Mechanism of Injury
Since the forces on the shoulder during throwing are so high, it is no surprise that most throwing injuries involve the shoulder. The rotator cuff and labrum are most often injured. If you understand a bit about how the shoulder works, this will make sense.
The shoulder has three mechanisms that control this instability in throwers. First is the ligament / labrum complex. The ligaments are like cables that allow the shoulder to rotate, but keep the humerus from sliding too much during this rotation. The labrum is a rubbery cartilage ring that surrounds the perimeter of the glenoid and acts like a bumper around the edge of the socket. It is also the spot where the ligaments attach to the socket. If these ligaments are loose, the pitcher can rotate further and throw harder, unless the joint gets too loose. Then the humerus will slide off-center and shear the labrum, at which point it will hurt too much to throw hard. It is no coincidence that so many dominant high school pitchers get shoulder problems. They tend to be the hardest throwers (with the loosest shoulders) and throw more innings. This combination can lead to the shoulder being overwhelmed and becoming painful from instability and labral tears.
The second mechanism that maintains shoulder stability is the rotator cuff. This combination of four muscles and tendons pulls the humerus into the center of the socket no matter what position the arm is in. If the ligaments are loose, the rotator cuff will have more work to do with every throw. Like any overworked muscle, the cuff may fatigue or get strained, making it weak and less able to center the shoulder. A cycle develops whereas the rotator cuff becomes inflamed through overwork, and thus painful and weak. An inflamed cuff is thus less effective in controlling the shoulder and the instability worsens. This allows the humerus to shift off-center and possibly tear the labrum or stretch the ligaments even more. This injures the labrum and ligaments which further loosens the shoulder, demanding more from the cuff, and so on. Obviously, this can be quite painful.
The third mechanism to maintain shoulder stability and performance is the complex of muscles that control the scapula (the “scapular rotators”). These include the trapezius, rhomboids and serratus anterior. Remember, the glenoid socket is part of the scapula, so movement of the scapula will cause the glenoid to move. We tend to focus on how the ball moves on the socket, but the socket itself moves too. For example, as you raise your hand over your head, about half of that motion is the ball of the humerus rotating on the glenoid while the other half comes from the scapula rotating on your rib cage. Think of it this way: if the humerus is slipping off the glenoid, the rotator cuff muscles and ligaments can help pull the humerus into the center of the glenoid while at the same time the muscles that move the scapula can help the glenoid follow the humerus. So all three work together to keep the humerus and glenoid aligned properly. The scapular muscles help in another way. When you raise your arm overhead a painful pinching of the rotator cuff between the humerus and the acromion (the bone on the top corner of your shoulder, also part of the scapula) called “impingement” can occur. If the scapular muscles are lazy or weak, this is much more frequent. If they are working well, they will rotate the scapula upward so that there is more clearance for the rotator cuff. This can be a little complicated, but suffice it to say that the muscles that control the scapula are critical for any overhead usage of the arm, throwing in particular.