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SLAP Injuries in the Baseball Player: Classification and Subjective Evaluation

A superior glenoid labrum injury is a common reason for shoulder pain in baseball players.

These injuries were initially explained by Dr. James Andrews in 1985 secondary to a biceps tendon origin pathology. The injury is described in that the biceps tendon exerted a traction-like force to actually “peel” the labrum off the glenoid structure, itself.

These superior labral tears, anterior to posterior (SLAP), have several different types can be stable or unstable that are under the umbrella that was previously described that result in the “peel back” etiology.

One can also back the argument that due to the high loads of eccentric contraction that occur in the late cocking phase of throwing most baseball players (pitchers in particular) have some level of a SLAP tear or superior labrum lesion that is, for the most part, asymptomatic in nature. Some SLAP tears may even be considered necessary adaptations to the throwing shoulder for baseball players.

Taking a brief look at the pathomechanics of the throwing motion and the underlying structures, one can quickly understand why the anterior portion of the labrum is under immense stress.

During throwing, particularly in late cocking phase at maximal external rotation, the humeral head will shift from a posterior to inferior direction. However, in these types of overhead athletes who likely already have anterior capsule laxity, the shoulder will have exhibit a greater willingness to shift the humeral head in a posterosuperior direction.

This directional-preference the shoulder takes subjects the superior labrum to greater load, particularly in a torsion-manner.

There’s several different types of mechanisms that can be explained by many different types of classification systems. For now, let’s take a look at the different mechanisms as described by Snyder for these types of injuries:

 

Classification of SLAP Tears

The original classification was proposed by Snyder in 1990, where he bunched SLAP tears into four primary mechanisms:

Type 1: These types of lesions are best characterized by degenerative fraying of the superior labrum with not only an intact, but stable biceps tendon. Snyder noted that this type of variant classification was present approximately 11% of the time. These types of lesions are most consistent with a rotator cuff pathology.

Type 2: These types of lesions include a detachment of the biceps anchor from the glenoid that are unstable in nature and are considered the most common variant encountered. Snyder stated that these represent 41% of all SLAP injuries.

Type 3: These types of lesions include a “buckethandle”-like tear of the superior labrum without disruption of the biceps tendon anchor. However, the bucket-handle can become mobile and become entrapped within the glenohumeral (shoulder) joint, creating a very painful clinical picture. This type of lesion is consistent with traumatic instability, in addition to a Type 4 tear.

Type 4: These types of lesions also include the “bucket handle”-like tear of the superior labrum. However, this type of tear also includes an unstable biceps tendon. It should be noted that the level of biceps involvement in this type of lesion can be extremely variable. This type of lesion is the least common variation of a SLAP tear.

 

Subjective Examination Pearls

Examination of an athlete with a SLAP tear that is pathological in nature can be very challenging. It’s likely that a patient who has sustained a SLAP lesion has another present injury. This may include a concomitant rotator cuff injury or even a Bankart lesion of the anterior labrum.

A great subjective examination is key to truly developing a great conclusion of what the patient has going on.

However, a subjective examination can become very difficult as the mechanism is often different in players. It may include a single traumatic event or can be gradual over time as part of an insidious etiology.

Amongst all athletes, the most common clinical symptom that will be apparent is anterior shoulder pain. These can lead to performance issues such as decreased control while throwing or even a decrease in throwing velocity.

The patient may also note an increase in clicking or other types of mechanical symptoms.

While certain types of labral injuries can involve concomitant instability, most SLAP tears in isolation are not unstable in nature.

Altogether, while the subjective examination can be challenging, it’s ultimately imperative to understand that these types of injuries are commonplace in baseball players. In many players, SLAP tears are asymptomatic and not a concern.

The Other Type of Shoulder Instability: Batter’s Shoulder

Several baseball injuries center around the inherent instability that the shoulder, or glenohumeral joint, provides.

The classic description of the golf ball (the humerus) sitting on the golf tee (the glenoid) is really a great description of how unstable the shoulder really is.

As described often on previous posts, throwing a baseball really isn’t good for you. This is evidenced by having over 7000 degrees of rotational motion in one’s shoulder when throwing.

This violent movement over the course of a season or career can begin to develop stress in the anterior compartment of the shoulder.

Whether this causes an injury as severe as a SLAP (superior labrum anterior to posterior) tear over time is up for debate, but it’s clear that this type of instability is the most common form in overhead throwers, such as baseball players.

