Originally Posted by
asm44
I don't think that's correct. Sure, sum forces so that SUM(F) = ma = 0, but that's only translational motion. You forgot the rotational component, SUM(M) = (I)(alpha), and that is definitely not equal to 0 in a lowbar squat, even when the knees are held in position after the initial descent.
And even if you are only looking at the translational component, a net 0 force does not mean the net effect is negligible. It just means the knees aren't accelerating anywhere. The internal forces (and torques) are significant and even desirable. If there were, as you say, negligible, the squat would not make your knees stronger, but it does.
It's been awhile, but I used to work in a lab as a undergrad mechanical engineering student where I wrote software to compute these internal forces and torques from motion capture data.