Abstract
Background
The relationships between shoulder abduction and external rotation with peak kinetic values at the shoulder and elbow in professional baseball pitchers are not well established.
Methods
Professional pitchers (n = 322) threw 8–12 fastballs under 3D motion analysis (480 Hz). Pitchers were stratified into quartiles by shoulder abduction and external rotation at distinct timepoints. Regression analyses were performed to quantify associations between shoulder position and kinetics.
Results
Shoulder abduction remained relatively consistent throughout the pitch (foot contact–ball release: 85.5 ± 11.1–90.7 ± 8.4°); shoulder external rotation increased dramatically (foot contact–ball release: 30.8 ± 24.6–165.2 ± 9.7°). For every 10° increase in maximum shoulder rotation, shoulder superior force increased by 2.3% body weight (p < 0.01), shoulder distraction force increased by 5.9% body weight (p < 0.01), and ball velocity increased by 0.60 m/s (p < 0.01). Shoulder abduction was significantly associated with shoulder superior force at all timepoints but not with ball velocity (p > 0.05). For every 10° increase in shoulder abduction at ball release, shoulder superior force increased by 3.7% body weight (p < 0.01) and shoulder distraction force increased by 11.7% body weight (p < 0.01).
Conclusion
Increased shoulder abduction at ball release and increased maximum shoulder external rotation were associated with greater superior and distraction forces in the shoulder. Pitchers can consider decreasing shoulder abduction at later stages of the pitch to around 80° in order to minimize shoulder superior force, with no impact on ball velocity.
Keywords: Motion capture, distraction force, ball velocity, shoulder injury
Introduction
With rising early sport specialization and increasing emphasis on pitch velocity, shoulder and elbow injury prevention in baseball pitchers is critical. As motion analysis and computational modeling capabilities have improved, a more sophisticated understanding of the relationships between arm position and kinetics at the shoulder and elbow can be achieved. In particular, shoulder abduction and external rotation are important kinematic parameters across the pitching motion that can affect forces and stresses in the shoulder and elbow.1–7
The existing literature on the associations between shoulder external rotation and abduction at various timepoints of the pitching motion with throwing arm kinetics is relatively sparse, with mixed results reported.2–7 Aguinaldo and Chambers 3 demonstrated maximum shoulder external rotation (MER) in adult motion capture studies to be a moderate correlator with elbow varus torque. Tanaka et al. 4 found in adolescent pitchers that shoulder inferior force increased with greater shoulder abduction. Matsuo et al. 5 noted shoulder abduction angles selected by professional pitchers were highly consistent with the angle that minimized elbow varus torque in model simulations.
The exact timepoints in the pitching motion when these relationships are the most pronounced are undefined, with varied results reported for elbow varus torque associations with shoulder abduction at foot contact (FC) and ball release (BR).4–7 Some authors have suggested that peak risk of shoulder injury occurs at BR,2,8,9 while others have proposed that maximum shoulder joint stress occurs at MER.10,11 Still, a number of kinematic studies have examined the point of FC,6,12–14 with shoulder abduction at FC in professional pitchers established as a significant variable in elbow varus torque prediction. 6 Contrarily, Matsuo et al. 7 demonstrated shoulder abduction at BR alone was not a predictor of peak elbow varus torque in collegiate pitchers.
Given seldom studies have compared these kinetic–kinematic relationships at multiple timepoints within the same cohort, the purpose of this study was to determine the associations between shoulder abduction and external rotation to throwing arm kinetics at FC, MER, and BR. We hypothesized that pitchers with shoulder abduction angles close to 90° and minimized shoulder external rotation throughout the pitch motion would experience the least cumulative loading on the shoulder joint.
Methods
Healthy professional baseball pitchers were included in this study. Inclusion criteria were: (1) at the time of testing, pitchers were currently on the Major League or Minor League (Low A, High A, AA, and AAA) roster; (2) pitchers had no record of serious injury (requiring > 2 weeks of rest or rehabilitation) in the past six months. All data were de-identified, and an exemption as granted by our institutional review board.