When we discuss shoulder instability, we often think of this certain type. Typically, the symptoms include anterior shoulder pain, perhaps from an acute dislocation, subluxation, or a general degeneration of the anterior capsule over the course of thousands of throws.

 

Introduction: Posterior Instability in Baseball

However, in baseball players, another type of shoulder instability is unique. This phenomenon is referred to as “batter’s shoulder.”

The injury, similar to anterior instability in pitchers, occurs mostly in hitters in their front shoulder from repetitive posterior capsule stresses incurred from swinging a bat. This stress is highlighted more so by instances of swinging and missing as well as swinging at outside pitches, where the front shoulder is subject to a greater moment of adduction.

This type of posterior instability is also seen in another type of unique population: football interior lineman, whose arms are always in a position of shoulder flexion with combined adduction holding onto the opponent’s shoulder pads. Through the continued line of force produced axially through their arms, posterior shoulder problems can arise.

For the purpose of this topic, we will focus on the baseball population. This type of injury is not nearly as common as anterior shoulder instability from throwing. Not often do we see player’s heading to the injured list for “batter’s shoulder” or “posterior shoulder instability,” yet it’s important from a clinician or rehab professional’s perspective to understand the inherent risks that our athletes face.

 

Phases of Batting

Just like throwing, hitting has phases, too. Shaffer and colleagues have stated that the traditional baseball swing goes through four distinct phases. These include the wind up, pre-swing, swing, and follow-through.

While throwing a baseball may be the fastest movement in sports, hitting a baseball can generate high shoulder movement speeds too. These same authors have stated that rotational velocity at the shoulder is 937 degrees per second–still a violent, powerful movement.

 

How Does Batter’s Shoulder Work?

Once we take this great amount of force produced at the shoulder and combine that with the mass of a baseball bat, albeit a small weight (typically around 2 pounds), we can begin to see how the repetitive amount of swings can impart stress onto the shoulder.

For another way to look at the effect of hitting, think about it like this. At the higher levels of competition, hitters swing the bat hundreds of times in a 5-day span, between game swings as well as the batting practice, etc., we can compare that to the amount of throws that pitchers throw over the same 5-day stretch.

In addition, it should be recognized that pitchers throw with a 5 oz. ball and hitters must use at least a 31 oz. bat in most cases. However, while a hitter uses an object that is 6x heavier than a baseball, the rotational velocity at the shoulder during pitching is approximately 7x that of swinging a bat.

It’s also important to note that anterior shoulder instability often occurs in the throwing arm of a baseball player. Posterior instability usually occurs in the front, non-throwing shoulder, unless the player hits on the opposite side of the plate, i.e., a right-handed thrower who hits from the left side.

This type of usage and repetitive force on the front shoulder that is not used for throwing will be critical when discussing the treatment options.

 

The 500-N Elephant in the Batter’s Box

As discussed earlier, swinging and missing on an outside pitch is often considered the main mechanism in a baseball setting for developing posterior instability. Kang and colleagues have determined that this type of circumstance can generate as much as 500 Newtons of “dynamic posterior pulling force” during a swing.

While 500 N may not sound like anything significant, this amount of force represents greater than about 110 pounds of force being pulled posteriorly for your shoulder to absorb. Imagine this amount of force produced over the course of a season or even a career.

While the weight of the bat, in addition to the rotational velocity and force at the posterior shoulder is important, the angle of the shoulder plays a great role in this equation.

As the shoulder is in increased horizontal abduction from a position of approximately 90 degrees of shoulder flexion, the shoulder is going to be in more a congruent, stable position with the glenoid, which is angled at about 30 degrees away from the body.

Once the shoulder moves into greater horizontal adduction (arm crossing the body) of approximately 105 degrees, as it would be the case in swinging for an outside pitch, it’s clear that the posterior shoulder would sustain more impact in this position.

The American Sports Medicine Institute group (ASMI) emphasized this arm angle with injury, stating that it may increase shear forces across the shoulder joint. This evidence truly underscores the stress that the shoulder can incur.

Swinging and missing can also develop more damage to the posterior capsule of the shoulder due to the shoulder muscle’s inability to contract as it normally would with making contact with the baseball.

 

So You Have This “Batter’s Shoulder,” Huh?

The typical presentation for batter’s shoulder often involves a feeling of instability, especially after reaching for an outside pitch or simply swinging and missing.

After these moments of instability, it’s also common for athletes to feel discomfort or pain with provocative positions, such as forward shoulder flexion, adduction across the body, and internal rotation, as they all function to stress the posterior shoulder capsule.

