The Oly Encyclopedia 73

Optimizing Various Aspects of the Jerk

Long Versus a Short Dip In The Jerk

The dip for the jerk is essentially a preparatory motion for imparting maximum vertical velocity to the bar. It is much like the bend in the knees that an athlete makes when he or she prepares to make a vertical jump, and therefore the same essential principles apply to both motions. The preparatory bending of the legs accomplishes a number of things. First, it lowers the center of gravity of the jumper's body (or the combined center of gravity of the lifter and the bar). This creates a distance over which the jumper or lifter can apply force and achieve acceleration. All things being equal, the longer the jumper/lifter can generate force, the greater the velocity achieved by the time the jumper or lifter loses contact with the ground.
In addition to the benefit of achieving a longer distance for and time of acceleration, a longer dip of the legs tends to increase the activation of the elastic properties of muscle tissue. When a muscle is made to contract in an eccentric manner immediately before it contracts concentrically, that muscle will contract with greater force than if it had not received the preliminary external force. Up to a point, a greater external force will elicit a greater eccentric contraction and hence greater subsequent concentric contractile force. Therefore, the preparatory bending of the legs both positions the legs so that a maximum duration of acceleration can take place and charges" the muscles with elastic energy so that the upward thrust will be greater than if the athlete had started from a dead stop.
Taken to their ultimate extreme, the preceding formulations imply that the greatest possible jumping height can be generated by dropping into a full squat position with maximum force and then jumping up. Clearly a drop into a full squat can both provide maximum stimulation of the downward force against the muscles that will subsequently contract and give the legs an opportunity to accelerate the body and bar over the greatest distance. Not surprisingly, the deep squat position is not the best method of achieving a maximum jumping height (or thrusting height in the jerk). This is because offsetting factors begin to overcome the advantages of a longer period of thrust and a greater elastic muscle "charge."
One problem with too long a dip is that while muscles can only achieve maximal contractile force if they are given sufficient time to do so, after a certain period of time the force of muscle contraction begins to diminish as a result of fatigue. Therefore, there is an optimum time of muscle contraction, and after that period force diminishes rather than grows. Another problem is that as the legs bend ever further in preparation for a jump or jerk drive, the amount of force required to straighten the legs increases, placing the muscles at a disadvantage. Still another problem is that as the depth of the bend grows, the extent of the movement of various joints increases, which tends to destabilize the pattern of movement and make it more difficult to maintain balance and control as the upward thrust is performed. Finally, greater downward force generated in the dip also means greater effort is required to reverse the direction of the bar.
Given these considerations, the depth of the dip will vary somewhat with the strength of a lifter's muscles, his or her body proportions and the style he or she uses to perform the drive (e.g., the speed of the dip and the speed with which the downward force is reversed). A faster dip and reversal can elicit the elastic properties of the leg muscles as much as a slower but deeper dip and reversal of the downward motion of the dip. For most lifters this depth will range from 8% to 12% of the lifter's height.

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