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.
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The player approaches the basket and obstruction, and then leaps. During flight, some portion of the player's body is elevated above the obstruction. This may entail raising the legs or some portion thereof in-air to soar over the obstruction. In other instances, the trunk-moves over an obstruction as the legs pass around it. Common obstructions include: motor vehicles; crouched, seated or standing person(s); ball rack; or other available objects.
To perform two-foot dunks, jumpers bend their knees very deeply and spend a lot more time on the ground loading the jump. This increased time during takeoff is useful because it allows the athlete to transfer force into the ground thereby improving height. Using this approach makes it more difficult to transform speed into jump height making a fast approach far less useful than in one-foot jumping.
Strength exercises include slow, controlled movements like squats, lunges, and weighted step-ups.  Power exercises require explosive, quick moves like those needed for plyometrics and power cleans. Plyometrics are explosive bounding, hopping and jumping drills that blend strength and speed. Finally, practicing maximum vertical jump will increase vertical jump.

Starting out, athletes should always err on the conservative side and only perform 10-20 maximal effort jumps in a training session. Because of the explosive nature of a vertical jump, the body can only perform a handful before performance starts to drop. Training beyond this point will not improve jumping height and will only lead to injury. At the completion of a training session, it is generally recommended to rest 48 hours before completing another intense training session.
I just turned 14 year old 5''''10-5''''11 8th grade 165 pounds and i''''m wondering what stretching exercises and weight lifting exercises i can do to increase my vertical its already at like 30-32 inches but i want maybe a 40 by high school and a 48 or more by the time i''m out of high school i''ve dunked over 15 times ( in practice and alone with one hand its effortless to touch rim with both feet and easier with one but i''m also wondering how to take off when i dunk because i stutter step and i want to get my explosiveness up. And i want to be able to dunk with two hands. Can anyone help me?

Discussed in this module are activities which when applied, modify a given dunk type. Modifier-activities occur prior to leaping or while airborne. Modifiers performed prior to leaping pertain to the manner of approach (e.g., locomotion or standstill), angle of approach (e.g., from the baseline), distance of leap from the basket, the addition of a pass (e.g., alley-oop), or some combination thereof. Modifiers performed while airborne pertain to bodily rotation (e.g., 360°), obstruction of own vision (e.g., arm-over-the-eyes), other bodily movements superfluous of dunk type (e.g., voluntary kicking of the legs), or some combination thereof. Dunk types can also be modified with obstructions (e.g., leaping over a car or person) which influence activities both prior to leaping and while airborne.

Some days I would be doing leg exercises — you want to give yourself a 48-hour break between heavy lifting involving any given part of your body, to give yourself time to recover. Some days, I’d go out to the track and do a track workout like sprints. I was avoiding doing long-distance running, because I didn’t want to develop slow-twitch muscles. I wanted to concentrate on fast-twitch muscles. And then for fun, on the weekends, I would play soccer or pickup basketball. I was becoming a better athlete because of this. Not only was I faster and stronger, but I was also more confident, in terms of just the run of play in any of these team sports, because I was more athletic than I had been.
variations: The vertical jump test can also be performed using a specialized apparatus called the Vertec. The procedure when using the Vertec is very similar to as described above. Jump height can also be measured using a jump mat which measures the displacement of the hips. To be accurate, you must ensure the feet land back on the mat with legs nearly fully extended. Vertical jump height can also be measured using a timing mat. The vertical jump test is usually performed with a counter movement, where there is bending of the knees immediately prior to the jump. The test can also be performed as a squat jump, starting from the position of knees being bent. Other test variations are to perform the test with no arm movement (one hand on hip, the other raised above the head) to isolate the leg muscles and reduce the effect of variations in coordination of the arm movements. The test can also be performed off one leg, with a step into the jump, or with a run-up off two feet or one foot, depending on the relevance to the sport involved. For more details see vertical jump technique.
Vertical jump measurements are used primarily in athletic circles to measure performance. The most common sports in which one's vertical jump is measured are track and field, netball, basketball, football, and volleyball, but many sports measure their players' vertical jumping ability during physical examinations. In addition, single and multiple vertical jumps are occasionally used to assess muscular strength and anaerobic power in athletes.[3]