Often, during a clinical examination, the jerk and Kim tests are very good to stress the shoulder. Tannenbaum and his colleagues have described both these special tests and Kim et. al determined that the combination of both these tests have a 97% sensitivity for posteroinferior labral lesion.

 

The normal course of action for this type of instability from a labral deficit often begins with conservative therapy. Kang described that interventions will focus on rotator cuff strengthening, scapular stabilizing, and improving generalized mobility.

In the event the patient does not improve with conservative treatment, surgery is indicated to arthroscopically repair the posterior labrum.

From here, the patient will undergo a period of protected motion, motion recovery, strengthening, plyometric training, and sport-specific training. Kang reports that patients can often progress to taking swings from live pitches at 6 months after surgery following an appropriate return to hitting program.

 

Let’s Step Out of the Batter’s Box

Kang reports that the average return to sport after surgery is approximately 6.5 months with very often, excellent outcomes, without any common significant complications.

Altogether, batter’s shoulder is very rare in the realm of musculoskeletal conditions, yet a common mechanism in a baseball population.

As a health care provider, it’s important to recognize posterior shoulder pathology and the mechanism in which it can occur. This can often occur by completing a thorough subjective history and being able to replicate the instability-provoking movements.

In-Season Training Metrics for the Baseball Player

The high school season is closing in! This was truly a remarkable off-season for me personally. We’ve had multiple athletes put on 15-20 pounds while simultaneously moving better and getting a lot stronger, and we’ve had athletes lose a few pounds while accomplishing the same things.

We said “see ya later” to our professional pitchers Joe Palumbo (Texas Rangers), Anthony Kay (New York Mets), Ben Brown (Philadelphia Phillies), Mike O’Reilly (St. Louis Cardinals), Kyle McGowin (Washington Nationals), and Bruce Kern (Lamingo Monkeys, Taiwan) and we are super excited to see the stellar season they are all going to have!

We also said “hello” to multiple high school athletes who are getting their names on the radar, and have some young men who might hear their name called in June for the 2019 MLB Amateur Draft.

With that being said, none of this will matter if training is put to a halt once March hits.

Fortunately, the official high school season start date was pushed back 2 weeks to accommodate for the brutal northeast weather. This means 2 more weeks of pre-season development!

The entire focus of in-season training is completely different from off-season and pre-season training, and I would like to highlight those differences in this post.

 

Injury Prevention

The baseball player’s workload volume increases a TON once practice starts. Throwing and swinging 5/6 times a week coupled with cold weather really do not mix well from an injury prevention standpoint.

Overuse injuries are real, and they could be hiding beneath the surface. One aspect of in-season training is maintenance of joint health. Here’s what gets abused repeatedly in the game of baseball:

  • The left ankle gets over worked when performing multiple base running drills, leaving the other muscles on the same side to become overworked, and leaving the other side of the body exposed
  • The pelvis tends to get dumped into an anterior tilt during the swing as well as an overactive quadratus lumborum from lateral tilt, which can lead to low back pain and stiffness during rotation
  • Hamstrings get tired and over worked from changing surfaces of sprinting: turf in the off-season to hard dirt with cleats during practice
  • The posterior shoulder takes a beating from both throwing and swinging a bat
  • The medial elbow also takes a beating from both throwing and swinging a bat
  • The thoracic spine (the mid back) gets out of whack and shows muscle imbalances from swinging, which can also lead to altered muscle firing patterns
  • The cervical spine (the neck) gets over worked from looking over one shoulder for multiple at bats

 

To prevent any injury from occurring, we must do 4 things religiously well:

  1. Mobilize/desensitize the overactive joints, which will be very common among most of the population
  2. Activate the problem areas associated with baseball movement patterns
  3. Learn how to reset the body and find peace of mind with relaxation techniques/breathing drills
  4. continue to get stronger, develop, and preserve your power stores

 

There is some great research from the American Sports Medicine Institute discussing movement of the shoulder and elbow and the risk of injury. See here and here.

  • Pitchers should have within 5 degrees of total rotational motionof both shoulders. If your throwing shoulder is significantly less than your other shoulder, you are 2.6 times more likely to sustain an injury.
  • Pitchers should have at least 5 degrees more external rotation in their throwing shoulder when compared to their nonthrowing shoulder. Pitchers with less than a 5 degree difference are 2 times more likely to sustain an injury.
  • Pitchers with a deficit of 5 degrees in shoulder flexion are 2.8 times more likely to sustain an elbow injury.

