Limited Total Arc Glenohumeral Rotation and Shoulder Biomechanics During Baseball Pitching

Baseball pitching mechanics consist of 6 phases, including the wind-up, stride, late cocking, acceleration, deceleration, and follow-through phases.¹ The late cocking, acceleration, and deceleration phases are the most stressful phases in the upper extremity because the shoulder and elbow undergo the greatest joint loading and maximal range of motion (ROM).² Specifically, the ulnar collateral ligament of the elbow, labrum, rotator cuff, and the joint capsule of the shoulder are susceptible to stress and potential injury during these phases.^3–6

To throw a baseball with increased velocity while decreasing the risk of injury, the kinetic chain must be efficient in each of its phases. The rapid rotational sequence of the body leads distal structures to temporally lag behind the proximal segments during the late cocking phase. The sequential rotation creates an effective transition of momentum to the throwing hand and allows pitchers to pitch a ball fast. Simultaneously, the sequence exposes the shoulder to the greatest maximal shoulder external rotation (ER), which exceed the clinically measured ER and the elbow to the highest valgus torque around 120 Nm.^7,8 Both the maximal shoulder ER and the valgus torque while pitching are positively associated with faster ball velocity, and ball velocity is the most common way to value a pitcher.^9,10

Pitching mechanics and ball velocity have been linked to a variety of upper extremity injuries and ulnar collateral ligament sprains of the elbow and superior labrum anterior-posterior (SLAP) tears of the shoulder.^2,3,10 Because of concerns for injury risks, authors of several studies have attempted to understand the underlying causes of upper extremity injury, with shoulder ROM as a major interest for researchers and clinicians.^11–14 To date, limited total arc glenohumeral ROM, internal rotation (IR), and ER measured in clinical evaluation have been reported as risk factors for injury.¹⁴ For instance, pitchers with limited total arc glenohumeral ROM demonstrated a higher risk of elbow injuries, but the physical characteristics may be modifiable using a variety of therapeutic interventions.^11,15 Thus, identification of such modifiable risk factors has been essential for injury risk mitigation. Recently, Bullock et al conducted a meta-analysis to understand shoulder ROM and baseball arm injuries and suggested 160° of total arc glenohumeral ROM could be a baseline for clinical screening to decrease the risk of upper extremity injuries.¹⁴ While it may be informative for clinicians to recognize a specific objective threshold for risk of injury, the behavioral aspect of how a limited total arc glenohumeral ROM may affect pitching biomechanics remains unclear. Understanding how this baseline relates to pitching mechanics is clinically meaningful to provide proper suggestions for pitchers and for clinicians to effectively implement therapeutic intervention. Therefore, the purpose of this study was to investigate differences in baseball pitchers with lower total arc glenohumeral ROM compared with pitchers with higher total arc ROM. Specifically, we aimed to examine if lower levels of total arc ROM demonstrated differences in shoulder characteristics and its effect on joint loading of the throwing shoulder and elbow as well as ball velocity. First, we hypothesized that pitchers with limited total arc ROM would display greater deficits between dominant and nondominant arms. Secondly, we hypothesized that pitchers with optimal total arc ROM would experience lower levels of joint loading, including IR torque and distraction force in the shoulder and valgus torque in the elbow, demonstrate less difference between maximal shoulder ER while pitching and clinically measured shoulder ER (MaxERDiff) with no difference in ball velocity than those with limited total arc ROM. We also investigated other shoulder and elbow characteristics considered risk factors for injury and performance factors, including elbow flexion angle at the late cocking phase and shoulder horizontal abduction at the stride phase.^2,7,16 We expected that our findings would facilitate clinicians to provide concise recommendations for baseball pitchers and enhance the quality of health care based on total arc glenohumeral ROM evaluated in clinical screening.

