Let’s take a look at one of the most famous pitchers in the game right now—Clayton Kershaw! He has a history of injuries - from his pitching shoulder to his back.
Improper pitching mechanics can lead to unnecessary stress on joints such as the shoulder, elbow, or low back. Pitching is made up of 6 key phases, with upper extremity injuries likely happening in the transition from late cocking to acceleration or in the deceleration phase.
At the late cocking phase, hip extension range of motion is critical to allow the forces generated from the legs to travel smoothly up the chain and ultimately into the hand and ball. If hip extension range is lacking (as seen here in Kershaw), the shoulder will have to take the brunt of the forces. The set up in the third picture is ideal for pitchers as they move into the acceleration phase where they will rotate their arms at >7000 degrees per second! If the shoulder isn’t strong enough or doesn’t have enough motion, the body will follow the path of least resistance, which may result in elbow or low back repetitive use injuries.
In the deceleration phase, the body has a lot of eccentric load to slow down the arm and torso. If the shoulder and planted leg stabilizer muscles (rotator cuff and gluteus medius) are not strong enough to control this eccentric load, it can dissipate forces in the wrong areas, possibly leading to injury.
This is why hip mobility and stability are vital components to look at in pitchers to protect them from shoulder and back injuries. A thorough PT exam should look at the strength of the rotator cuff and shoulder blade stabilizers, as well as hip mechanics.