Shoulder Range of Motion and Baseball Arm Injuries: A Systematic Review and Meta-Analysis

Upper extremity injuries in baseball players are a persistent and significant problem. Such injuries are common,^1–3 and injury rates are increasing.⁴ In the last decade, the injury rates of high school, collegiate, and professional baseball players were 4.0, 5.8, and 3.61, respectively, per 1000 athlete-exposures, with the greatest incidence of injury at the shoulder and elbow.^1–3 These injuries can be attributed to increases in overall playing volume⁵ and changes in shoulder range of motion (ROM)⁶ among other factors. As a result, clinicians have investigated shoulder ROM with the aims of understanding and predicting upper extremity injuries in baseball athletes.⁷

Throwing is a complex and high-demand movement that produces high segmental velocities and joint forces,⁸ with the highest proportion of forces occurring in the upper extremity.^9–12 Previous researchers^13–20 proposed that pitching forces can affect the osseous and soft tissue structures of the upper extremity. The large stresses caused by external-rotation (ER) torque during the pitching motion could induce adaptations in the humeral epiphyseal cartilage, orienting it to a more posterior and medial position.^15,17 Due to throwing-shoulder structural alterations,^13–15,21 some ROM differences between the throwing and nonthrowing limbs are necessary adaptations in high-level throwers.²² Although long-term, osseous, throwing-specific ROM adaptations occur,^13–15,22 short-term soft tissue ROM changes have also been observed^18,19,23 and have demonstrated disparate results in terms of upper extremity injuries.^18,23 Given the multiple factors that affect shoulder ROM^{13–15,18,22,23} and differing conclusions concerning shoulder ROM changes,^18,23 shoulder ROM changes have many confounding factors and are poorly understood.

Soft tissue pitching adaptations can be modified through specific interventions.^24–26 The sleeper stretch reduced shoulder ROM recovery time compared with time alone.²⁵ Instrumented soft tissue mobilization combined with self-stretching decreased shoulder ROM risk factors compared with self-stretching alone in baseball players.²⁶ Due to the heterogeneity of studies investigating the association between shoulder ROM and injury in baseball players^18,27 as well as the ability to effect soft tissue ROM adaptations through specific interventions,^24–26 we need to investigate and summarize the evidence.

Shoulder injuries in baseball continue to be a significant problem for athletes, coaches, parents, and sports medicine providers.^1–4 The identification of modifiable risk factors, including ROM, is essential for injury prevention.^1,7 Several groups^1,5,7,18,27 have prospectively examined modifiable risk factors for upper extremity injuries, but varied results made it unclear if shoulder ROM was a risk factor for upper extremity injury. This has limited the development and integration of effective upper extremity injury-prevention strategies for baseball players. Therefore, the purpose of our systematic review and meta-analysis was to critically assess the methodologic quality and level of evidence in the literature and to investigate the relationship between shoulder ROM and the risk of upper extremity injuries in baseball players. We anticipated that this systematic review would provide clinicians with evidence-based insights that could be used to develop and integrate injury-prevention strategies.^24,25

METHODS

Study Selection

Two reviewers (G.S.B., M.S.F.) assessed the titles and abstracts for adherence to the inclusion and exclusion criteria. Full-text documents identified by either reviewer as possibly applicable were then held for further examination (Figure 1). In case of disagreement between the 2 reviewers, a third reviewer (T.C.S.) was asked to resolve the discrepancy.

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RESULTS

A total of 707 studies were identified in an initial exploration of database and reference searches. After duplicates were removed (n = 287), 420 titles were screened. Eligibility for inclusion was assessed in 75 abstracts, and 18 studies were evaluated via full-text review. Six studies were included in the quality assessment and analysis (n = 1056 participants).

Study Quality Assessment

All 6 studies reviewed^{1,6,7,18,27,29} were prospective; therefore, all provided level 2b evidence (Table 1).³⁰ Study quality scores (modified Downs and Black scale²⁸) ranged from 11 to 14 out of a maximum of 15. Comparison groups were included in all 6 studies. The authors of all 6 studies compared throwing with nonthrowing arms.^{1,6,7,18,27,29} Three sets of authors^1,7,18 blinded examiners to the participant's throwing arm. One group²⁷ considered confounding variables (eg, pitch count) in the data analysis. Lastly, investigators in 3 studies^1,18,27 calculated sample size a priori, and only 2 studies^18,27 had sufficient power (Table 2).

