Front Squat 1-1-1-1-1-1-1 reps
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[T]he front squat, high-bar back squat, and low-bar back squat are all done with different back angles. Each causes the skeleton to move in a different way between the top and the bottom of the movement. This is because the bar position on the trunk varies relative to the hip and knee. . . . A front squat places the bar at roughly the same level as a high-bar squat, but across the depth of the chest on the anterior deltoids, requiring the back angle to be so steep as to approach vertical.
A near-vertical back will result in a knee position much forward of that in a low-bar back squat; the high-bar squat is intermediate between the two. A knees-forward back-vertical position at the bottom produces a more acute knee angle, an already shortened hamstring, and a more extended hip. The front squat is therefore a much more quad- and glute-dominant movement than the low-bar back squat, since these are the muscles that remain in a position to contribute to the lift. This is also what‚Äôs wrong with knees too far forward in the back squat: it is undesirable not because having the knees too far forward will destroy them but because of the detrimental effect it has on hip extension.
The vertical back position of the front squat seems like it would result in a more direct compressional load on the spine than the back squat‚Äôs inclined angle would produce. This is partially true. The lower back is in a nearly vertical position, but the upper back has a much tougher job because the load it is holding up is farther away from the spine. The bar in a back squat, low-bar or Olympic, sits directly on top of the muscles that are holding it directly over the mid-foot. The front squat places the bar all the way across the depth of the chest, which in a bigger guy might be 12 inches away from where it would be sitting in a back squat. This is a much longer lever arm than no inches at all and presents a mechanical challenge to the muscles that maintain thoracic extension (it is very common to get pretty sore between and just below the shoulder blades when first starting the exercise). So while the lower back is vertically compressed, unless you are flexible enough to be capable of actually leaning back a little with the bar on your anterior delts, your thoracic erector muscles have a lot of work to do. This results in a gradual shift from compression to torque from low back to upper back, so things are not as simple as they may seem. The load on the lumbar spine in the front squat is friendlier (partly because it will be lighter) as long as the upper erectors are able to maintain position, and for this reason many people find it easier on the low back to front squat. And anything that gets too heavy gets dropped automatically before death can occur.
–Mark Rippetoe, “Popular Biomechanics,” CrossFit Journal, Issue 65