METHODS

Procedures

Participants were evaluated using both clinical and biomechanics pitching evaluations on the same day. Participants first answered a custom demographics questionnaire, including age, weight, height, and hand dominance before beginning with the clinical evaluation. All clinical measurements were performed before participants warmed up for the pitching biomechanical evaluation. Glenohumeral ROM for the throwing and nonthrowing arms were measured by 2 clinicians with a standard goniometer. Participants were positioned supine on a standard plinth, and the shoulder was abducted at 90° with elbow flexed at 90°. The upper arm was maintained parallel to the table so that the shoulder was not horizontally abducted. To measure IR, the anterior shoulder was stabilized, and the arm was rotated internally until a firm end feel was noted (Figure 1).¹⁷ After measuring IR, the evaluators measured ER with the participant in a similar position. However, instead of an anterior stabilization, the lateral border of the scapula was stabilized by the clinician’s hand before moving the arm externally to achieve a firm end feel. The ER and IR values were added to determine the total arc glenohumeral ROM for both the dominant and nondominant arms. Each measurement was conducted once, and the peak-end ROMs were used for data analysis.

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After the clinical evaluation, 41 reflective markers were placed on bony landmarks of the body.¹⁸ A 20-high-speed-camera motion analysis system was used with sampling at 320 Hz (Qualisys AB). After the markers were placed, participants warmed up using their own self-prescribed routine. Participants threw a baseball from a custom instrumented force plate (Bertec) pitching mound toward a frame object placed at 17 m away in the pitching laboratory, with force plate sampling frequency set at 1280 Hz. Participants pitched roughly 20 balls during the pitching evaluation, and approximately 10 of those pitches were 4-seam fastballs. The markers were tracked in Qualisys Track Manager before being exported through the project automation framework. Tracked markers were attenuated by a low pass 6th-order Butterworth filter with a cutoff frequency of 20 Hz. Kinematic outcomes were measured at stride and late cocking phases, and kinetic outcomes were evaluated at the late cocking and deceleration phases. The stride phase was determined as the time that the stride foot fully contacted the pitching mound, and the late cocking was defined as the moment of the throwing shoulder reaching maximal shoulder ER (Figure 2). Kinematic and kinetic outcomes included maximal shoulder ER (°), horizontal abduction (°) at the stride, elbow flexion angle (°) at the late cocking, peak shoulder distraction force (BW%), peak IR torque (Nm), and peak elbow valgus torque (Nm).

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RESULTS

Physical Characteristics

Fifty-seven baseball pitchers participated in this study. Thirty-eight participants were allocated into the control group (age = 20.9 ± 1.6 years, height = 186.0 ± 5.7 cm, and mass = 91.1 ± 9.6 kg), and 19 participants were assigned for the lower ROM group (age = 20.1 ± 1.8 years, height = 188.1 ± 6.5 cm, and mass = 90.7 ± 10.6 kg). No statistically significant difference was found in the demographics (Table 1). Only dominant ER is not normally distributed between groups (U = 85.5, P < .001), but other independent variables met the assumption of homogeneity. The lower ROM group had significantly more deficits in ER, IR, and total arc glenohumeral ROM than the control group (mean ER deficit: lower ROM = 3.5° ± 6.2°, control = 7.8° ± 9.6°, P = .041; IR deficit: lower ROM = −15.9° ± 14.2°, control = −9.2° ± 10.7°, P = .027; total arc deficit: lower ROM = −12.4° ± 13.9°, control = −1.5° ± 10.0°, P < .001; Table 2).

Table 1. Physical Characteristics

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Table 2. Glenohumeral ROM Deficits (in °)^a

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The control group had less overstretching in ER with the throwing shoulder. The lower ROM group had significantly less maximal ER while pitching than the control group (lower ROM = 160.1° ± 11.0°, control = 170.5° ± 13.3°, P = .002; Table 3). The lower ROM group also demonstrated significantly greater MaxERDiff (lower ROM = 64.4° ± 12.1°, control = 55.8° ± 16.6°, P = .025; Figure 3).

