This post is not intended to be an exhaustive study of proper squat technique. I'm just going to mention one unintended consequence that I've seen in group exercise classes when the following cue is given all by itself.
Most of us have heard that cue. I've given it. And it's not technically wrong. But I don't like it any more.
Here's what I see a lot of. This is me. I didn't inflict it on anyone else! I'm going to follow the knees-behind-toes cue, and see how far that cue gets me absent other information.
The knees are well behind the toes just like the cue says. Problem is, in over-focusing on keeping the knee back, I've shut down mobility (in this case, flexion) in my ankle. When my ankle stays relatively locked and my knee bends, the only way I can keep squatting without falling over is to over-hinge in my hip joint, putting un-needed stress on my low back. My video shows me keeping my head up and shoulderblades down, but sometimes this squat is accompanied with a head drop and a lurch forward in the upper back. Ouch!
What really drives me nuts is when people get down into this squat and pulse there by moving only their knee joints; the glutes bounce up and down but the back stays low and the ankles stay locked.
This is a problem for several reasons.
1) The most obvious is the stress on the low back.
2) A quieter problem, but still a problem, is the lack of mobility in the ankle. We need ankle mobility and calf flexibility to stand, to walk, for basic function. A well-done squat promotes mobility in the ankle.
3) It's not functional! OK, maybe if you're are a jockey. But if you are not a jockey, the risks of hurting the back and reduced ankle mobility outweigh, in my opinion, the benefits of the glute bounce.
Let's view a better squat (from a knee perspective) for a moment. Thank you, B. and T. for letting me use you as examples.
Here, the knee is still behind the toes for the duration of the movement, but notice that it gets closer to, and further away from, the toe throughout the squat. The whole motion is like the coiling and rebounding of a spring - three joints are moving - the ankle, the knee, and the hip. Since the ankle is also moving, the back isn't as hinged over, creating less strain on the low back. The concept is called triple flexion. All three joints share the load.
How could / should this be cued?
Here are a few things that I do to get safe and effective squats in group exercise situations where I don't have time to lecture for long or spend one-on-one time (unless someone is acting really dangerously, then I will correct them).
1) I pre-educate members about "triple flexion" before class, and then I can say "triple flexion" as a quick cue that reminds them about the shared load on all joints.
2) Or I'll say - "Bend three joints - hip / knee / ankle and keep your chest up." Sometimes it's easier to cue a more obvious part of the body. When I say chest up, I really mean flex the ankles. If they can't keep their chest up, chances are that they're not flexing the ankles. (note: this cue works most of the time but sometimes people will flex at the ankle and the knee but not at the hip, then we get knees way out in front of the toes, so it's important to emphasize all three)
3) Or I'll say - "Drop the hips back and keep the chest up."
4) This one depends on how good of a sense of humor your class has. "You have to go potty, but the toilet seat is dirty so you have to hover over the seat. BUT, you don't want to pee on the back of your shoes. Don't look down!!!" This actually gets great compliance but don't say it if it's going to get you fired.
What's the best cue ever? There's not just one. But good cues are:
Meaningful to your audience.
Concise, yet complete.
Gets the highest rate of compliance, without creating safety issues, dysfunctional movement, or offense.