As a baseball pitcher transitions from the stride phase to the cocking phase, the torso begins to rotate towards home plate and the throwing shoulder begins to maximally externally rotate. This is one of the most critical points, from a risk of injury standpoint, in the pitching delivery. It has been shown that baseball pitchers routinely externally rotate the shoulder in excess of 165 degrees by the end of the cocking phase (6). To put this in perspective, it is accepted that normal shoulder external rotation in the general population is only 90 degrees. These extreme ranges of motion, when combined with the torques involved in the pitching delivery, challenge the physiologic limits of the surrounding tissues. The thrower’s shoulder must be lax enough to allow for them to achieve these extreme ranges of motion but must also be stable enough to keep the ball of the humerus centered in the socket of the glenohumeral joint. This requires a delicate balance between mobility and stability, and potentially a little help from some structural adaptations that are frequently seen in overhead athletes.
One way to identify some of the physiologic adaptations that baseball pitchers undergo is by simply comparing their dominant shoulder to their nondominant one. Studies have shown that baseball pitchers possess between 7 and 10 degrees more shoulder external rotation in their throwing arm than their nonthrowing arm (3,11-13). This is a positive adaptation that allows them to achieve the layback position of maximal external rotation. In the throwing population, this increase in throwing shoulder external rotation typically comes with a subsequent decrease in internal rotation. Studies have shown that baseball pitchers display about 7 degrees less internal rotation in their throwing arm compared to their nonthrowing arm (3,7,11-13). Historically, there has been a lot of discussion about how this decrease in internal rotation may increase injury risk. This led to rehab programs that emphasized aggressive stretching of the shoulder into internal rotation to try to create better symmetry side-to-side. While well intentioned, it probably wasn’t necessary. Let’s explain.
If you’re doing the math, then it should come as no surprise that the total range of motion (adding internal plus external shoulder rotation) has not been found to be significantly different on one side versus the other in throwers (3,7,11-13). Total range of motion being similar bilaterally points to some structural changes, rather than capsular laxity, that occur over time with repetitive throwing. Humeral retroversion, which refers to the warping of the humerus in the direction of external rotation, is one of those structural changes. In the overhead throwing population, it has been found that the throwing arm has significantly more retroversion than the nonthrowing arm (3,7-9,11). This adaptation of humeral retroversion has been shown to be between 10 and 14 degrees in throwers (8,9,11). Sound familiar? That’s about the same amount of motion that pitchers have been shown to gain in external rotation and lose in internal rotation. In fact, when humeral retroversion is taken into account, it has been shown that there is no appreciable difference between internal and external rotation when comparing shoulders (7,11).
The warping of the humerus often confuses us when attempting to consider shoulder range of motion in overhead athletes. Because the adaptation is occurring at the humerus and not the glenohumeral joint, this means that when the shoulder is resting in a neutral position, the rest of the arm appears to be in a position of external rotation. The throwing shoulder itself, however, is achieving relatively the same degrees of internal and external range of motion as the opposite shoulder. This allows a pitcher to achieve what looks like excessive external rotation without actually putting added stress on the shoulder capsule to get there.
How Humeral Retroversion Develops
Retroversion is actually a very normal finding in the general population. In fact, the average adult will typically possess between 25 and 30 degrees of humeral retroversion by the time they are done growing (5). We aren’t born like this, though. At birth, humeral retroversion is high, typically around 65 degrees in the first four years of life (5). As we begin to grow and develop, however, our degree of retroversion begins to decrease, moving towards a position of neutral rotation. By age 11, humeral retroversion is decreased to about 38 degrees (5). Past the age of 11, this process begins to significantly slow down, but continues to occur, nonetheless. Between the ages of 16 and 19, the process of developing humeral retroversion is mostly complete, with adult values having been reached. *
Derotation of the humerus, though, can be affected by external forces acting around the epiphyseal plate, or growth plate, during skeletal maturation. Overhead throwing is an example of an external force that could have an effect on humeral growth. As we throw, the external rotation forces at the shoulder act in the opposite direction of the derotation occurring at the humerus. What’s interesting is that it appears these forces do not actually increase the degree of humeral retroversion, but rather, they impede the progress of derotation on the throwing arm and result in an asymmetry in retroversion when comparing the overhead thrower’s dominant arm to their nondominant arm (13).
In 2014, Hibberd and colleagues performed a study that illustrated the course of acquiring humeral retroversion asymmetries during the baseball playing career (7). The study examined 287 baseball players between the ages of 6 and 18 that were divided into four different groups based on their age. Prior to the start of the season, range of motion and humeral retroversion measurements were taken and compared between groups. The varsity group (ages 16-18) had a significantly greater degree of humeral retroversion than the youth (ages 6-10) and junior high (ages 11-13) groups when the dominant arm was compared to the nondominant arm. In addition, the junior varsity group (ages 14-16) had a significantly greater degree of humeral retroversion than the youth group, but not the junior high group. This study helped to illustrate the steady progression of acquiring humeral retroversion throughout the beginning parts of a baseball career.
Another interesting finding in this study was that the degree of humeral retroversion was actually greater in the youth group than it was in the varsity group. As the athletes aged, the degree of retroversion continued to decrease, however, the asymmetry between dominant and nondominant arms grew. This speaks to the point made earlier that the forces acting on the shoulder may not increase the degree of retroversion at the humerus, but instead impede the derotation process thus creating a bigger asymmetry between limbs.