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Table 3. Mean Differences in Pitching Metrics

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The control group had statistically significant faster ball velocity than the lower ROM group (control = 85.0 ± 4.3 mph, lower ROM = 82.4 ± 4.8 mph, P = .024; Figure 4). Additionally, the control group demonstrated significantly greater shoulder horizontal abduction at the stride phase (control = 21.0° ± 13.7°, lower ROM = 7.3° ± 14.3°, P < .001). No statistically significant difference was found between groups in shoulder distraction force, shoulder internal torque, elbow flexion angle, and elbow valgus torque (Table 3).

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DISCUSSION

The current study investigated whether pitchers with less than 160° of total arc glenohumeral ROM had differences in stress in the throwing shoulder and elbow while pitching compared with those with 160° or greater total arc ROM. This threshold was determined based on findings from a previous meta-analysis, and we aimed to examine how pitchers with limited total arc ROM differed in their pitching biomechanics from those with optimal total arc ROM.¹⁴ We found pitchers with 160° or greater total arc ROM exhibited less deficits in shoulder ROM and less difference in range between maximal shoulder ER while pitching and clinically measured ER. These findings confirm our hypotheses that pitchers with ideal total arc ROM present with less deficits in shoulder ROM and less difference between maximal shoulder ER while pitching and clinically measured shoulder ER in the shoulder. Contrary to our hypothesis, we did not find significant differences in shoulder and elbow kinetics. Finally, pitchers with optimal total arc ROM pitched balls faster than those with limited total arc ROM.

The lower ROM group had significantly greater total arc glenohumeral ROM deficits than the control group. As we defined deficit as the difference in ROM between dominant and nondominant arms, our findings suggest that pitchers with less than 160° of total arc ROM may present with decreased ROM due to the underlying musculoskeletal tissue mechanics. Early baseball researchers postulated that glenohumeral IR deficit (GIRD) was linked to a variety of upper extremity injuries.^5,20 Recently, Wilk et al suggested a total rotational motion concept, in which pitchers with deficit in total arc ROM less than −5° (calculated by dominant arm − nondominant arm) had a significantly higher injury rate that resulted in missing time of playing in professional baseball players.²¹ Soft tissue and osseous adaptations play a role in glenohumeral ROM. Paul et al found that humeral retrotorsion and posterior capsule thickness might be primary physical adaptations responsible for preseason IR ROM.²² Additionally, the posterior rotator cuff is more related to acute loss of IR after pitching.²³ On the other hand, humeral retrotorsion contributes to an increase of ER and decrease of IR.^24,25 Because we summated IR and ER for total arc ROM and conducted the clinical evaluation before the pitching biomechanics evaluation, posterior capsule thickness might be the underlying cause of the loss of ROM in the lower ROM group.

The throwing shoulder typically externally rotates back between 170° and 190° of shoulder ER during the late cocking phase, which is significantly greater than clinically measured ER.^7,14 When the throwing shoulder is experiencing extreme ER, the biceps tendon plays a critical role in the protection. Due to the extreme ER creating powerful tension, the superior labrum gets pulled by the long head of the biceps posteriorly, which leads to shear stress on the superior labrum (a potential mechanism for SLAP tears).^2,26,27 We observed the lower ROM group demonstrate a greater difference in motion between maximal shoulder ER while pitching and clinically measured ER. Pitchers with limited total arc ROM may undergo more tension on the long biceps tendon and peel-back stress.⁵ Although it may be necessary to consider thoracic extension and scapulothoracic movement, the discrepancy indicates higher risk of shoulder injury.²

