The Dip Phase of the Jerk
Not all aspects of the Dip and Drive are well understood. Easy to understand, but not easily achieved, is the need to elevate the bar in one direction only – vertically upwards. This directional control requires an absolute avoidance of any rotation of the upper body during the dip, in other words the body must stay totally vertical as displayed in Figure 5. It is critical for the athlete to ‘bear down’ through the heels in the Dip to prevent forward movement of weight distribution to the front of the foot.
A common fault in the dip is an inability to “dip straight” (keep the torso vertical). This incorrect action, displayed in Figure 6, will send the barbell forwards. Even a slight rotation of the body will have an undesirable effect. Once the barbell is moving forwards it gains momentum and becomes very hard, and often impossible, to stop. In Figure 6 it can also be seen that the forward movement of barbell is so dramatic that even at the bottom of the dip the barbell is no longer over the base of support.
Another common fault in the jerk is forward movement of the ‘centre of pressure’ during the dip. The ‘centre of pressure’ is the effective point of application of force through the foot into the floor. Figure 7 portrays the common fault where the athlete starts the dip with the centre of pressure toward the heel but as the bottom of the dip is reached, the centre of pressure has moved forwards to the ball of the foot. The probable effect of this is forward movement of the body during the dip and as a consequence the bar gains forward momentum. This issue is also the root cause of the problem depicted in Figure 6 above.
Other aspects of the Dip and Drive for the Jerk are not well understood. In particular, the velocity of the dip is critical to success. The higher the downward velocity of the bar, the greater ‘impact’ of the bar at the bottom of the dip. The impact, that is the abrupt stopping of the bar and changing its direction to upwards, must be absorbed by the body. The effect of this impact can be seen in slow-motion video and is typically manifest in the ‘buckling’ (loss of rigidity) of the torso, that is bending of the spine, dropping of elbows and partial collapse of chest. On the other hand, there are also beneficial effects of absorbing the kinetic energy of the barbell in the dip. To some extent, the body acts like a spring and the ‘recoil’ is the upward movement of the bar (The Drive). Therefore, if the velocity of the dip is too slow, the athlete looses some of the beneficial spring effect. Furthermore, there is also the complicated issue of the spring in the bar, which increases in magnitude the greater the weight. It is important to control the dip velocity. If it is too rapid or too slow, there can negative consequences.
In Figure 8 below, X is the start of the Dip and Z is the bottom of the Dip. Y is an arbitrary point at which barbell downward velocity begins to slow. The distance between X and Y (marked in Figure 8 as the distance d1) is the “fall” distance. This is not a free fall as the athlete is resisting the downward movement of the bar. However during the “fall”, the bar will be accelerating. The distance between Y and Z (marked in Figure 8 as d2) is the “impact” distance and is characterised by deceleration of the bar until it comes to a stop at Z. During the “impact” distance, the athlete is exerting great force to decelerate the barbell to zero and change its direction to upward.
The following spreadsheet illustrates that the consequence of a more rapid dip during the fall distance (in the Jerk) is that more force is required to decelerate and change the direction of the bar. The difference between Case 1 and Case 2, is that in Case 2 the fall time is 0.25 sec or 25% longer duration than Case 1 which is 0.20 sec. Therefore the downward velocity over the “fall distance” is faster in Case 1 (1.18 m/sec) than in Case 2 (0.944 m/sec). The result of the slow dip speed in Case 2, is that force required to turn the bar around is 717 Newtons, and this is significantly less than the force required in Case 1 (897 Newtons).
There may be a limit to which each athlete can produce sufficient force at the bottom of the Dip without significant “buckling” of the body. The issue is that coaches and athletes are not aware that a fast Dip can be problematic. The incorrect assumption made is that a faster Dip stores more kinetic energy, and therefore a more ballistic Dip and Drive gives the athlete a better chance of elevating the bar to the height needed. However this assumption does not take account of the athlete’s limited ability to avoid “buckling” which causes the kinetic energy to be absorbed in body position changes rather than transmitted to the barbell.