The Shoulder, Part III: Internal Impingement

Table of Contents

    In part 2 of our shoulder series we discussed the topic of “shoulder impingement”, which is commonly understood as the mechanical compression of the tissues beneath the acromion process, resulting in pain or dysfunction — better known as external impingement. We described the biomechanical theory behind this diagnosis, and examined the available research evidence on the topic. Ultimately, we found a lack of compelling evidence — and in fact, a substantial amount of contradictory evidence — for our historical mechanically-focused understanding of this topic.

    Another lesser-known type of impingement, known as “internal” impingement, is diagnosed in overhead and throwing sports like baseball, racket sports, volleyball, water polo, etc. In a similar fashion to external impingement, this is another situation that has been historically pathologized — in other words, labeled as a problem that needs to be fixed / treated. But is this really the case? Or, as we have seen in so many other contexts, could it represent normal activity-related adaptation that gets blamed for pain and dysfunction in the athlete? Let’s examine this topic.Imagine the late-cocking phase of pitching, with the arm abducted and externally rotated (image 3 above):

    In this phase of the throwing movement, the greater tuberosity of the humerus is thought to come into contact with the posterior-superior glenoid rim.1 The assumed purpose of this mechanical phenomenon is to limit excessive external rotation of the shoulder. Some argue that repeated bouts of overhead throwing movements can lead to “internal impingement” of the labrum and rotator cuff, between the humeral greater tuberosity and glenoid rim (see image) 1


    It is hypothesized that as a result of this “impingement,” these athletes develop symptoms such as shoulder pain, dysfunction, and decreased performance. Spiegl describes the clinical presentation as “nonspecific and unyielding … Most athletes will present with chronic, diffuse, posterior shoulder pain that is exacerbated by activities requiring abduction and external rotation.”1

    The larger issue here is that shoulder symptoms are complex and multifactorial; but as is often the case, the narrative surrounding symptomatic internal impingement often becomes purely mechanical in nature. We illustrated the associated problems and consequences of delivering such narratives to patients in part 2 of this article series, where patient understanding of their condition influenced willingness to engage in continued exercise or physical therapy.

    The clinical examination typically involves evaluation of internal and external rotation ranges of motion, as well as orthopedic tests like the “posterior impingement sign”. If the exam reveals an apparent range of motion deficit with a positive posterior impingement test (i.e., posterior shoulder pain while placing the shoulder in late-cocking phase position), then radiologic imaging is typically obtained to evaluate for pathology.

    As usual in musculoskeletal practice, symptoms are being assigned to a specific structure/tissue without recognizing the complex, multifactorial aspect of the patient presentation. Additionally, we can’t ignore the possibility that what is seen on imaging may represent normative sport-specific adaptations (more on this later) or common age-related incidental findings that are being inappropriately labeled for symptoms.

    Many clinicians and coaches argue for pre-participation screening and assessment of the shoulder of overhead athletes to prevent such “injuries”, despite a 2018 review by Asker et al finding little support for this approach. The authors found limited evidence on any currently researched risk factor for shoulder injury, and even less evidence on preventing injuries in this cohort.2 Their conclusions:

    “All investigated potential risk factors for shoulder injury in overhead sports had limited evidence, and most were non-modifiable (eg, sex). There is also limited evidence for the effect of shoulder injury prevention measures in overhead sports.”

    The usual mechanical culprits attributed to this symptom presentation in overhead athletes include the following:

    • Bennett’s Lesion (posteroinferior glenoid exostosis)
    • Anterior glenohumeral laxity
    • Glenohumeral Internal Rotation Deficit (GIRD)
    • SLAP (superior labrum anterior to posterior) tears due to “peel-back mechanism” leading to “pseudolaxity”
    • Scapular malposition resulting in “SICK scapula syndrome” (prominent inferomedial border of scapula, coracoid tenderness, and scapular dyskinesis, which is considered an overuse syndrome that leads to increased risk for SLAP tears and internal impingement. See our previous blog on scapular dyskinesis.)
    • Humeral and glenoid retroversion (“posterior shift” of structures)

    This list continues to grow as we approach a complex pain-based issue through a structural biomedical lens. Overall, there remains a lack of agreement on pathoanatomical (abnormal structure) or kinesiopathologic (abnormal movement) shoulder issues relevant to internal impingement. Spiegel et al state:

    “Owing to the large body of evidence suggesting various factors that may be involved with the development of symptomatic internal impingement and its corresponding pathologic lesions, it is most likely a complex, multifactorial process that has yet to be completely elucidated. Further research is needed to more clearly demonstrate the association between the various pathophysiological adaptations in the throwing shoulder with the pathologic lesions seen in patients with symptomatic internal impingement.”

