When an athlete throws at high speeds and repetitions, the anatomy that keeps the humeral head centered in the glenoid is heavily loaded.

Of the five phases that make up the pitching motion, the late lift and follow-through phases exert the greatest force on the shoulder.

• Late elevation phase. The arms and hands should be raised and positioned behind the body during the late lift phase to produce maximum pitch velocity. This extreme arm external rotation helps to develop ball speed, but it also pushes the humeral head forward and puts a lot of strain on the ligaments in the front of the shoulder. After a while, the ligaments relax and the increased external rotation results in faster pitching speed but less shoulder stability.

• follow-through phase. During acceleration, the arm rotates internally rapidly. When the ball is released, the follow-through begins, with the ligaments and rotator cuffs behind the shoulder bearing the heavy load, slowing the arm and controlling the humeral head.

When one structure, such as a ligamentous system, is weakened by repeated loading, other structures are stressed. As a result, a variety of shoulder injuries occur in throwing athletes.

The rotator cuff and glenoid labrum are the most commonly damaged shoulder structures in pitching injuries.