The results also align with findings that limited total arc glenohumeral ROM is more susceptible to upper extremity injuries. Wilk et al reported that professional baseball pitchers with deficits of less than −5° in total arc ROM in their dominant shoulders had a 2.6 times greater risk of shoulder injuries and surgery.¹⁵ Our control group exhibited with −1.5° ± 10.0°, whereas the risk group had −12.4° ± 13.9°. As Wilk et al suggested pitchers with deficit in −5° of total arc ROM as a risk indicator, pitchers with less than 160° may tend to display greater deficits. Regarding shoulder ER, a statistically significant difference was found in ER between groups (control: 114.7 ± 13.2, lower ROM: 95.7 ± 8.5, P < .001). Professional baseball players with insufficient ER were 2.2 times more likely to suffer from a shoulder injury and 4.0 times more likely to require shoulder surgery than pitchers with optimal shoulder ER.¹² Additionally, authors of a prospective study demonstrated that, if the pitcher’s throwing arm does not have 5° or greater ER than the nonthrowing arm, it could be an injury risk factor.¹³ In our study, the lower ROM group falls into the suboptimal range (3.5° ± 6.2°), whereas the control group had greater than 5° difference between dominant and nondominant arms (7.8° ± 9.6°).

Regarding elbow loading, we observed 7.2% increased elbow valgus torque for the lower ROM group compared with the control group, yet the mean difference was not statistically significant (P = .133). Noonan et al reported that pitchers who suffered from shoulder injury sustained less humeral retrotorsion, whereas pitchers who injured their throwing elbow displayed increased humeral retrotorsion.²⁸ In another study, less humeral retrotorsion inversely correlated with more severe upper extremity injuries.²⁹ The measurement of humoral retrotorsion is not clinically accessible for everyone due to the need for training and a device such as a diagnostic ultrasound. Although traditional goniometric ROM measures may detect these changes, we did not directly measure the osseus adaptation. Thus, this may be a limitation of this study, and future researchers may want to consider direct measures of humeral torsion which may reveal the further effects of total arc ROM on medial elbow stress.

Interestingly, pitchers with greater total arc glenohumeral ROM demonstrated significantly faster ball velocity than the lower ROM group. Additionally, the control group had significantly greater horizontal abduction at the stride phase than lower ROM group. Both maximal shoulder ER at the late cocking and horizontal abduction at the stride phases are positively correlated with ball velocity, and 16° to 21° of the horizontal abduction is a typical range.^7,16 Although the interaction between glenohumeral ROM and horizontal abduction is not well understood, our finding implies that limited total arc ROM may affect pitching mechanics and performance. Thus, further research is needed to understand how glenohumeral ROM affects other anatomical planes during the pitching motion besides sagittal plane.

This study has several limitations that we must acknowledge. Our sample size with a 1:2 ratio (38 participants in the control group and 19 participants in the risk group) lacked the statistical power to determine differences in kinetic variables. Nevertheless, we observed differences in glenohumeral characteristics, shoulder kinematics while pitching, and ball velocity. Secondly, participation occurred anytime throughout the period of data collection. Thus, we did not consider timing of season in our cross-sectional study. Glenohumeral ROM is known to adapt repetitive stress from throwing, so it may be ideal to conduct this type of study with all participants before or after the official baseball season. Still, 160° can be considered a threshold to monitor degradation or progression of the ROM because pitchers may demonstrate more than 160° but display with less than 160° due to adaptation according to the amount of pitching load.

CONCLUSIONS

Pitchers with less than 160° of total arc glenohumeral ROM may undergo overstretching toward ER in the shoulder during the late cocking phase. Pitchers with greater levels of total arc ROM can pitch at the same or faster ball speed without increasing loading in the upper extremity. While it is paramount to pay attention to ER and IR individually, athletic trainers can also combine both measurements to reveal total arc glenohumeral ROM. Monitoring total arc ROM may help identify pitchers who are at higher risk of shoulder injuries, initiate preventive exercises or therapeutic interventions, and monitor arm health over time. Future researchers should consider the timing of evaluation, other clinical characteristics, such as shoulder strength and lower extremity, bony anatomical changes, and pitching habits, including pitch counts per game and rest between games in conjunction with examination of glenohumeral ROM.