    Labeling tissue changes as pathological in these scenarios is a typical oversimplification of a complex, multifactorial issue. For example, Pennock et al’s study Shoulder MRI Abnormalities in Asymptomatic Little League Baseball Players recruited 23 asymptomatic male little league baseball players (average age 11.4 years) and assessed range of motion, strength, and stability of their arms.3 Following physical examination, subjects underwent MRI and ultrasonography of both shoulders. Participants with a prior history of baseball-related shoulder pain/injury were then compared to participants without such issues. Further comparison was performed between players with dominant-arm MRI abnormalities versus those without.

    Physical examination revealed asymmetrical shoulder range of motion between dominant and non-dominant arms (See Table 2). Specifically, internal rotation was decreased by 7 degrees in the dominant arm and up to 10 degrees in 26% of the included players. 5 degrees of retrotorsion of the dominant arm vs nondominant arm was also identified.

    The MRIs revealed 17 asymmetric “abnormalities” in dominant shoulders of 12 players (52%). 4 out of the 12 players had multiple abnormalities. The non-dominant arm MRIs had 2 abnormalities (labral tears).

    Overall, there was a 6-fold higher rate of “abnormalities” in the throwing arm of the athletes. The most common abnormalities included:

      • Edema/widening of proximal humeral physis (5 players)
      • Labral tear (4 players)
      • Partial rotator thickness tear (4 players)
      • Hypertrophy of acromioclavicular joint capsule (2 players)
      • Subacromial bursitis (1 player)
      • Cystic change of greater tuberosity (1 player)

    Of note: subjects who were 1) year-round players, OR 2) single-sport athletes were more likely to have MRI abnormalities. Players with one of these two risk factors had a 71% chance of MRI abnormality, and players withboth risk factors had a 100% chance of an MRI abnormality.

    The argument could be made that these sorts of imaging findings increase the risk of developing symptoms in the future. But so far, that doesn’t appear to be the case for most shoulder imaging findings either. Tran et al’s 2018 paper What Imaging-Detected Pathologies Are Associated With Shoulder Symptoms and Their Persistence? A Systematic Literature Review provides some support.4 The authors compiled 56 studies and assessed the evidence linking imaging findings to symptoms and examined if any evidence supports development of symptoms.

    From the cross-sectional data:

    Structure Finding
    Rotator Cuff Tears 1 high quality study demonstrated no association with pain or function. The remaining studies on rotator cuff tears were of, “mixed quality, unadjusted, and reported conflicting findings.”4
    Tendinopathy The evidence is mixed and demonstrated conflicting findings regarding the relationship between tendinopathy and pain/dysfunction. Meaning, we can’t draw conclusions from the available data.
    Subacromial Bursitis No relationship found with shoulder symptoms.
    Osteoarthritis No relationship found between presence of radiographic osteoarthritis in the acromioclavicular joint or glenohumeral joint and shoulder pain symptoms.
    Calcification No relationship found with shoulder symptoms.
    Acromion pathology Mixed findings regarding the acromial index (“lateral extension of the acromion relative to humeral head”) and Acromial Humeral Distance (AHD). However, scapuloacromial angle and acromion shaped demonstrated no relationship with symptoms. In other words, the acromion shape as hooked, straight, or curved is not well supported as it relates to symptom presentation.
    Adhesive Capsulitis

    Imaging findings such as capsular enhancement did correlate with symptoms.


    There was insufficient longitudinal data examining association of imaging findings and symptom persistence/progression over time to draw strong conclusions for any of these supposed pathologies.

