Shoulder Range of Motion Changes in Baseball Players: GIRD, Retrotorsion, and More

Brooks Klein Baseball

Baseball players consistently demonstrate greater shoulder external rotation range of motion (ROM) in their throwing arms and decreased internal rotation ROM. These changes have been documented as early as 1969 in scientific journals (King 1969). The cause of these range of motion adaptations has been debated since. 

 

Are These Changes Caused by the Shoulder Capsule? 

Initially, researchers proposed capsular adaptations in the throwing shoulder as the reason for these ROM changes. 

Some authors attributed increases in shoulder external rotation to repetitive anterior capsule “microtrauma.” Other authors theorized that external rotation gains and internal rotation deficits could be attributed to a tight posterior shoulder capsule. 

In a popular article series by Burkhart, Morgan, and Kibler (2003), the authors theorized that a tight posteroinferior capsule resulted in glenohumeral internal rotation deficit (GIRD) leading to a “pathologic cascade” in the throwing shoulder. They originally defined pathological GIRD as a difference greater than 25° between the throwing shoulder and non-throwing shoulder. 

This theory lead to popularization of the sleeper stretch. The authors believed daily posteroinferior capsular stretching would minimize GIRD and reduce the risk for shoulder injury in baseball players. 

However, further research has not supported the idea of a tight capsule. While the posterior capsule has been shown to be thicker in the throwing shoulder of baseball players, side-to-side differences in capsule thickness are not correlated with differences in shoulder internal rotation ROM. (Hibberd 2015)

Studies measuring joint translation have not found side-to-side differences or correlations with shoulder range of motion in baseball players either (Borsa 2005). Again, the capsule does not seem to be the cause of these ROM changes.

An argument could be made that capsule thickness is not as important as capsule stiffness when assessing the throwing arm of baseball players. A study by Takenaga et al.(2015) found that differences in posterior and posteroinferior capsule stiffness between arms were negatively correlated with internal rotation ROM. Basically, the stiffer the capsule the less internal rotation ROM. 

Well, wouldn’t that mean that the capsule does play a role in shoulder ROM changes in baseball players? 

Maybe…

But, I think this correlation was probably because of the methods of the study. All participants had their shoulders measured in the same position. Normally, this is important for standardizing the methods so measurements are consistent for each participant. In this case, it may have been a limitation. 

If all the shoulders were measured with the same degree of internal rotation, then differences in bone structure may have confounded the results by tensioning the capsule differently.

 

How Do Changes in Bone Structure Affect Shoulder Range of Motion?

Changes in the shape of the humerus have consistently been demonstrated in the throwing arm of baseball players (Greenberg 2015). Baseball players have a different amount of twist in the humerus of their throwing arms. The twist in the humerus is referred to as torsion. 

Typically, the throwing arm of baseball players will have greater humeral retrotorsion. Picture this as the bone twisting around its longitudinal axis to allow more external rotation when throwing. See the image below.

Source: Keith Allison on FlickrKevin Gausman, diagrams added by Brooks Klein, CC BY-SA 2.0

 

Changes in humeral torsion would then account for the increase in shoulder external rotation ROM in throwers while also explaining loss of shoulder internal rotation ROM. 

Side-to-side differences in torsion for skeletally mature baseball players have been reported between about 10 to 20 degrees (Greenberg 2015). These torsion differences have been associated with ROM changes in the throwing arm of baseball players as well. (Hibberd 2015Lee 2016)

In skeletally immature baseball players, the humeral torsion difference between arms is still developing, and torsion changes will depend on age and playing level. The following chart shows average side-to-side difference in humeral torsion grouped by various competitive levels.

Age Group

Mean Side-to-Side Difference in

Humeral Retroversion (degrees)

Youth (6-10 years) 7.5 ± 10.1
Junior High (11-13 years) 10.7 ± 9.9
Junior Varsity (14-16 years) 15.3 ± 11.1
Varsity (16-18 years) 16.2 ± 11.4

Source: Greenberg 2015

An interesting note, the humerus of the throwing arm does not actually twist back into humeral retrotorsion as baseball players age. Rather, the humerus actually stops twisting forward. We naturally develop with our humerus in greater retrotorsion, and then the humerus twists forward as we age. In baseball players, throwing actually slows down this natural rotation forward. So, technically the humerus is not actually twisting back into retrotorsion. The humerus of the throwing arm is actually prevented from twisting forward into natural torsion, while the non-throwing arm twists forward. This creates the side-to-side differences in humeral torsion in baseball players.

 

What Are Some Other Factors That May Affect Range of Motion?

While bony changes can account for some of the shoulder ROM changes in baseball players, they do not completely account for these changes. Some other factors that may affect shoulder ROM include muscle and the nervous system.

Bony changes are gradual and take time to develop, but there are several studies demonstrating immediate ROM changes after throwing. Reinold et al. (2008) showed that pitchers lost shoulder internal rotation, total rotational motion, and elbow extension immediately after pitching, and these motion deficits persisted for 24 hours after pitching. Another study by Escamilla et al. (2017) found similar results. They found that pitchers lost shoulder internal rotation, external rotation, and total rotational motion after throwing. ROM was quickly restored to pre-pitching levels after a quick stretching routine (two-out drill). ROM changes that occur that quickly are likely not changes in bone structure. 

Reinold et al. attributed these ROM changes to acute musculotendinous adaptations due to the high eccentric stress on shoulder musculature during throwing.

Another factor affecting shoulder ROM may be the nervous system. A study by Laudner et al. (2014) reported changes in shoulder internal rotation ROM after a brief 40-second instrument assisted soft tissue mobilization treatment to the posterior shoulder muscles. It is extremely unlikely that 40 seconds of IASTM is enough to significantly change muscle structure. A possible explanation may be some underlying nervous system changes, such as a decrease in reflexive muscle guarding. 

The mechanism of ROM change for these studies is still unclear, but we know that bony changes cannot completely explain changes in ROM.

 

How Does This Affect Training and Rehabilitation?

The biggest takeaway is that asymmetry is normal for baseball players. We should not expect internal rotation and external rotation ROM to be the same between arms. Much of these changes shoulder ROM can be attributed to bony changes. Bony changes are clearly not going to be affected by stretching and soft tissue treatments.

While asymmetry is normal, we should still expect similar values when comparing total rotational motion between sides. Increased humeral retrotorsion would only shift shoulder total rotational motion back into external rotation. It would not create side-to-side differences in total rotational motion. If side-to-side deficits in total arc of motion exist, we should address these deficits.

Source: Keith Allison on FlickrKevin Gausman, diagrams and text added by Brooks Klein, CC BY-SA 2.0

 

Admittedly, the benefits of restoring shoulder ROM have not been tested. While deficits in shoulder ROM have been prospectively linked with injury, the effects of ROM restoration and injury risk are unclear. We may just be finding that deficits in shoulder total rotational motion are a proxy for overuse in throwers. Throwing at high volume, frequency, and/or intensity leads to shoulder ROM loss and injury. The ROM loss may not be the cause of injury but an effect of too much throwing.

Does this mean it is a bad idea to spend valuable treatment time on improving shoulder ROM? No…

As stated before, soft tissue treatments can change shoulder ROM very quickly. I think addressing shoulder ROM changes should still be a component of performance training and rehab for baseball players.

Again, the biggest takeaway is that asymmetry is 100% normal for baseball players. Don’t crank on someone’s shoulder until their internal rotation is the exact same side to side. 

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