Table 1 Study Demographics and Design

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Table 2 Methodologic Quality Scores of Included Studies (Modified Downs and Black Scale)

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Meta-Analysis

We analyzed the absolute shoulder ROM pooled variance for IR, ER, and TROM from 3 studies^1,7,18 and HA from 2 studies.^1,7 The pooled ROM bilateral deficit was assessed for IR^1,7,18 and TROM.^1,7,18 The overall pooled assessment demonstrated that the absolute shoulder IR ROM was a predictor of injury (P < .001; –5.93 [95% CI = –9.43, –2.43]) that exhibited homogeneity (P = .77; I² = 0%; Figure 2).^1,7,18 The IR shoulder ROM pooled bilateral deficit was a predictor of injury (P = .02; 4.28 [95% CI = 0.71, 7.86]) that had homogeneous data (P = .41; I² = 0%; Figure 3).^1,7,18 The overall pooled assessment for absolute shoulder TROM was a predictor of injury (P = .003; –6.19 [95% CI = –10.28, –2.10]) that had homogeneous data (P =.78; I² = 0%; Figure 2).^1,7,18 The shoulder TROM pooled bilateral deficit was not a predictor of injury (P = .60; 1.04 [95% CI = –2.84, 4.91]); the data were homogeneous, with a moderate heterogeneous effect (P = .19; I² = 39%; Figure 3).^1,7,18 The overall pooled assessment for absolute shoulder ER ROM was not a predictor of injury (P = .13; –2.86 [95% CI = –6.56, 0.83]), which had highly heterogeneous data (P = .003; I² = 83%; Figure 2).^1,7,18 The overall pooled assessment for absolute shoulder HA was a predictor of injury (P < .001; –8.32 [95% CI = –12.08, –4.56]); the data were homogeneous, with a moderate heterogenic effect (P = .16; I² = 50%; Figure 2).^1,7

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DISCUSSION

The purpose of this systematic review was to evaluate the methodologic quality and level of evidence in the literature and to investigate the relationship between shoulder ROM and the risk of upper extremity injuries among baseball players. A total of 6 articles met the inclusion criteria for the review. We found that absolute shoulder IR and TROM and the throwing versus nonthrowing upper extremity deficit were associated with time-loss upper extremity injuries. The meta-analysis revealed absolute shoulder IR and TROM less than 44° and 160° and side-to-side deficits in excess of 5° and 8,° respectively, which should be considered when designing upper extremity injury-prevention programs. These values should be measured as part of baseline injury and return-to-sport screening in an effort to decrease the upper extremity injury risk.

All 6 groups^{1,6,7,18,27,29} used prospective cohort designs, and their study scores ranged from 11 to 15 on the modified Downs and Black scale.²⁸ Investigators were blinded in only 3 studies,^1,7,18 indicating that the other studies may have had potential investigator bias. Only 1 group²⁷ accounted for a confounding variable: pitch count, which has been associated with fatigue and pain.^5,31 Furthermore, the number of pitches and pitch types thrown during a game and over a season were associated with pain.⁵ Finally, only 3 sets of authors quantified power a priori,^1,18,27 and only 2 had sufficient samples.^18,27 This shows that the majority of studies lacked ample sample size to adequately compare shoulder ROM and injuries.

Our pooled analysis demonstrated that absolute IR <44° and side-to-side dominant-shoulder deficit >5° indicated a greater risk of arm injury. Decreased IR has been associated with posterior shoulder tightness and increased humeral retrotorsion.^32,33 Pitchers with internal impingement developed greater posterior shoulder tightness (deficits in HA and IR) than healthy pitchers.³² In addition, increased posterior shoulder tightness may affect the biomechanics by increasing the subacromial contact pressure and contact area during pitching.³⁴ The increased contact pressure and greater contact area may contribute to the risk of rotator cuff injury during pitching.³⁴

The studies not included in the meta-analysis had conflicting results. Tyler et al²⁷ observed an injury incidence of 1.14/1000 pitches for the below-normative mean, whereas Wilk et al^6,29 did not detect a relationship between shoulder IR ROM deficit and injury. This disparity may have been because their participants were professional pitchers rather than adolescent amateur athletes. Previous authors^35,36 have observed that athletes at higher competition levels have greater proficiency in fundamental movements. Also, pitchers at higher competition levels may receive more specifically tailored screening for shoulder IR ROM discrepancies.

Our pooled results further showed that absolute TROM <160° and side-to-side deficits >10° increased the arm injury risk. During the pitching motion, the greatest degree of ER occurs during the late-cocking phase.³⁷ The extreme ER is followed by an acute acceleration and subsequent IR torque, increasing the susceptibility to injury.⁸ Other researchers³⁸ detected that a decrease in TROM correlated with a decrease in IR ROM. As stated earlier, IR has been shown to be a factor in pitching injuries.^1,7,18,27 This highlights the fact that TROM may also be a risk factor in upper extremity injuries among baseball players.