    The authors go on to state:

    “Structure-pain relationships are complex. There is the possibility that there may be no relationship between imaging findings and symptoms, and imaging findings need to be considered as part of a wider pain construct. Other factors that may be associated with musculoskeletal symptoms include age, sex, body mass index, activity, mental health, and central sensitization. Only 6 studies adjusted for age and sex when evaluating the relationship between shoulder pain and imaging, and none adjusted for psychological factors. Other adjustments included occupation, arm dominance, and comorbidity.”4

    They conclude:

    “This review found that increasing [rotator cuff] tear and certain imaging features found in adhesive capsulitis may be associated with symptoms. Further high-quality, adjusted prospective studies evaluating the role of multiple imaging pathologies and other extrinsic factors are required to understand the role of imaging in shoulder pain care pathways.” 4

    So 56 studies later, we haven’t learned much other than shoulder imaging doesn’t appear to tell us much about symptoms, prognosis, or treatment. And observing the trends in clinical practice, it seems to create more unnecessary problems than it solves.

    Going back to the above-cited Pennock study, the argument can also be made that the reported imaging findings are actually sport-specific adaptations that we “label” as pathologic using biomedical terminology on radiology reports. Since we can observe these findings as early on as in Little League players, as well as later in baseball careers (asymptomatic MLB draft picks), these may very well represent adaptive changes for the demands placed upon these shoulders.5 Further, baseball isn’t the only overhead sport where we find asymptomatic alterations on shoulder imaging, either. We have similar studies for:

    Elite level volleyball players

    Elite level tennis players

    Division IA collegiate volleyball players, swimmers (n=6), and gymnasts6,7,8

    As well as a 5 year follow-up on shoulder imaging in asymptomatic overhead athletes. The authors’ findings 5 years after imaging:

    “None of the athletes interviewed 5 years later had any subjective symptoms or had required any evaluation or treatment for shoulder-related problems during the study period.” 9

    Is there a “natural history” effect to these MRI findings? Schar et al. set out to answer this question: Many Shoulder MRI Findings in Elite Professional Throwing Athletes Resolve After Retirement: A Clinical and Radiographic Study.10

    Overall, it appears to be a mixed bag. The authors completed MRIs on two separate groups.

    The first group received initial MRI scans of their throwing and non-throwing shoulders, and these were repeated six years after their retirement from sport. A second group who were not initially scanned were recruited and underwent bilateral shoulder MRI scans 15 years after their retirement from sport. MRI Findings:

    The red boxes signify findings that worsened in group 1 from initial scan to the scan 6 years later. The most interesting aspect of these findings is that issues still appeared to worsen in the non-dominant shoulders, and this may be further evidence of normative age-related changes. Granted, there are several issues that didn’t worsen — or even improved — on MRI imaging. Whether some of these findings have a natural history is difficult to state based on these data alone. Even more intriguing is that range of motion and pain didn’t change in a clinically meaningful way.

    Where does this leave us?

    Hopefully with a better understanding for the need to deemphasize rigidly biomedical narratives that are overly focused on structural and/or mechanical “abnormalities”. We have little supportive data on a pathoanatomical or kinesiological reason for symptomatic internal impingement. However, we have good quality research evidence on efficacious ways to mitigate the risk for development of symptoms in the overhead athlete.

    We should likely begin by mitigating early sport specialization (recall the data on year-round, single-sport players cited above), given that symptomatic internal impingement appears to be an overuse “injury” to the shoulder. We have further support for this line of thinking via a recent systematic review and meta-analysis by Bell et al: the authors found athletes who were highly specialized were twice as likely to sustain an overuse injury when compared to athletes with low specialization.11 Analysis also revealed risk for overuse injuries increased in a stepwise fashion with increasing levels of specialization.

    Early sport specialization goes hand-in-hand with load and fatigue management. Jones et al discussed the influence of training load and fatigue on athlete injury and illness.12

    The authors recommend training load and fatigue should be monitored and altered accordingly. Fatigue is defined as “the decrease in the pre-match/baseline psychological and physiological function of the athlete.”

    They go on to state,  “An accumulation of fatigue can result in overtraining, which has a significant negative impact on performance.”

    “Training load” is multifactorial, but includes variables such as session Rate of Perceived Exertion (sRPE), volume, intensity, and acute-to-chronic workload ratios (ACWR). For an in-depth review of the supportive research on the strong associations between training load and injury, see Eckard et al.13

    We also recently recorded a podcast on this topic HERE.

