This phase begins with the athlete at the bottom of the jump, just as he begins exploding upwards towards the takeoff. The force-time graph shows that the athlete reaches peak forces shortly after reaching the lowest point of the jump. He then further accelerates until his feet leave the ground and there are no more ground reaction forces measurable.
The simplest method to measure an athlete's vertical jump is to get the athlete to reach up against a flat wall, with a flat surface under his/her feet (such as a gym floor or concrete) and record the highest point he/she can reach flat-footed (the height of this point from the ground is referred to as "standing reach"); fingertips powdered with chalk can facilitate the determination of points touched on the wall. The athlete then makes an effort to jump up with the goal of touching the highest point on the wall that he or she can reach; the athlete can perform these jumps as many times as needed. The height of the highest point the athlete touches is recorded. The difference between this height and the standing reach is the athlete's vertical jump.
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