contractions switch to the concentric or shortening regime. However, for maximum return of energy, minimum time must elapse from when the force is received to when they are returned. The greater the time between receiving the forces and giving them back, the less is the return and the less the height that can be achieved in the jump. Most of the lengthening and shortening occurs in the respective muscle tendons which have greater elasticity. Another way of saying this is that the faster the switching from the eccentric to the concentric contraction, the greater will be the force produced and the greater the return movement. The speed of the switching is extremely fast, 0.20 seconds or less. For example, high-level sprinters execute the switch from the eccentric contraction that occurs when the foot hits the ground to the concentric contraction when the foot breaks contact with the ground in less than 0.10 seconds. In world-class sprinters, the time is approximately 0.08 seconds. The exact platform height used by most athletes in the depth jump should be less than in the early stages of training. Most athletes start at approximately after doing some jump training. They then gradually work up to and then to 30 inches depending upon how well the jumps are executed. The main criterion is that the athlete is jumping as high as possible on every jump. If the athlete gradually improves his jump height, the same platform height is continued until increases in jump height are no longer observed. At this time, takeoff height is increased by a few inches. If the athlete continually fails to jump very high, the height of the drop-down is lowered somewhat. Most important here is how high the athlete jumps after the drop-down. The maximum platform height used by a high level athlete is no more than . Rather than developing greater explosive power this height leads to more eccentric strength development. Going higher than is usually counterproductive and may lead to injury. This occurs when the intensity of the forced involuntary eccentric contraction upon landing is greater than the muscles can withstand. In addition, the athlete will not be able to execute a quick return (fast transition between muscular contractions), which is the key to successful execution of explosive plyometrics. Because of the forces involved and the quickness of execution, the central nervous system is strongly involved. It is important