Starting Strength

on the wrench. A longer wrench works better than a shorter one because the longer length creates a longer moment arm if the angle of the pull remains efficient. The moment arm’s length is determined by both the length of the segment and the angle of the pull. A long wrench pulled from an angle that is less than 90 degrees will not turn the bolt well because the horizontal distance between the pull and the bolt is not as long as the wrench; i.e., you have created a short moment arm. Likewise, a short wrench pulled at 90 degrees is not an effective tool for a tight bolt because of the short moment arm.

    Figure 4-16 . The important mechanical concept of the moment arm, as illustrated by the wrench and bolt.

    This fact applies to all situations where a weight is lifted by the back, i.e., pulling or squatting. Gravity operates in a straight vertical line in the direction we call “down.” A bar in the hands always pulls straight down, so the moment arm in this system is always measured from the bar horizontally. A short back at a more horizontal angle might have the same moment arm length as a longer back at a more vertical angle. The best setup would seem to be a short back at a vertical angle, but we are, unfortunately, limited by the other physical constraints on the system in our ability to make our pulling mechanics more favorable. If the back is short relative to the legs, making the back vertical will drop the hips, which shoves the knees forward, which inclines the shins, which pushes the bar forward. This sequence puts the bar forward of the mid-foot and puts the shoulders behind the bar, neither of which will work at heavy weights, for reasons we shall soon investigate.
    A wrench-and-bolt model works just fine for simply describing a moment arm, but it’s not really an accurate depiction of what happens at the hip joints in a deadlift. There is another way of describing the mechanics of the pull. The hip and the spine held rigid by your trunk muscles form a Class 1 lever. To refresh your memory, a Class 1 lever places the fulcrum between the load and the force that moves it, with the rigid member being the object that transmits the force, like a seesaw ( Figure 4-17 ). The moment arms are the segments of the rigid member on either side of the fulcrum. If they are the same length, the force applied to the load is the same as the weight of the load if the system is in balance, and the distance each side moves is the same. If one side is shorter and the other side longer, the short side moves a shorter distance, more slowly, while the longer side moves a longer distance more quickly. But the speed at the longer end comes at the expense of higher force at the shorter end, with the force at the short end being multiplied by the length of the bar at the long end. So, a Class 1 lever can move a heavy weight a short distance more slowly if you push (or pull) down on the long side, like a crowbar prying loose a nail. Or it can move a light weight faster if you push (or pull) down hard on the short side, like stepping on a rake and having the handle hit you in the face, or the way a trebuchet worked in the olden days of siege warfare.

    Figure 4-17. The Class 1 lever.
    Because our muscles can contract only a small percentage of their length, our skeletal system is composed of levers that multiply the distance of their contraction at the expense of an increased force production requirement. The human hip is a Class 1 lever. The back and the pelvis form the rigid segment; the hip joint is the fulcrum; the hamstrings, glutes, and adductors of the posterior chain are the force pulling down behind the hips (the short segment); and the load in your hands is the force pulling down in front of the hips (the long segment) ( Figure 4-18 ). If the force generated by the posterior chain is high enough – if you are strong enough – the short segment behind the hips can lever up the long segment in front, even with a heavy weight. The simultaneously extending knees complicate the system, but not much. If we could design the system to deadlift heavy weights, we’d put the hips closer to the bar. But since we can’t, we have to design the pull to make the most of the mechanics we have, and this is why we keep the bar as close to the hips as we can get it. Some advanced lifters use an intentionally rounded upper back to shorten the distance between their hips and the bar. As we’ll see, this is properly the job of the