It appears that encouraging young, aspiring baseball players to throw early and often, within safe parameters, may actually give them a competitive advantage in their sport. As we have seen, the best time to acquire humeral retroversion in the throwing arm is early in one’s baseball career. Attempting to save a child’s arm until they are physically matured, in high school or later, may actually have some unintended consequences, as changes in humeral retroversion are not expected after skeletal maturity is reached (3,5). Failure to acquire retroversion has the potential to decrease throwing performance and increase the amount of stress experienced at the shoulder, which could predispose the throwing athlete to an increased risk of injury.
*This process is similar, but opposite in direction, to what occurs at our hips. When we are born, our hips rest in a high degree of anteversion, meaning the hip is oriented in the direction of hip internal rotation. As we age, the hip begins to derotate towards a position of neutral rotation.
How Humeral Retroversion Impacts Performance
One benefit to acquiring a greater degree of humeral retroversion in the throwing arm is increased throwing velocity. Greater degrees of humeral retroversion allow for more external rotation of the throwing shoulder before being restricted by the anterior capsule or glenohumeral ligaments (8). This increase in external rotation enables the athlete to extend the late cocking and early acceleration phases of the pitching delivery. This is important because it allows the pitcher to increase the overall throwing arc, which allows for more time to accelerate the arm and generate greater velocities prior to ball release (9).
How Humeral Retroversion Impacts Shoulder Health
Not only can increases in humeral retroversion potentially improve throwing performance, it may also serve as a protective mechanism for the shoulder and its surrounding tissues. One study examining professional baseball players found that lower degrees of humeral retroversion in the throwing shoulder was correlated with more severe injury, defined as greater than 30 days of missed activities (9). In this study, it was found that for every 10 degree increase in humeral retroversion, the risk of injury decreased by 10%. Additionally, smaller side-to-side differences in retroversion were correlated with more severe injury. Admittedly, the sample size for this study was not great enough to draw significance, however, the trends observed are worth noting. Another study, which included 183 professional baseball pitchers, found that pitchers who suffered a shoulder injury displayed 3.5 degrees less humeral retroversion than those who did not suffer an injury (10). Once again, this number was not considered significant but is worth noting.
A third study examined 51 handball players (8). Handball can be described as land-based water polo, where players are running on a basketball-esque court attempting to score by throwing the ball into a goal. Of the 51 players examined, 38 reported no history of shoulder problems. When examined, the asymptomatic group demonstrated an average of 14.4 degrees more humeral retroversion than the nonthrowing arm. This was in stark contrast to the chronic pain group, which actually showed an average of 5.2 degrees less humeral retroversion in the throwing arm compared to the nonthrowing one. That’s a difference of 19.6 degrees!
One logical explanation for why humeral retroversion may be protective at the shoulder is the relative motion that occurs at the shoulder while throwing. Because humeral retroversion is acquired distal to the head of the humerus, there is actually less external rotation of the humeral head relative to the glenoid (shoulder capsule) in the throwers possessing greater degrees of humeral retroversion. This allows for an increase in external rotation without relying on an increase in capsular laxity. The result is a decrease in twisting and shear forces on the long head of the biceps tendons, superior glenoid labrum, and rotator cuff muscles (8-10).
How Humeral Retroversion Impacts Elbow Health
As with most things, more is not always better. While retroversion may be protective at the shoulder, it also has the potential to increase the risk of injury at the elbow. Pitchers with increased humeral retroversion may have a higher incidence of ulnar collateral ligament (UCL) injury (10). One possible explanation for this are the prolonged late cocking and early acceleration phases that occur as a result of increased retroversion. These phases of the pitching delivery have been shown to stress the UCL more than any other phase (10). Increasing the length of these phases places additional stress on the UCL and thus may increase the risk of injury. So, for the same reason that increased humeral retroversion can help to increase fastball velocity, it may also increase the risk of injury at the elbow. This does not necessarily come as a surprise, however, as several studies have shown that increased fastball velocities are correlated with a higher incidence of UCL injury (1,2,4).
In a study published by Noonan and colleagues in 2016, it was found that pitchers with a UCL injury possessed 3.5 degrees more humeral retroversion in the throwing arm and 5.2 degrees less retroversion in the nondominant arm compared to those pitchers without an injury (10). There are a couple conclusions we can draw from this data. First, the UCL injury group had, on average, an 8.4 degree greater humeral retroversion asymmetry in their throwing arm when compared to their nonthrowing arm (23.2 degrees in the injured group versus 14.8 degrees in the uninjured group). Based on this data, it appears that greater side-to-side asymmetries may not always be a good thing. Second, the decreased retroversion on the nonthrowing arm points to a lack of genetically acquired humeral retroversion in the UCL injury group, as the nonthrowing arm is not experiencing the same torques that lead to adaptation in the throwing arm. It appears that possessing more inherited retroversion may be an advantage for the baseball pitcher. Pitchers who possess less genetic retroversion will either fail to develop the retroversion necessary to protect the throwing shoulder or they will develop the necessary retroversion for throwing and be more susceptible to a UCL injury (10).
Summary
The pitching delivery requires extreme ranges of motion to successfully achieve high velocities. In order to achieve these extreme ranges of motion, the baseball pitcher develops structural adaptations to safely achieve these ranges. One of those adaptations is humeral retroversion. Acquiring humeral retroversion can help to increase pitching performance while protecting the shoulder joint in the process. Unfortunately, it also appears that excessive humeral retroversion may increase the risk of injury at the elbow. Like most things, it appears that there may be a range of humeral retroversion that is protective of both the shoulder and the elbow. One study suggests that this sweet spot lies between 30 and 45 degrees (9). The baseball pitcher, however, must acquire these adaptations early in their career as these adaptations must occur prior to the completion of growth. For this reason, it is recommended that young baseball players throw early and often, within safe parameters, in order to give them a competitive advantage in their sport while mitigating the risk of injury.
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