In the squat chapter of the book, you mention that the partial squat can be bad for the knees. The reason being that in a partial squat the posterior and anterior forces on the patella are not balanced causing a shear.
I understand this, but given that the range of motion of the full squat passes through that of a partial squat on the way up, doesn't the same imbalance occur for a time during full squats too?
Is it just a matter of the length of time exposed to this imbalance when it occurs at the bottom of the squat that is the issue?
No. The partial squat is a knee bend, while the squat activates the hamstrings as the movement is initiated from the hips.
I had this same question, and my tentative answer to it was twofold:
I say tentative, because this is not a fully analyzed intuition. Would be nice to have any engineers/physicists to chime in here.
Regarding force balances...the force acting on the bar due to gravity is a constant throughout the squat. The force you apply to you it is not. The bar wants to fall to the earth at 9.8m/s2. While holding the bar you apply an equal and opposite force through your skeletal system and finally to the ground.
Upon initiating the squat you begin to apply a force to resist the bar at something just less than that so that the bar "falls" in a controlled manner. Ideally the delta between gravity and your applied force is minimal so that the bar does not achieve any type of noticeable acceleration towards the earth...crushing you into the floor. The mechanics of the squat will then change the parts of the anatomy that apply the required force (from initially the skeleton to the posterior chain then back when you stand back up).
During the ascent, or concentric component, of the squat, the force produced must be greater that of the bar. Otherwise it will not accelerate in the vertical direction...the greater the force you produce, the more acceleration you will apply to the bar. So there is a change in the total force production from eccentric to concentric.
I'll let others more competent and well versed in the function of the anatomy than I discuss how this change in the force applied that occurs primarily in the quads and across the knee is detrimental to your knees. My observation would be that a partial squat places all of the load (and probably a load heavier than should be done) through the quads and primarily across the knee and patellar tendon while the correctly performed LBBS transfers some of the load responsibility and force production to the hamstrings. To perform a partial squat while engaging the hamstrings would probably result in the bar out over midfoot creating new problems.
In Starting Strength you say that heavy weighted back extensions are best performed for higher reps because they are hard on the knees. Can you explain why?
I've been doing 3x10 but have on and off knee pain, and I'm not sure if I should quit doing them.
Because they put a huge shearing load on the knees, which are stabilized in shear by the cruciate ligaments and no skeletal structures.
Ah. I guess that is due to the moment/torque from the pads on either end of the leg?
Would weighted situps also cause a significant shearing load on the knees, except in the opposite direction(s)?
I assume this would be theoretically applicable to GHRs as well, but they are safer because you are not nearly going to load as much weight as you can on a back extension.
Weighted situps still affect the knees through shear, but the quads seem much better able to stabilize the knee than the hamstrings. I have done 135 x 5 GHRs all the way up, so I'm not sure that back extensions are that much lighter. With either exercise, if the distance between ankle roller and hip pad is short, the knees get a lot of moment force and shear.
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