This is from one of Kelly’s Q and A’s off of www.higher-faster-sports.com and sums up what you need to do nicely:
"When it comes to evaluating where to direct your training efforts it is very individual. You only have about .25 seconds in the vertical jump to apply as much force as possible to elevate your body in the air. In simplistic terms, to increase this proficiency you can either:
A. Increase the speed and rate at which you apply your force in the movement. (rate of force development)
B. Keep velocity and rate of force development constant, while increasing the amount of force you have to draw from.
Option A would call for explosive training (lighter weight with greater acceleration), plyometrics (depth jumps), and other rate of force development methods.
Option B would call for muscular development training and strength training.
You can increase your vertical jump either way but it’s best to find your weaknesses and train for them. The same program approach will not work for everyone. One person may be able to put 20 inches on their vertical jump with nary a plyometric drill in sight, while others need the exact opposite approach.
What I’d suggest you do is test your reactive (plyometric efficiency) with a version of Schmidbleicher’s reactive vertical jump test. At your height and jumping ability, stand on about an 18-inch box and perform a depth jump, measuring your height jumped.
Step off the box about the same as the height of the box. (so land about 18 inches away) Try to use the depth jump to enhance your height jumped and measure it. Compare the height of this jump to your regular stationary jump (down and up).
Stepping off the box and hitting the ground allows you to build up more force which your body must absorb, stabilize, and react to. This is the definition of “plyometric” so performing a depth jump automatically boosts plyometric/reactive contributions.
If the depth jump is higher than the stationary jump this will usually indicate well-developed reactive/plyometric functionality, with a need to improve the amount of force you have to draw from. (Strength training and perhaps muscle growth training supplemented with shock jumps).
If the depth jump is lower than the stationary jump this indicates an untapped plyometric potential and reactive training should be emphasized.
So you will need more “spring-like” training rather than muscular training. If the 2 are fairly close together you will find a well rounded approach (strength, explosive strength, and reactive efficiency) will give you the best results."