Unfortunately, in isometrics training the resulting neurological and strength augmentations are essentially restricted to the angle at which you are performing the hold. If you were going to practice your vertical leap you might go down into it by bending your knees to a particular degree, but as you explode up and away from that certain angle you won't see the benefits of your isometric training any longer.
Because of this, some vertical jump programs recommend doing isometric contractions in a power rack in a number of different angles in the jump ranging from the bottom, to the half way point, to just before the top of the movement. The purpose of this is to train your body to recruit the extra fibers across the full range of the jumping motion. While this may sound plausible, I don't recommend it. Why waste your time? Resistance training will give you more explosion than isometric training ever will.
Vertical jump training studies with isometrics
As I mentioned, traditional resistance training, especially with extremely heavy weights, will most successfully train your body to recruit more muscle fibers. While isometrics training strength gains stagnate after about six to eight weeks, resistance training will keep your gains coming.
There have actually been several studies done on isometrics and vertical leaping. One study completed at Australia's Southern Cross University compared the training effects of heavy weights to isometric training to increase strength and power. While both the isometrics and heavy weight groups showed an increase in strength, only the heavy weight training group-not the isometrics trainers-also showed considerable changes for the better in a counter movement jump.
Meanwhile, a study completed at Oregon State University compared regular barbell squatting to isometrics. Both groups again improved strength, but the dynamic squatting group increased vertical leap power by double-that's right, double-that of the isometrics athletes.
Isometrics should not be used to jump higher
Isometrics are not usually the way most vertical jump athletes and coaches train. It's clear why this should be avoided. One, traditional heavy and explosive weight training provides more concrete and proven improvements. Two, traditional heavy weights across a full range of movement is quicker, easier, and more effective since you do not need to constantly stop and reset the height of the bar in order to train various angles. This not only saves time but also makes sure there are no holes in the lift range of your strength training.
While isometrics may offer some benefits for athletes training purely for strength or shape, they should not be used with athletes who wish to jump higher. Resistance training will more effectively provide the benefits that athletes in using isometrics training are seeking.