Pitching evaluations were conducted as previously discussed. 15 Demographic data, arm dominance, experience, history of injury, height, and weight were recorded. The pitcher was given unlimited time to warm-up with his preferred routine (i.e., arm bands, stretching, plyometric care, long-toss, etc.). Once the pitcher was ready, 46 reflective markers were placed on anatomical landmarks as previously described by Luera et al. 15 An 8-camera Raptor-E motion analysis system (Motion Analysis Corp, Santa Rosa, CA, USA) was used to record the marker data at 480 Hz.
At regulation distance (18.4 m) throwing from a mound, pitchers were instructed to pitch between 8–12 fastballs with game-like effort to a catcher behind home plate. Pitchers were allowed to pitch either the stretch or the wind-up. Ball velocity was collected with a radar gun located behind the pitcher (Stalker Sports Radar, Richardson, TX, USA).
Kinematic and kinetic analyses were performed using custom scripts in MATLAB (The Mathworks, Natick, MA, USA) as previously described by Luera et al. 15 Data from the markers were filtered by a low-pass filter (fourth order, zero lag Butterworth filter, 13.4 Hz cutoff frequency). 16 Pitch time began at FC and ended at BR. FC was defined as the first frame when the lead toe or heel had reached the minimum in the Z axis. MER was established as the maximum angle created between the forearm and an anterior–posterior line created in the z axis of the elbow joint. BR was established at 0.01 s after the wrist passed the elbow in the forward direction.16,17 Shoulder abduction was the angle between the distal direction of the upper arm and the line between the two shoulders with respect to the frontal plane. Shoulder rotation was the angle created between the forearm and the anterior-posterior line created in the z axis of the elbow joint. 4 Kinetic values collected included elbow varus torque, shoulder distractive force, shoulder anterior force, shoulder superior force, shoulder horizontal adduction torque, and shoulder internal rotation torque. All forces and torque values were normalized by player weight and player weight multiplied by player height, respectively.16,18
Pitches were averaged per player for all calculations, with both strikes and balls included. Pitchers were stratified into quartiles based on shoulder abduction at FC, MER, and BR. Across the quartiles, kinematic outcomes and ball velocity were compared using analysis of variance (ANOVA). Similarly, pitchers were stratified into quartiles based on shoulder external rotation at FC and MER. External rotation was not assessed at BR as the shoulder significantly reduces the degree of external rotation at BR. Post-hoc analysis included a two-sample t-test and linear regression correlation coefficients for variables that derived significance from initial ANOVA. Correlation coefficients were utilized to quantify the degree of correlation between shoulder abduction and external rotation. Bonferroni corrections were performed for ANOVA and post-hoc analysis was set an alpha value of 0.05. Statistical analyses were performed using MATLAB version R2020a.
Results
A total of 322 professional pitchers were included in this study (age: 21.9 ± 2.1 years; 189.7 ± 5.7 cm; 94.8 ± 9.5 kg; ball velocity: 38.4 ± 1.7 m/s). Shoulder abduction was relatively consistent throughout the pitch, with means of 85.5 ± 11.1° at FC, 91.7 ± 8.7° at MER, and 90.7 ± 8.4° at BR. Among all pitchers, at FC shoulder external rotation was 30.9 ± 24.6° and proceeded to externally rotate to 165.2 ± 9.7° at MER. Shoulder abduction and shoulder external rotation throughout the pitch motion are presented in Figure 1.
Figure 1.
Average shoulder abduction (a) and external rotation (b) angles throughout pitch motion from maximum knee height to ball release (t = 0). Shaded region represents standard deviation of entire cohort. MER: maximum external rotation.
Pitcher subdivision into quartiles based on shoulder abduction at FC is shown in Table 1. Decreased shoulder superior force was noted for Q1 vs. Q3 and Q4 (13.8 ± 8.5%BW vs. 18.9 ± 8.1, 23.0 ± 7.8%BW, respectively). Additionally, decreased shoulder superior force was noted for Q2 vs. Q3, Q4 as well as for Q3 vs. Q4.
Table 1.