Interestingly, shoulder ER was not related to upper extremity injury. This supports previous retrospective and cross-sectional studies.^39,40 In all of the studies included in the meta-analysis, ER for total and increased ROM was analyzed.^1,7,18 However, a deficit in ER ROM was associated with injury in 1 study⁶ that was not included in the meta-analysis. Pitchers who had less than 5° greater ER in their throwing versus nonthrowing shoulder were more likely to incur an upper extremity injury.⁶ It has been hypothesized²² that some ROM changes between the throwing and nonthrowing limbs are a necessary adaptation in high-level throwers. Previous authors^41,42 have described an inverse relationship between humeral retrotorsion and shoulder injuries in overhead athletes. These findings suggest that humeral retrotorsion offers protection to the shoulder joint in overhead throwers.^6,41,42 Furthermore, humeral retrotorsion explained 24% of the difference in ER and 16% of the difference in IR between limbs.⁴³ Thus, while screening for the risk of arm injury, measuring ER alone may not be sufficient; further inquiry into the relationship between ER and humeral retrotorsion as well as pitching development is needed.

Absolute shoulder HA had a statistically significant correlation with injury. However, the effect was heterogeneous, which reflects the data inconsistencies. The discrepancy within the meta-analysis may be due to the smaller samples that were assessed for HA: only 2 groups^1,7 assessed HA. Other researchers²⁷ assessed HA but did not find any association with injury. Loss of shoulder HA ROM has been postulated to be due primarily to shoulder capsule and rotator cuff decrements, which hinder humeral head translation during pitching and cause subsequent injury.⁴⁴ In other studies,^32,45 decreased HA with shoulder impingement symptoms was noted. Although decreased HA ROM has been associated with shoulder injury in individual studies, data discrepancies mean that the current evidence is insufficient to support this relationship.

Of the 2 sets of authors^6,29 who measured shoulder flexion, one²⁹ found a relationship only with elbow injuries in professional pitchers. The authors hypothesized that this relationship may have been due to decreased tissue flexibility, specifically within the latissimus dorsi,²⁹ which is highly active during pitching.⁴⁶ The greatest latissimus dorsi electromyography activity occurs during the late-cocking and acceleration phases of pitching, with the highest power generation during the latter.⁴⁶ Due to the latissimus dorsi's contribution to pitching power production, decreased flexibility could correlate with a deficit in shoulder flexion, a measurement that may be clinically important.

These summary results should be considered in light of the limitations of the available studies. Only English-language articles were used in this review, which is a publication bias, and only American baseball players were investigated. As a result, these data cannot be generalized to other countries or cultures. Most investigators followed the athletes for only 1 season. Little is known about how changes in shoulder ROM over multiple years affect injury rates. Shanley et al¹⁹ observed that shoulder ROM in professional pitchers changed over the course of 2 seasons. These findings suggest that shoulder ROM alters over time; however, to date, no researchers have looked at the effects of these changes on upper extremity injuries. Most authors assessed athletes at only 1 competition level, the majority of whom were adolescents. Pitchers at higher competition levels throw at greater velocities and play more games in a season compared with those at lower competition levels.^47,48 Furthermore, the studies we reviewed included players from young adolescents to professionals. Due to the physical differences among the varying age ranges, these findings should not be considered ubiquitous for all baseball player ages and competition levels. Understanding age- and competition-level normative data for shoulder ROM may be beneficial in elucidating specific athletes' prophylactic needs. Finally, the cut-off values of 5° and 8° were statistically significant in the larger samples and our pooled analysis. However, it is likely that 10° of IR and 15° of TROM are the smallest changes that can be measured clinically.⁴⁹ Clinicians should carefully consider their ROM assessment techniques to optimize the meaningfulness of their measures. Given that the purpose of screening is to identify athletes who may be at risk and to provide relatively simple, noninvasive, effective stretching interventions, smaller cut-offs may be considered.

In summary, the moderate to high methodologic scores demonstrated that a relationship was present between IR and TROM and upper extremity injury in baseball pitchers. Overall, ER did not have a significant correlation with injury; however, 1 group⁶ observed that a deficit in ER ROM was associated with injury. Although HA and shoulder flexion ROM were associated with injury, an association between HA and shoulder flexion and injury cannot be claimed because of data heterogeneity and low sampling power. Future research is necessary to confirm these results in larger prospective studies. Also, more work is required to understand the effect of multiple years of shoulder ROM changes in relation to injury and normative shoulder ROM data for athletes at different competition levels. This systematic review and meta-analysis demonstrated important relationships between shoulder ROM and the future risk of upper extremity injury. These data provide the foundation for screening tools and the design of individualized or team-based injury-prevention programs.