One way to measure the explosive upper and lower body power of the athlete is by testing the vertical jump. And the reason we want to measure that? Because the ability of the athlete to overcome inertia in an explosive manner is necessary for most athletic endeavors.
The vertical jump necessitates an athlete using numerous muscles of the body to allow the coordinated movement of many joints in order to propel the athlete with the highest velocity possible off the ground and into the air. Translation: there is a lot involved. The more force that the athlete can impart into the ground, the equal and opposite force will be imparted back; the higher the initial take off velocity and the higher the athlete will jump.
And the reason you care about that? Because explosive force can be trained, and explosive force is a foundation skill for the majority of sports.
Training the Vertical Jump
The height attained during the vertical jump is due to the power of both the lower and upper body. The muscles of the posterior chain (gluteals, hamstrings, calves and lower back muscles) are very important to the jump; therefore these are the muscles that should receive extra attention during training.
If the athlete does not have an initial high strength level, obviously additional strength training, concentrating on the posterior chain, will improve their jump. But improvement with only strength work is not going to last forever. What you are really going for is increasing the maximum rate of force development. (The time is takes the athlete to produce a maximal force wth the muscles he or she is developing.) The goal is to produce an explosion of strength in minimal time to fire from the ground to attain maximal height.
The jumping technique is exactly that, a technique. Practicing the proper form will let the athlete develop the proper patterns increasing against contractions and decreasing the resistance of the antagonistic muscles. Keys to good technique are bringing the arms down as rapidly as possible and bending only slightly at the hip, knee, and ankle. This rapid descent causes the stretch reflex to be activated and repeated quick stretches inhibit the inhibitory golgi tendon reflex from responding. As the athlete explodes upward with the resultant concentric contraction, the elastic energy stored during the stretch shortening cycle is also released.
Landing technique is especially important to prevent injury.