 

Performance Enhancement

“If you don’t use it, you lose it”. 

This is the biggest concept to understand when training in-season. You spent all winter developing your strength and power to get ready for the long season.

For every week taken off from training, will be lost, dependent on training age. For the entire high school season being at least 12 weeks, that is at least 36% of your strength gains out the window.

You could train ONCE a week during the season and further maintain and improve your strength numbers to keep your strength around. Easy fix.

However, speed on the other hand tends to be lost a lot quicker. You will be displaying your speed multiple times throughout the week from base running and getting to balls over your head, but if you’re not training your speed at some point then you are limiting yourself as a baseball player.

On the other hand, let’s just think about the aggressive action of the upper body during the throwing motion. Repeated eccentric stress will result in a loss of strength and mobility in some joints. To prevent this loss from being chronic, you need to strengthen these areas and try to get as close to your “baseline” as possible.

I’m a big believer in auto-regulation ever since I got my hands on the research in college. I saw first-hand a group of trainees get stronger and put on even MORE muscle by following an auto-regulated training model as compared to a fixed periodization scheme.

To put it in the simplest terms, auto-regulation can be thought as listening to your body. You are the only one who knows your body better than anyone else. Rather than chasing the numbers on the bar, or the percentages of your 1-RM, try chasing your rate of perceived exertion (RPE) during the season.

Not only will you further enhance your strength and power, but you will also reduce the likelihood of being fatigued from your lift. Get in and get out!

 

Recovery Management

At Infiniti Sports Performance, we like to use restorative techniques and modalities in our popular recovery room during the season, taking “arm care” to a whole other level. These techniques include, but are not limited to:

  • Myofascial massaging
  • Foam rolling and lacrosse ball trigger point release
  • Cupping
  • Voodoo wrapping
  • Vibration massage
  • Marc Pro systems
  • Mobility training

 

There are a ton of modalities that we can use to promote recovery for the baseball player, and we usually recommend a recovery session within 48 hours of your last pitch.

While the body should be doing most of the work itself when it comes to recovery, we assist in the process with the techniques and tools listed above.

However, since there are many ways to instill “recovery” work with the baseball player, we do not want them to get married to these concepts. Here is why.

Our body craves adaptation, and it also can get habituated to the same stimulus. Just like how we periodize and vary our strength programs, the same concept goes for recovery work.

 

Conclusion

Every baseball player needs to take advantage of in-season training. To sum it up, here is why:

  1. Overuse injuries are real, and they could be hiding beneath the surface. One aspect of in-season training is maintenance of joint health
  2. Learn how to reset your body in conjunction with getting stronger and more resilient
  3. Loss of range of motion in the throwing shoulder can lead to an increased injury risk of up to 2.8 times greater than pitchers without motion loss.
  4. For every week taken off from training, usually 3-5% of your strength gains will be lost, dependent on training age.
  5. Following an auto-regulated training model allows the player to still develop while minimizing fatigue

Regaining Shoulder Range of Motion in Baseball Players

Regaining motion following heavy doses of high intensity throwing cannot be overlooked.

Throwing causes significant range of motion changes from eccentric stress. Any movement activity causing significant muscle damage can alter movement in the days following.

Pitchers throwing a large amount of pitches suffer significant eccentric stress, not only in the arm/shoulder but throughout the entire body. Joint distraction forces at the shoulder and elbow rapidly increase to 1–1.5 times the body mass to decelerate the arm.

Mike Reinold, in a 2008 study, showed significant decreases in shoulder internal rotation, total motion, and elbow extension following pitching in the dominant shoulder.  These changes were noted immediately post pitching and continued to exist 24 hours after.  These motion losses begin acutely but can progress into long term alterations.

Many of us in the baseball field have long known about glenohumeral internal rotation deficit (GIRD) when it comes to the throwing shoulder, but several recent studies are linking injuries to other movement loss within the glenohumeral joint.

Over 6 years, Dr. Chris Camp and others studied range of motion on all invited pitchers from one professional organization, totaling 81 pitchers followed and accounting for 132 pitcher seasons.

The authors looked at shoulder flexion, horizontal adduction, external rotation, internal rotation, as well as elbow flexion and extension in both the dominant and non-dominant arm during the pre-season.

