Starting Strength

the segment length changes and the angle is held constant ( top panels ), or if the angle changes and the segment length is held constant ( bottom panels ), the moment arms can be varied.

The moment arm – the horizontal distance – between hips and barbell in both positions may indeed be the same length. But we don’t use the low-bar position because it reduces moment force on the back segment; we use it because the more horizontal back angle, closed hip angle, and open knee angle place the hips further behind the mid-foot balance point, so the hamstrings, glutes, and adductors have to work harder to maintain the angle and come up out of the bottom than they do when the knees are farther forward and the hips are closer to the bar. This anatomical manipulation adds their mass to the muscles moving the load, and thus also enables heavier weight to be used.
    There is another way to consider the moments active in the lifter/barbell system. In each case, a moment arm involves a force on one end, a point of rotation on the other end, and a segment transmitting the force in between. Consider the effect of the bar on your shoulders as it relates to the balance point at the mid-foot. If the bar moves forward or backward from its ideal position directly over the mid-foot – i.e., you apply any force horizontally to the bar – and the mid-foot balance point is thought of as a point of rotation, then between the bar and the mid-foot, a rotational force is created that acts along the whole system. This horizontal force creates a moment arm that is expressed vertically along the body between mid-foot and barbell.
    Now, it is true that the foot is a flat surface (the sole of your shoe) in contact with another flat surface (the floor), and the actual point of rotation nearest the floor would be the ankle. But given that the calf stabilizes the ankle, that the load shifts in relation to the mid-foot if the bar and your body move forward or backward, and that the greater the weight and distance, the larger the effect, the system behaves like a moment arm acting on a point of rotation at the mid-foot. This leverage has the potential to add quite a bit to the force needed to overcome the weight of the bar, which happens as the bar moves forward of the balance point.
    Forward is the usual direction of off-balance movement due to the vagaries of human anatomy – the ankle is behind the mid-foot, the knees articulate forward, and the eyes are forward-directed. Most people who have been training for more than a couple of weeks will not put themselves in the rather awkward position of moving back with the bar on the shoulders. And since the body is in an asymmetrical position at the bottom of a squat or a deadlift, with more of the body behind the bar than in front of it, it would be simplistic to conclude that the same amount of movement forward and backward from the mid-foot would affect the system symmetrically, i.e., that a forward bar movement of 3 inches would have to be reacted against with the same force as a backward bar path deviation of the same 3 inches.
    Considered in this context, the term “out of balance” means that a moment (rotational force) exists between the bar and the mid-foot vertically along the body, and this moment must be controlled with an amount of force necessary to cancel its effects. This is force that could be more productively used to lift more weight on the bar if it were “in balance.” So your ability to control the moment between bar and mid-foot – your ability to maintain a vertical relationship between barbell and mid-foot – is your ability to use good technique in lifting.

    Figure 2-30. “Balance” defined as the absence of a horizontal moment arm along a vertically-oriented system.

    We must consider the effects of two systems of leverage while we squat. The moments operating horizontally along the segments of the body are produced by the force of gravity acting on the load. They are inherent in squatting down and standing back up under a heavy barbell; they make up the resistance against which we work to get strong. The moment operating vertically between the bar and the mid-foot balance point, however, must be kept at ZERO to avoid wasting force that could otherwise be used to lift more weight. Both of these moments must be considered when you’re analyzing the biomechanics of the system.
     

    Figure 2-31. Good technique in the squat is the ability to maintain zero moment