Starting Strength

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 into existence along the back, thigh, and shank segments in the squat, moment arms come into existence between the end points of these segments and the location of the barbell relative to the segment and the balance point under the mid-foot. The force of gravity always operates straight down − the hand turning this particular wrench is gravity, and it’s always pulling straight down from the bar. So we can calculate the moment arms along the segments as measured perpendicular to the bar.
    This means that the length of the moment arms along the back segment in the squat will always be the horizontal distance between the bar and the hips.

    Figure 2-28 .The moment arm along the back segment in the squat. ( M.A. = moment arm )

For the thigh segment, the moment arms will be the horizontal distance between the bar and the hips, and the bar and the knees, since the femur is bisected by the gravity vector, and the moment arm can be considered from either hip or knee. The hip extensors “see” the femur moment arm between the hip and the bar, and the knee extensors “see” the femur moment between the knee and the bar. Likewise, along the shank segment between knees and ankles, the moments can be regarded as between bar and ankles, and between bar and knees.
    The moment arm between the bar and the hips will thus vary with the bar position on the back and the angle at which the back is inclined. If the bar is in the low position advocated here, the distance between hips and bar is shorter than it would be if the bar were in the higher position. But since the bar must be maintained over the mid-foot balance point, the lower bar position requires a more horizontal back angle. And for the same reason, the more vertical back angle compensates for the longer distance between bar and hips in the high-bar position.

    Figure 2-29. The moment arm varies in length with angle and segment length. If