Starting Strength, 3rd Edition - Mark Rippetoe [10]
These angles describe the relationships of their constituent segments to each other under the load of the barbell. The back angle is said to be either more vertical or more horizontal, while the knee and hip angles are either more open or more closed. Control of the position of these angles depends on the muscles operating the bones that form the angles. We know that the lifter/barbell system will be in balance when the bar is directly over the middle of the foot, and the heavier the bar, the more precisely this position must be kept. Even if the weight is light enough to remain in a position of imbalance, the lifter will expend more energy than he would if the bar were in balance.
Figure 2-7. Extra work that must be done on an out-of-balance bar.
If the bar is on the front of the shoulders, as in the front squat, this bar position will require a very vertical back angle if the bar is to be kept over the mid-foot, as Figure 2-8 illustrates. Notice the knee angle made necessary by this position: it is very closed. And notice the hip angle: it is much more open than it would be with a more horizontal back angle. In this position, the hamstrings are shortened because their proximal attachments on the pelvis and their distal attachments at the knee are as close together as they can be at the bottom of a squat. Here, the hamstrings are functioning isometrically to hold the torso in the nearly vertical position required of the front squat, a much easier position to hold than a more horizontal back angle because of the reduced leverage against the hips (much more on this later). But when the hamstrings are shortened, there is not enough contractile capacity left to contribute much to hip extension. In essence, the hamstrings are already contracted in the bottom of the front squat and can’t contract much more. This leaves the glutes and adductors on their own to produce hip extension, and this is why your butt gets so sore when you front-squat heavy: it’s having to do all the work the hamstrings normally help with in a squat.
Figure 2-8. Squat variations commonly seen in the gym. (A) The low-bar squat, our preferred position and the form referred to in this text as “the squat.” (B) The front squat, used to catch and recover from a clean and as an assistance exercise by Olympic weightlifters.
The upshot of this situation is that the front squat leaves out much of the hamstrings’ function, and we’d like to use the hamstrings when we squat so that we can get them strong. The front squat is therefore a poor choice for training the posterior chain. To best recruit the hamstrings, and let them contribute the most they can to hip extension, we need to use a squat form that produces a more closed hip angle and a more open knee angle. At the bottom of this squat, the hamstrings are contracted isometrically – that is, they are stretched out proximally, by the attachments at the pelvis, even as they are shortened distally because of the flexing knees. As the knees and hips extend during the ascent, the hamstrings have to work hard to maintain tension on the pelvis, and to control the effects of the increased leverage demands of the more-horizontal back angle. The back angle largely determines the hip angle, and the back angle enables the hamstrings to contribute more force to the squat.
And when we use that more horizontal back angle, the bar must be placed on the back such that the bar is over the middle of the foot. The lower the bar is on the back, the more horizontal the back angle can