    Finally, strength training should be incorporated as a mechanism of protection and injury risk reduction for these athletes.14  Although full discussion of this approach is beyond the scope of this article, we encourage the readers to review the citations for more information.

    In closing, symptomatic internal impingement describes a non-traumatic pain-based issue that has little evidence supporting a clear structural pathology. The findings on radiologic imaging once a person becomes symptomatic likely represent sport-specific adaptations, rather than primary drivers of patient symptoms. Furthermore, we have strong evidential support for ways to mitigate the development of symptoms. However, once symptomatic, time can be utilized educating the person on pain and how to mitigate symptom development while regression to the mean takes place.15

    Next in the series: rotator cuff tendinopathy / tears.


    1. Spiegl UJ, Warth RJ, Millett PJ. Symptomatic internal impingement of the shoulder in overhead athletes. Sports medicine and arthroscopy review. 2014; 22(2):120-9.
    2. Asker M, Brooke HL, Waldén M, et al. Risk factors for, and prevention of, shoulder injuries in overhead sports: a systematic review with best-evidence synthesis. British journal of sports medicine. 2018;
    3. Pennock AT, Dwek J, Levy E, et al. Shoulder MRI Abnormalities in Asymptomatic Little League Baseball Players Orthopaedic Journal of Sports Medicine. 2018; 6(2):232596711875682-.
    4. Tran G, Cowling P, Smith T, et al. What Imaging-Detected Pathologies Are Associated With Shoulder Symptoms and Their Persistence? A Systematic Literature Review. Arthritis care & research. 2018; 70(8):1169-1184.
    5. Del Grande F, Aro M, Jalali Farahani S, Cosgarea A, Wilckens J, Carrino JA. High-Resolution 3-T Magnetic Resonance Imaging of the Shoulder in Nonsymptomatic Professional Baseball Pitcher Draft Picks. Journal of computer assisted tomography. ; 40(1):118-25.
    6. Lee CS, Stetson WB, Goldhaber NH, Davis SM, Brock A, Wosmek J. Magnetic Resonance Imaging Findings in Asymptomatic Elite Volleyball Players Arthroscopy: The Journal of Arthroscopic & Related Surgery. 2017; 33(10):e58-e59.
    8. Fredericson M, Ho C, Waite B, et al. Magnetic resonance imaging abnormalities in the shoulder and wrist joints of asymptomatic elite athletes. PM & R : the journal of injury, function, and rehabilitation. 2009; 1(2):107-16.
    9. Connor PM, Banks DM, Tyson AB, Coumas JS, D’Alessandro DF. Magnetic resonance imaging of the asymptomatic shoulder of overhead athletes: a 5-year follow-up study. The American journal of sports medicine. ; 31(5):724-7.
    10. Schär MO, Dellenbach S, Pfirrmann CW, Raniga S, Jost B, Zumstein MA. Many Shoulder MRI Findings in Elite Professional Throwing Athletes Resolve After Retirement: A Clinical and Radiographic Study. Clinical orthopaedics and related research. 2018; 476(3):620-631.
    11. Bell DR, Post EG, Biese K, Bay C, Valovich McLeod T. Sport Specialization and Risk of Overuse Injuries: A Systematic Review With Meta-analysis. Pediatrics. 2018; 142(3):.
    12. Jones CM, Griffiths PC, Mellalieu SD. Training Load and Fatigue Marker Associations with Injury and Illness: A Systematic Review of Longitudinal Studies. Sports medicine (Auckland, N.Z.). 2017; 47(5):943-974.
    13. Eckard TG, Padua DA, Hearn DW, Pexa BS, Frank BS. The Relationship Between Training Load and Injury in Athletes: A Systematic Review Sports Med. 2018; 48(8):1929-1961.
    14. Lauersen JB, Andersen TE, Andersen LB. Strength training as superior, dose-dependent and safe prevention of acute and overuse sports injuries: a systematic review, qualitative analysis and meta-analysis. British journal of sports medicine. 2018.
    15. Whitney CW, Von Korff M. Regression to the mean in treated versus untreated chronic pain. Pain. 1992; 50(3):281-5.
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