Read Online Starting Strength by Mark Rippetoe - Free Book Online
Authors: Mark Rippetoe
Tags: strength training
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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 the segment length changes and the angle is held constant ( top panels ), or if the angle changes and the segment length is held constant ( bottom panels ), the moment arms can be varied.
The moment arm – the horizontal distance – between hips and barbell in both positions may indeed be the same length. But we don’t use the low-bar position because it reduces moment force on the back segment; we use it because the more horizontal back angle, closed hip angle, and open knee angle place the hips further behind the mid-foot balance point, so the hamstrings, glutes, and adductors have to work harder to maintain the angle and come up out of the bottom than they do when the knees are farther forward and the hips are closer to the bar. This anatomical manipulation adds their mass to the muscles moving the load, and thus also enables heavier weight to be used.
There is another way to consider the moments active in the lifter/barbell system. In each case, a moment arm involves a force on one end, a point of rotation on the other end, and a segment transmitting the force in between. Consider the effect of the bar on your shoulders as it relates to the balance point at the mid-foot. If the bar moves forward or backward from its ideal position directly over the mid-foot – i.e., you apply any force horizontally to the bar – and the mid-foot balance point is thought of as a point of rotation, then between the bar and the mid-foot, a rotational force is created that acts along the whole system. This horizontal force creates a moment arm that is expressed vertically along the body between mid-foot and barbell.
Now, it is true that the foot is a flat surface (the sole of your shoe) in contact with another flat surface (the floor), and the actual point of rotation nearest the floor would be the ankle. But given that the calf stabilizes the ankle, that the load shifts in relation to the mid-foot if the bar and your body move forward or backward, and that the
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 the segment length changes and the angle is held constant ( top panels ), or if the angle changes and the segment length is held constant ( bottom panels ), the moment arms can be varied.
The moment arm – the horizontal distance – between hips and barbell in both positions may indeed be the same length. But we don’t use the low-bar position because it reduces moment force on the back segment; we use it because the more horizontal back angle, closed hip angle, and open knee angle place the hips further behind the mid-foot balance point, so the hamstrings, glutes, and adductors have to work harder to maintain the angle and come up out of the bottom than they do when the knees are farther forward and the hips are closer to the bar. This anatomical manipulation adds their mass to the muscles moving the load, and thus also enables heavier weight to be used.
There is another way to consider the moments active in the lifter/barbell system. In each case, a moment arm involves a force on one end, a point of rotation on the other end, and a segment transmitting the force in between. Consider the effect of the bar on your shoulders as it relates to the balance point at the mid-foot. If the bar moves forward or backward from its ideal position directly over the mid-foot – i.e., you apply any force horizontally to the bar – and the mid-foot balance point is thought of as a point of rotation, then between the bar and the mid-foot, a rotational force is created that acts along the whole system. This horizontal force creates a moment arm that is expressed vertically along the body between mid-foot and barbell.
Now, it is true that the foot is a flat surface (the sole of your shoe) in contact with another flat surface (the floor), and the actual point of rotation nearest the floor would be the ankle. But given that the calf stabilizes the ankle, that the load shifts in relation to the mid-foot if the bar and your body move forward or backward, and that the
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