Out of the 53 injuries they encountered over the 6-year period, the most significant risk factors for elbow injury were loss of shoulder external rotation and loss of shoulder flexion. Injury risk at the elbow went up 7% for each degree of external rotation loss and 9% for each degree of flexion loss.

None of the range of motion measures correlated with shoulder injuries. The study shows that altered kinematics at the shoulder have a significant effect down the kinetic chain at the elbow.

In the Camp study, the presence of an internal rotation deficit did not correlate to any increased risk of shoulder or elbow injuries.

However, the predominant overall theme remains in that pitching causes significant stresses to the arm and body, and that stress can cause significant motion losses. Motion losses are regularly tied into shoulder and arm injuries.

Regaining full overhead capacity can be much more than simply soft tissue restrictions but an easy place to start is by mobilizing the tissue around the scapula and shoulder. Gaining full overhead motion can be a combination of many factors, and it often starts at form and function when it comes to global movement.

Body and anatomical positions determine form and form dictates function.  The pelvis stuck in a poor position affects the lumbar spine which affects the thoracic spine orientation which 100% affects the scapular position which affects the shoulder.  The following list to regaining overhead motion is a good place to start but is not an exhaustive list by any means.

 

Positions – Form and Function

1. Pelvic control

Pelvic control in my opinion is the foundation of all patterns within the body.  The inability to control the pelvis stems from poor glute and abdominal motor control.

Compensation patterns result from this lack of control in the form of extended spinal positions, especially at the lumbar spine.  An over extended lumbar spine creates the rib-flair up the kinetic chain, and the rib-flair allows for false motion with overhead athletes.

Developing the ability to control the pelvis should be a priority for efficient overhead motion.  Pelvic stability should revolve around the rectus abdominus, obliques, and glute complex.  Teach and train a neutral pelvis along with anti-extension throughout the torso.

 

2. Thoracic Extension

The ability of the t-spine to extend assists in humeral flexion through efficient scapular mechanics.

A poor position places the scapula at a disadvantage.

A forward head posture with excessive rounding of the upper back drives the scapula forward into anterior tilt.

An athlete unable to extend through the t-spine will be unable to effectively upwardly rotate, and posteriorly tilt the scapula resulting in less overhead ability.

A study by Strunce in 2009 showed large increases in shoulder range of motion following thoracic spine and rib manipulations.

Although not on an athletic population, the results demonstrated the immediate benefits of positioning when it comes to the thoracic cage.

Active range of motion at the shoulder improved in flexion by 38° and improved total rotation by 30° after manual manipulation.

Results from another study by Edmonston in 2012 confirmed that extension of the thoracic spine does occur during arm flexion. The ranges of thoracic spine extension with humeral flexion was an average of 12.8 degrees, showing the importance of the kinetic chain in overhead motion.

 

3. Scapular Positioning

For the shoulder to function correctly, it relies on assistance from the scapula to get overhead.  Upward rotation and posterior tilt are significant factors in flexion.

For the arm to get a full 180 degrees of motion, the scapula must contribute 60 degrees while the glenohumeral joint handles the other 120 degrees. Without that 60 degrees, motion is compromised.

Focusing on the lower trap and serratus anterior are a good place to start as both are responsible for upward rotation, while the lower trap is key in posterior tilt.  Both are the most commonly inhibited muscles in overhead athletes due to posture, lifestyle, training methods, etc.

 

4. Soft Tissue Restrictions

Obviously, the muscles attaching to the scapula and humerus play huge roles in not only creating motion but restricting motion as well. Overactive or tight tissues limit motion.

Some muscles that have great influence on scapular positioning and subsequently on motion include the latissimus dorsi, pec major and minor, rhomboids, and levator scapula (to name a few).

Manipulating these common problem areas can go to great lengths in restoring fluid movement and positioning. However, muscles aren’t the only factor.

Capsular restrictions around the glenohumeral joint itself can factor into the equation as well.

 

Soft Tissue for Scapular Upward Rotation

 

T-Spine Extension for Overhead Athletes

 

The “overhead” in overhead athletes tells us all we need to know. The overhead position matters to throwers and being able to get there the right way is key. Don’t let flexion disappear or become a compensation pattern over the course of a long season. Keep moving efficiently for long term health when it comes to the shoulder and elbow.

 

Sources:

Camp, Christopher L., et al. “Decreased Shoulder External Rotation and Flexion Are Greater Predictors of Injury than Internal Rotation Deficits: Analysis of 132 Pitcher-Seasons in Professional Baseball.” Orthopaedic Journal of Sports Medicine, vol. 5, no. 7_suppl6, 2017, doi:10.1177/2325967117s00221.

