Starting Strength

peak velocity at the jump.
    The leverage produced by the moment arm of the back can be thought of in two ways. (Remember that the moment arm along the back is the horizontal distance between the load and the hips, not the length of the back itself.) The pessimist will view the load hanging from the arms as moment force against the hips that would be better configured as a shorter moment arm with a more vertical back angle in order to take the “load” off the hips and low back. The effective lifter will see the moment arm of the back as a tool with which to accelerate the barbell more effectively. The same way a pitcher throws a ball by using the moment arm provided by the length of his forearm (no one would argue that a short arm is an advantage to a pitcher), the lifter accelerates the bar through the middle of the pull by using the length of the moment arm along the back as a tool. Back strength makes this possible and is one of the ways the deadlift is useful for heavy cleans.
    The wrench analogy was used to illustrate the concept of moment force, with the bar on the shoulders being the force that turns the bolt, the back being the wrench handle, and the hips being the bolt.

    Figure 6-29. The important mechanical concept of the moment arm, as illustrated by the wrench and bolt.

But in this specific application, the force gets directed from the hips to the bar, and the moment arm is the tool used to command the bar to move faster with the force generated by the muscles that open the hip angle. When we squat, the muscles of the hips and back are used to resist the rotation that could be produced by the loaded bar on the way down. But when we clean, we are using the muscles of the hips and back to produce the rotation along the back that is required to accelerate the bar upward.
    Remember that the human hip is a Class 1 lever. The back and the pelvis form the rigid segment; the hip joints are the fulcrum; the hamstrings, glutes, and adductors of the posterior chain are the force pulling down behind the hips; and the load in your hands is the force pulling down in front of the hips ( Figure 6-31 ).

    Figure 6-30. The Class 1 lever.

    Figure 6-31. The human hip, a Class 1 lever.

Because our muscles can only contract a small percentage of their length, our skeletal levers must multiply this distance if we're going to move anything efficiently. This amplification of muscle contractile distance comes at the expense of greater force production. If you are strong enough that the force generated by the posterior chain is high enough, the short segment behind the hip joint – the ischium of the pelvis – can lever up the long segment, and the length of the back multiplies the velocity of the hip rotation . The short side moving a short distance with enough force can make the long side accelerate its load over a long distance. This acceleration starts during the middle of the pull, as the bar approaches the knees, when the back angle changes to make the moment arm between the bar and the hips much shorter. This change in angle whips the back through perhaps 60 degrees of angle in a fraction of a second, making the bar accelerate along with it.

    Figure 6-32. The long moment arm of the back in the clean is used to accelerate the load hanging from the arms, taking advantage of the “whip” that can be produced around the hips. Like the trebuchet, a medieval siege engine that uses this same leverage principle, we can aggressively use the leverage available along the relatively longer back segment, instead of trying to shorten the leverage by becoming more erect before the acceleration occurs. ( M.A. = moment arm )

    As the angle of the back becomes more vertical, faster, the angular velocity – the rate at which the angle described by the plane of the back changes around the axis of the hips – increases. As this occurs, the linear velocity of the bar hanging from the arms increases as well. The bar hanging at the end of the arms increases its velocity with the angular acceleration of the back angle, just like the ball thrown from a forearm whipping through the angle it makes when the upper arm accelerates into internal rotation.

    Figure 6-33. (Figures) The sequence of force production, acceleration, and bar velocity in the power clean. (Graph) Velocity of the bar through time during the power clean, with the corresponding positions during the pull noted on the graph.

A looping bar path would take further