Starting Strength, 3rd Edition - Mark Rippetoe [21]
Figure 2-25. Tension, compression, and moment are the expressions of the force of gravity across the lifter/barbell system.
When the bar is carried on the back, or overhead in the lockout position of the press, the force it applies is compression. When the bar hangs from the arms in a deadlift or a clean, the force along the arms is tension. The bones transmit compressive force, and the connective tissues and muscles transmit tension. Both the connective tissues and the bones working together transmit moment (leverage). If the bar is supported overhead and then lowered in an arc to the hang position of the deadlift, all three forces – compression at the top, moment as the arms travel through the arc to the body, and tension as the bar comes to rest on the legs – can be experienced in that order.
Figure 2-26. Compression, moment, and tension expressed through the upper body with a loaded bar.
A moment arm is the distance between a point of rotation and the point at which the rotational force is applied, measured at 90 degrees from the point of the force application. When you’re using a wrench, for example, the moment arm is the distance along the handle, between the point of rotation (the bolt) and the force that causes the rotation (your hand), measured at 90 degrees to the force. Moment is the force transmitted along a rigid bar to act on a pivot, or fulcrum. The moment arm (the term “lever arm” is synonymous) is essentially a way to calculate the amount of moment force generated by a lever: the moment force is the force applied to the bar multiplied by the length of the moment arm. At one end of the system, force is being applied to the bar. At the other end of the system, the turning force is being resisted by the object being turned, so that along the rigid bar, force is acting in two directions. (For this reason, moment is a shear force, in contrast to the axial forces of tension and compression.) The “moment arm” is the effective distance over which the system operates. The longer the moment arm is, the more turning force is produced by the actual force applied to the bar.
The most effective angle to pull on the wrench handle is perpendicular to it. This is intuitively obvious to anyone who has ever used the device; you adjust the position of the jaws on the conveniently designed hexagonal head – shaped this way for just this purpose – so that you can pull on the wrench at right angles to it, regardless of the angle at which the job causes the wrench to fit on the bolt. If you pull at any angle other than 90 degrees, some of the force will be either compression or tension along the wrench handle – 90 degrees is the only angle at which all of the pulling force causes the wrench to turn the bolt. Since 90 degrees is the most effective angle at which to pull, any other angle is only as effective as the distance along the moment arm measured at 90 degrees, thus the convention of measuring its length at this angle (see Figure 2-27).
Figure 2-27. The moment arm is the distance between the point of rotation and the point of the application of force along a rigid segment, measured at 90 degrees from the point of force application. In barbell training, gravity provides the force, and gravity always acts vertically and down.
The amount of turning force that can be applied to the bolt varies with the length of the moment arm (the distance from the working end of the wrench to your grip, measured at 90 degrees to your pull) and the amount of force applied to it (how hard you pull on the wrench). You can increase the amount of turning force either by pulling harder or by lengthening the handle – by getting a longer wrench or extending its length with a “cheater pipe.”
In barbell training, the turning force is the force of gravity acting on the barbell, and the moment arms are the horizontal distances between barbell and joint along the segments of the body over which this force acts. The instant the knees and hips are unlocked and our diagnostic angles come