Starting Strength, 3rd Edition - Mark Rippetoe [156]
Since the front squat has such radically different form, you might expect that it should produce a different result than the squat. It does, for the back, hips, and legs. The vertical back position of the front squat seems like it would result in a more direct compressional load on the spine than the squat’s more horizontal 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, forward. The bar in a back squat, low-bar or high-bar, sits right on top of the muscles that are holding it up. The front squat places the bar all the way across the depth of the chest, which in a bigger guy might be 12 or more inches. This distance is a moment arm that presents a mechanical challenge to the muscles that maintain thoracic extension (it is very common for lifters to get pretty sore between the shoulder blades when first starting the exercise). And since the bar is forward of the hips, too, there’s also a moment arm against the hips, although probably not as long as in a squat, and certainly under a lighter load. So while the lower back is vertically positioned, your thoracic erector muscles have a lot of work to do. What actually happens is a gradual shift from compression to moment, 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 (because it will be lighter) as long as the upper erectors can maintain position, and for this reason, many people find front squats to be easier on the low back. But this also means that the front squat is a less effective back exercise than the squat.
When you front-squat, don’t worry about your back; worry about your knees. To facilitate the vertical back, they have to track forward so that the hips can stay directly under the bar. This means that the shins will be much more horizontal at the bottom of the front squat, with a closed knee angle, dorsiflexed ankles, and a lot more moment force operating along the tibias than there is in a squat. For most people, these factors will mean contact between calf muscles and hamstrings at the bottom, and sometimes a rather dynamic loading of the Achilles tendons and quadriceps. For some people, this closed knee angle will create enough “wedging” on the posterior knee cartilage that it can produce instability and injury, a situation that essentially never happens in a properly performed low-bar squat. For everybody, the job of opening the knee angle through a much greater ROM, with muscles working against more moment force because of a more horizontal shin, is much harder.
Figure 7-10. The knee position in the front squat, necessitated by the vertical back position, produces a moment arm along the tibias, a phenomenon that is not significant in the squat. (M.A. = moment arm)
Since the front squat places the knees so much farther forward than they are at the bottom of the squat, the hamstrings are not nearly as involved in the hip extension. In the front squat, the vertical back and pelvic position and the acute angle of the tibias place the hamstrings in a position where the