Starting Strength, 3rd Edition - Mark Rippetoe [72]
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 lats.
Figure 4-18. The human hip, a Class 1 lever.
This leverage system operates when you deadlift. But if you’re strong enough, the moment arm works the other way, too; the short side moving a short distance with enough force can make the long side accelerate its load over a long distance. This is what happens in a clean or snatch.
The bar path in a heavy deadlift should theoretically be straight, because that is the shortest, most efficient way to move an object through space from one point to another, and vertically up, because that is opposite to the direction in which gravity is pulling the barbell. Work is defined as force (in the case of work against gravity, the force of gravity acting on the mass of the loaded barbell) multiplied by distance (the measured distance the barbell has to travel),