Edmondston, Stephen, et al. “Clinical and Radiological Investigation of Thoracic Spine Extension Motion During Bilateral Arm Elevation.” Journal of Orthopaedic & Sports Physical Therapy, vol. 42, no. 10, 2012, pp. 861–869., doi:10.2519/jospt.2012.4164.

Reinold, Michael M., et al. “Changes in Shoulder and Elbow Passive Range of Motion after Pitching in Professional Baseball Players.” The American Journal of Sports Medicine, vol. 36, no. 3, 2008, pp. 523–527., doi:10.1177/0363546507308935.

Strunce, Joseph B., et al. “The Immediate Effects of Thoracic Spine and Rib Manipulation on Subjects with Primary Complaints of Shoulder Pain.” Journal of Manual & Manipulative Therapy, vol. 17, no. 4, 2009, pp. 230–236., doi:10.1179/106698109791352102.

 

Humeral Retroversion Occurs in Youth Baseball Players as Young as Fourth Grade

A recent research report out of Japan continues to add to the body of evidence regarding humeral retroversion in the dominant shoulder in baseball players.  Essentially, the upper arm in baseball players has boney adaptations that occur while throwing as a youth with open growth plates.  This allows for external rotation (layback) while throwing, and is also responsible for the shift in total rotational range of motion and subsequent loss of internal rotation.

While this has been known for some time, we continue to see that this happens at an early age.  The authors of this study note that this occurred in youth athletes as young as in the 4th grade.

This boney adaptation is a requirement for throwing later in lift, showing the importance of needing to throw as a youth as young fourth grade.

Differences in humeral retroversion in dominant and nondominant sides of young baseball players.

J Shoulder Elbow Surg. 2017 Jan 25. pii: S1058-2746(16)30624-3. doi: 10.1016/j.jse.2016.11.051.

Kurokawa D1, Yamamoto N2, Ishikawa H3, Nagamoto H4, Takahashi H5, Muraki T4, Tanaka M6, Sato K6, Itoi E7.

Author information

  • 1Department of Sports Medicine and Orthopaedic Surgery, Tohoku Rosai Hospital, Sendai, Japan; Department of Orthopaedic Surgery, Tohoku University School of Medicine, Sendai, Japan.
  • 2Department of Orthopaedic Surgery, Tohoku University School of Medicine, Sendai, Japan.
  • 3Department of Physical Medicine and Rehabilitation, Tohoku University School of Medicine, Sendai, Japan.
  • 4Department of Orthopaedic Surgery, Kurihara Central Hospital, Kurihara, Miyagi, Japan.
  • 5Department of Orthopaedic Surgery, Kesen-numa City Hospital, Kesen-numa, Miyagi, Japan.
  • 6Department of Sports Medicine and Orthopaedic Surgery, Tohoku Rosai Hospital, Sendai, Japan.
  • 7Department of Orthopaedic Surgery, Tohoku University School of Medicine, Sendai, Japan. Electronic address: [email protected]

Abstract

BACKGROUND:

The relationship between the disabled throwing shoulder and humeral retroversion has recently attracted a great deal of attention. However, none of the previous studies clarified when the side-to-side difference of humeral retroversion in young baseball players would start. This study aimed to clarify when the difference of humeral retroversion in the dominant and nondominant sides appeared in baseball players.

METHODS:

The bicipital-forearm angle in bilateral shoulders of 172 elementary school baseball players was measured by ultrasound. The bicipital-forearm angle was defined as an angle between the perpendicular line to the bicipital groove and the ulnar long axis with the elbow flexed at 90°. The correlation between the bicipital-forearm angle and the grade and the difference of the bicipital-forearm angle between the dominant and nondominant sides were analyzed.

RESULTS:

In the nondominant shoulders, the bicipital-forearm angle increased with the grade in school (r = 0.32, P < .0001), but this was not observed in the dominant shoulders. In the fourth to sixth graders, the bicipital-forearm angles were significantly smaller in the dominant shoulders than in the nondominant shoulders.

CONCLUSION:

Our findings indicated that humeral retroversion decreased with age in the nonthrowing side but not in the throwing side and that the side-to-side difference of humeral retroversion in the baseball players became obvious from the fourth grade. We assume that the repetitive throwing motion restricts the physiologic humeral derotation process and the difference became apparent from the fourth grade when the growth spurt begins in boys.