telling you that Citizen Kane is better than Battlefield Earth and that I watch Love Actually at least 12 times a year. I'm also damn comfortable telling you that if your squat pattern keeps a pretty neutral spine, active hips, an upright chest, and looks athletic, then it probably is. That's because I've seen a whole lotta movies and a damn sight more squats.
Just Tell Me How to Squat!
There is no correct squat. There is only the best you can do on that day with the load you've got in the position you've got it in for the reps that you can do it. And as long as every time you move you're trying your damnedest to do the Citizen Kane of squats, then I know you'll get stronger and I very much doubt you'll get hurt. So forget about "correct," CTFD, keep calm and squat.