Hip range of motion is critical for pitchers to maintain the health of their throwing arm and to attain peak performance. Any pitcher experiencing shoulder or elbow pain should look toward the hips, and all higher level pitchers (say, over the age of 16) should sporadically check on a few key issues.
Pitching demands adequate hip internal and external range of motion on both sides. There are two moments where hip function particularly comes into play:
1. The moment of shoulder/pelvis seperation. This is when the upper half and lower half of the body are (very) temporarily rotating in opposite directions, effectively winding up stored elastic energy from the powerful hip and trunk muscles to be transfered to the scapula and shoulder.
If the trail leg (right leg on a righty) doesn't have sufficient hip external rotation range of motion, the pitcher will often appear to be "opening up" his torso too early. In addition, he may shorten his stride length because the trail leg simply cannot push off over a long distance as the upper body moves down the mound.
The player who is told to "quit flying open too early" may become frustrated when he struggles to correct his
form. When he is finally able to stay "closed" longer, the
extensive adjustments needed to make that happen come at the expense of velocity and accuracy. This pitcher may need to devote a few weeks or even months to some serious hip mobility work in order to sync everything up.
2. The moment of leverage and then deceleration off the front leg during and after ball release.
Adequate hip internal rotation of the lead leg is needed for the pitcher to efficiently block with the lead leg and "catapult" the force generated in the lower half to the throwing arm. Inadequate internal rotation will cause the pitcher to cut their arm path short and/or shorten their stride length.
Lastly, it's good to remember that even in an otherwise healthy person, pitching creates what pitching demands. Pitching is a high speed, asymmetrical activity that does create problems over a bazillion reps.
The lead hip that is internally rotated and flexed forward all the time gradually loses hip extension and often pulls that half of the pelvis into a posterior tilt. The trail hip that is extended and externally rotated all the time often loses some flexion range of motion and pulls that half of the pelvis into an anterior tilt. For this reason, many pitchers will create a situation where their dominant leg is functionally longer than their stride (forward) leg. This is usually an easy fix, as the pitcher basically stretches each hip into the opposite planes of motion that they go through when throwing.
Part II - Stretches and strengthening to gain and maintain adequate hip mobility.