We have discussed the analogy in the past that your trunk is like your internet connection (see Sparta Point 11/12/09), in that greater strength in this region allows for a faster transmission of movement between the lower and upper body limbs. Perhaps the simplest example is how you can jump higher using an arm swing as opposed to keeping your arms at your side. It is hard to picture your upper body and trunk playing such a crucial role in jumping and sprinting, but in some cases, it might be more important than strengthening your legs.
A 2010 study out of Texas Tech University found that athletes using their arms slowed down their hip extension, or straightening of the legs, to allow more force production into the ground. Remember a greater force production down into the ground (see SpartaPoint 12/8/10) causes a greater movement in the opposite direction, a higher jump in this case.
So as our volleyball players pursue greater jump heights, upper body and trunk strength play a crucial role, particularly increasing the ability to produce force. And no movement is more effective than pull-up variations as they involve the greatest amount of musculature above the waist.
For our athletes that do not jump as a primary sports skill, such as baseball or football players, this vertical jump still represents one of the simplest assessments of force production, and our rationale for using the test on our force plate. This increased force production in a jump will carry over to faster sprints, better fitness, and harder throws.
So whether you jump for a living or are just looking to increase your overall force production, pull-ups can be a huge boost to your game. You can also just avoid training your upper body, just make sure you run, jump, and throw without using arms as well.