Starting Strength, 3rd Edition - Mark Rippetoe [8]
Figure 2-3. Gravity acts vertically, and only vertically. Any work done against gravity will be done in a direction opposite to its force, i.e. straight up. Any horizontal component to a barbell movement is not work done against gravity.
Next, when a barbell is supported by a human body, the lifter and the barbell must be considered as a system for any analysis that applies to their combined mass. The center of mass (COM) of the human body in the standing “normal anatomical position” is a point in the middle of the hips, approximately level with the sacrum. When you squat down below parallel, the geometry of the system changes to place the COM in the air somewhere between your thighs and your torso. The COM of the loaded bar is in the middle of the bar on your back. The lifter/barbell system has a COM somewhere between the two. As the weight of the bar increases, the system’s COM moves closer to the bar until, at very heavy weights, the barbell itself approximates the system’s COM. For practical purposes, we will assume that the barbell will be loaded with heavy weights and that the barbell is usually the object that we must be concerned with balancing as we move it through the range of motion of the exercise.
Figure 2-4. The COM shifts up toward the bar as the mass of the barbell increases.
Notice that in Figure 2-5, a dashed line illustrates a vertical relationship between the barbell on the back and the middle of the foot against the floor. It should be intuitively obvious that the lifter/barbell system will be in balance when it is directly over the middle of the foot, with the mid-foot position – right under the arch of the foot – being the point of interaction with the ground that is the farthest away from both the forward and rearward edges of contact. Very simply, the mid-foot is exactly halfway between either end of the sole of the shoe. It is therefore the most stable position, the one which would take the most movement to disrupt, and therefore the one naturally favored by the body, loaded or not. The heavier the weight on the bar, the more precisely the bar position calibrates to the mid-foot. In other words, at light weights, where the mass is primarily that of the body itself, the bar may be forward of the mid-foot in a position of stability, and as the weight increases, the bar comes into balance more directly over the mid-foot.
Figure 2-5. The diagnostic angles for the squat. The hip angle is formed by the plane of the torso and the femur. The knee angle is formed by the femur and the tibia. The back angle is formed by the plane of the torso and the floor. Note that the barbell is directly over the mid-foot and is therefore in balance.
The body prefers stability to pretty much everything else. For example, the ankle joint – the actual point of rotation – is behind the mid-foot, and the calf muscles attach at the heel at about the same distance behind the ankle as the mid-foot is in front of it. The calf muscles exert tension on the heel behind the ankle to counter the effects of the leverage between the ankle and the mid-foot (Figure 2-6). The body selects the mid-foot as the balance point by inclining the shins and doing the calf work necessary to maintain this more stable position. In addition, the gastrocnemius, the hamstrings,