The defining characteristic of the depth jump is that the jump is preceded with the strong eccentric (negative) muscle action caused by dropping down from a raised surface, as opposed to a standard box jump where you start on the floor. This makes the depth jump a true plyometric movement, where the muscles are stretched suddenly (by the impact of the landing), producing a powerful shortening of the muscle fibers.
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Technique Tip: Determining how far out in front of you to place your front foot may require some trial and error. At the bottom of the motion, your front knee should be somewhere above your heel to mid foot. If your knee is behind your heel, your foot is too far forward; if it’s out over your toes, step out further. One trick to find the right distance is to start in the bottom position and adjust your stance from there. Then stand up and have someone hand you the dumbbells.
Of course, these forces increase linearly with increasing body weight. Therefore Olympic high-jumpers are usually build more like marathon runners and less like football players. Every unnecessary pound adds to the forces during take-off, and at some point the muscles and tendons of the jumping leg are just not strong enough any more to support all the weight.
The first thing they have to do is improve their flexibility, for a couple of reasons. They need to be flexible to undertake the kind of exercises they need to be able to jump higher. They also just need to be able to increase their flexibility, because in the short sprints you take when you try to dunk a basketball, if you can imagine yourself running up to try to dunk on the rim, the higher you can bring your knees in a sprint, just like a sprinter running the hundred meters, the greater force you’ll be able to exert on the ground, especially with your leaping step.
An important component of maximizing height in a vertical jump is attributed to the use of counter-movements of the legs and arm swings prior to take off, as both of these actions have been shown to significantly increase the body’s center of mass rise. The counter-movement of the legs, a quick bend of the knees which lowers the center of mass prior to springing upwards, has been shown to improve jump height by 12% compared to jumping without the counter-movement. This is attributed to the stretch shortening cycle of the leg muscles enabling the muscles to create more contractile energy. Furthermore, jump height can be increased another 10% by executing arm swings during the take off phase of the jump compared to if no arm swings are utilized. This involves lowering the arms distally and posteriorly during the leg counter-movements, and powerfully thrusting the arms up and over the head as the leg extension phase begins. As the arms complete the swinging movement they pull up on the lower body causing the lower musculature to contract more rapidly, hence aiding in greater jump height. Despite these increases due to technical adjustments, it appears as if optimizing both the force producing and elastic properties of the musculotendinous system in the lower limbs is largely determined by genetics and partially mutable through resistance exercise training.