Starting Strength, 3rd Edition - Mark Rippetoe [71]
Back injuries often occur during lifting, and most usually occur when someone is lifting incorrectly. But even when this does occur, the circumstances are markedly different from those in which a hamstring tears. A leg muscle tears during a long angular contraction that involves a significant change in the muscle belly’s length over a long ROM, whereas a back injury occurs over a small intervertebral ROM that may involve little or no movement within the erector belly. Even if the entire lumbar musculature completely relaxes, not much movement will have occurred, certainly not when compared to a sprint stride. This makes it highly unlikely that you will actually rupture a back muscle belly while picking up a sack of groceries, yet these low-force, low-velocity types of activities are precisely where most back injuries happen. In the absence of blunt trauma, true back muscle ruptures are quite rare.
Most back injuries are, unfortunately, spinal in nature. Think of them as joint injuries, like a knee injury. The intervertebral discs and facet joints are quite susceptible to loaded abnormal intervertebral movement, the kind of movement that back muscle contraction is supposed to prevent. Strong back muscles developed through correct lifting technique are perhaps the best preventative for back injuries, since the habits you form while lifting correctly contribute to spinal safety just as much as the strength it produces does. Knowing this, pay extra attention to form while learning to pull off the floor; it will come in handy. That’s a promise.
Pulling Mechanics
First, let’s make a few general observations about the behavior of the physical system we’re working with here. Moment, or rotating force (sometimes the term torque is used), is the force applied along a rigid bar that makes an object at the end of the bar turn around an axis. Moment is at its maximum when applied at 90 degrees to the thing being rotated. Think about turning a nut with a wrench; your hand placed at a weird angle to the wrench is not strong, and the strongest position is one in which your hand is at a right angle to the wrench. This is why a mechanic always wants to have enough room to get his arm at right angles to his wrench on a stuck bolt.
Moment also increases with distance away from the thing being turned. A grip on the wrench turns the bolt more easily the farther it is from the bolt. The moment arm is the distance between the bolt and your hand on the wrench, measured at right angles between the bolt and the direction you’re pulling 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