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Shoulder Instability in Baseball Players

Updated: May 5, 2022

Shoulder Anatomy

Shoulder instability is a common issue in baseball players. I want to review the anatomy of the shoulder so that we can understand what causes instability and its effects. First, let’s review some basic shoulder anatomy with a couple of images. The first is from and the second is from

shoulder hypermobility shoulder instability baseball players thrower's shoulder batter's shoulder

shoulder hypermobility shoulder instability baseball players thrower's shoulder batter's shoulder

The first image highlights some of the passive structures that are important to understand. The glenohumeral joint is the main joint of the shoulder. It is a ball-in-socket joint, but it is more analogous to a golf ball resting on a tee. This allows for a significant range of motion, but there is a greater chance of instability relative to other muscles. The labrum is a structure that provides greater depth to the socket of the glenohumeral joint, allowing for more stability. It is a passive structure, meaning that it does not contract or move with motion.

The second image showcases the rotator cuff muscles, which are active stabilizers of the glenohumeral joint. The rotator cuff muscles help with moving the shoulder, but their primary function is to stabilize the head of the humerus in the glenohumeral joint. They are muscles, so they are active structures and change their length and tension with movement.

There are other passive and active constraints involved in the shoulder joint, but I want to highlight the importance of the labrum and the rotator cuff. They play the largest role in instability of the shoulder, so they are important to understand.

What Causes Shoulder Instability?

Instability of the shoulder can be caused by several factors. The first cause of shoulder instability can be an acute injury such as a traumatic fall. This is not commonly the cause in baseball, but it can happen occasionally when a fielder dives for a ball or a runner dives into a base. The shoulder can become subluxed or dislocated, which traumatizes the structures around it. The labrum or rotator cuff muscles may tear so that even if the joint is put back into place, damage exists. Once there is damage to these restraints, it can be difficult to achieve stability again.

Genetics are also an important factor in instability. Some people are genetically hypermobile, meaning that their joints are more lax than normal. This can be beneficial for certain sports including baseball, as it allows you to get your shoulder into positions that others cannot. However, it does remove passive support from the structure. The best way to think about hypermobility is that the passive support is significantly decreased. If active stability is maintained, then the person may be perfectly fine. However, they have less room for error when it comes to instability. A person can have hypermobility throughout their body or in certain joints. The Beighton Scale is one component of measuring hypermobility. It involves a series of simple tests including those pictured below courtesy of

shoulder hypermobility shoulder instability baseball players thrower's shoulder batter's shoulder

The first test involves bending your pinky on each hand backwards. If you can reach beyond 90 degrees on one hand you get a point, if you can do it on two hands you get two points. The second test involves trying to touch your thumb to your forearm while your wrist is flexed. The third test looks for hyperextension of your elbow beyond 10 degrees and the fourth is the same for your knee. The final test only allows for one point and it involves touching your palms flat on the ground with your knees fully extended. There is no consensus for a threshold score, but a 4 or more out of 9 is typically classified as being hypermobile.

A study by Liaghat, et al. showed that joint hypermobility in baseball players leads to three times the risk of a shoulder injury. As I mentioned before, hypermobility can actually be helpful to allow players to assume positions that others would not be able to, such as layback when pitching, but it opens up increased injury risk that needs to be monitored. A genetically hypermobile athlete should be monitored closely for injury risk.

The final cause of instability can be chronic exposure to end-range positions. Think of a pitcher in full layback as shown below:

shoulder hypermobility shoulder instability baseball players thrower's shoulder batter's shoulder

They repeatedly assume this position, which is the farthest your shoulder can rotate. This puts compression on the back of your shoulder and stretches the front of your shoulder in ways that most other motions do not. Gradually, there will be wearing away of structures needed to support the shoulder when this happens. Part of this can be beneficial, as you can gain greater range of motion to throw the ball harder, but it comes with risk. Wearing of the labrum and rotator cuff can push your mobility, but everyone has an eventual limit. I want to show two common causes of shoulder instability in baseball players: pitching and hitting.

Thrower’s Shoulder

Thrower’s shoulder is a catch-all term for the wear and tear that is seen in throwing athlete’s shoulders, particularly baseball pitchers. It occurs in the throwing arm, and is usually a wear and tear injury that can worsen over time. As I mentioned above, external rotation of your shoulder stretches the front of the shoulder while compressing the back. Issues can occur in the front of the shoulder with stretching of the labrum in addition to your subscapularis, which is the only rotator cuff muscle resisting forward motion of the humerus. This stretching can lead to pain, instability, or tearing in the structures.

Problems can also occur in the back of the shoulder due to compression. The shoulder joint has multiple small openings for tendons and other structures to pass through, and this is noticeable for the rotator cuff tendons in the back of the shoulder. Repeated compression in the back of the shoulder can lead to internal impingement, which refers to contact between the rotator cuff tendon and the back of the humerus. The rotator cuff tendon can suffer repeated trauma resulting in inflammation and degradation. This can lead to weakness of the muscle, which can worsen symptoms. The rotator cuff in the back of the shoulder is made up of the supraspinatus, infraspinatus, and teres minor and is responsible for pulling the humeral head backwards. If this structure is damaged, then the humerus may migrate further forward, exacerbating the condition of the shoulder.

Batter’s Shoulder

A less common shoulder instability is known as batter’s shoulder. This occurs in the front shoulder for batters and involves posterior labral tears (in the back of the shoulder). The injury is essentially the inverse of a throwing injury. Repeated bat swings create trauma in the back of the front shoulder from slowing the forward momentum of the back. Eventually, the labrum can wear down and there is greater instability and damage. As I mentioned, this is a less common injury, but it is still a result of instability from repetitive baseball movements. A recent example of this injury occurred to Fernando Tatis, Jr. The image below shows the moment when force is greatest on the posterior lead shoulder to stop the bat momentum courtesy of

shoulder hypermobility shoulder instability baseball players thrower's shoulder batter's shoulder

Rehabilitation Options for Shoulder Instability in Baseball Players

Rehabilitation options for shoulder stability are numerous, and they depend on the injured tissue. As I mentioned before, the instability can cause damage to the labrum, rotator cuff, or other structures and that is usually the cause of pain and disability. Labral tears are treated non-operatively and operatively differently from rotator cuff tears, so it is important to accurately diagnose the injury.<