Knees sliding forward on the bottom of the squat

[semiok45]

Rip,

Recently you helped me with my squat by telling me that my knees were sliding forward at the bottom. I’ve been working on it. Original video here, recent video here.

After reading/re-reading your books, articles, forum, etc., I’ve learned that that the knees slide forward in the bottom of a squat if a lifter loses the control/tension necessary to keep the pelvis fixed in its proper position, then the pelvis tilts forward, the hamstrings slack and lose tension, and the quad tension wins out and pulls the tibia enough to cause the knees to slide forward. This is bad because hip drive is lost and knees can ache. So shove your knees out and forward early so that they are in good position during the bottom 2/3 of the squat, and keep your hips tight. I think I get it.

However, do you ever see knees sliding forward at the bottom for other reasons? I was thinking maybe the lifter maintains his pelvic position, but tries to reach his final depth with his hips but not the bar. Say, a lifter who’s low back is weak and he avoids leaning over all the way in the bottom, so his back angle steepens subtly and his knees slide forward.

Shit, then again a weak low-back would probably lead to the first scenario and my whole thought process is moot. There are reasons why I ask this, but I’ll spare you.

[Mark Rippetoe]

If the knee angle becomes more closed, and the quads control knee extension, it's certainly going to involve the quads.