Pitchers subdivided by shoulder abduction quartiles at foot contact.
| Q1 (n = 81) | Q2 (n = 80) | Q3 (n = 81) | Q4 (n = 80) | Significance | |
|---|---|---|---|---|---|
| Kinematics | |||||
| Shoulder abduction at FC (deg) | 69.9 ± 5.3 | 79.5 ± 2.1 | 86.9 ± 2.2 | 97.6 ± 5.5 | a,b,c,d,e,f |
| Ball velocity (m/s) | 38.0 ± 1.8 | 38.5 ± 1.8 | 38.5 ± 1.9 | 38.3 ± 1.7 | |
| Peak kinetics | |||||
| Elbow varus torque (%BW × BH) | 4.7 ± 0.7 | 4.9 ± 0.8 | 5.1 ± 0.8 | 5.1 ± 0.8 | |
| Shoulder distractive force (%BW) | 114.1 ± 8.9 | 114.9 ± 14.6 | 113.8 ± 4.6 | 118.9 ± 15.5 | |
| Shoulder anterior force (%BW) | 41.6 ± 6.4 | 41.9 ± 7.6 | 42.2 ± 6.8 | 43.0 ± 8.7 | |
| Shoulder superior force (%BW) | 13.8 ± 8.5 | 15.0 ± 7.8 | 18.9 ± 8.1 | 23.0 ± 7.8 | b,c,d,e,f |
| Shoulder horizontal adduction torque (%BW × BH) | 5.5 ± 0.9 | 5.6 ± 1.0 | 5.7 ± 1.1 | 5.7 ± 1.0 | |
| Shoulder internal rotation torque (%BW × BH | 4.8 ± 0.7 | 5.0 ± 0.8 | 5.1 ± 0.8 | 5.1 ± 0.7 |
FC: foot contact; deg: degrees; BH: body height; BW: body weight.
Values are presented as mean ± standard deviation; significant differences (p < 0.01) between (a) Q1 and Q2, (b) Q1 and Q3, (c) Q1 and Q4, (d) Q2 and Q3, (e) Q2 and Q4, (f) Q3 and Q4.
When examining pitchers based on shoulder abduction at MER, there was no difference in ball velocity and only shoulder superior force had significant differences between quartiles (Table 2). Decreased shoulder superior force was noted for Q1 vs. Q2, Q3 and Q4 (12.5 ± 8.5 vs. 16.1 ± 7.1, 20.4 ± 8.1, 21.6 ± 8.1%BW, respectively) as well as for Q2 vs. Q3 and Q4.
Table 2.
Pitchers subdivided by shoulder abduction quartiles at maximum shoulder external rotation.
| Q1 (n = 81) | Q2 (n = 80) | Q3 (n = 81) | Q4 (n = 80) | Significance | |
|---|---|---|---|---|---|
| Kinematics | |||||
| Shoulder abduction at FC (deg) | 80.7 ± 4.0 | 88.8 ± 1.9 | 94.4 ± 1.8 | 102.7 ± 3.6 | a,b,c,d,e,f |
| Ball velocity (m/s) | 38.5 ± 2.1 | 38.2 ± 1.8 | 38.4 ± 1.7 | 38.2 ± 1.8 | |
| Peak kinetics | |||||
| Elbow varus torque (%BW × BH) | 5.1 ± 0.8 | 5.0 ± 0.8 | 5 ± 0.8 | 4.7 ± 0.7 | |
| Shoulder distractive force (%BW) | 111.9 ± 16.0 | 114.6 ± 15.9 | 116.2 ± 12.2 | 119.0 ± 18.9 | |
| Shoulder anterior force (%BW) | 43.3 ± 6.8 | 42.7 ± 7.4 | 43.2 ± 7.1 | 40.0 ± 8.0 | |
| Shoulder superior force (%BW) | 12.5 ± 8.5 | 16.1 ± 7.1 | 20.4 ± 8.1 | 21.6 ± 8.1 | a,b,c,d,e |
| Shoulder horizontal adduction torque (%BW × BH) | 5.7 ± 1.0 | 5.5 ± 0.9 | 5.9 ± 1.0 | 5.5 ± 1.1 | |
| Shoulder internal rotation torque (%BW × BH) | 5.2 ± 0.8 | 5.0 ± 0.8 | 5.1 ± 0.8 | 4.8 ± 0.7 |
FC: foot contact; deg: degrees; BH: body height; BW: body weight.
Values are presented as mean ± standard deviation; significant differences (p < 0.01) between (a) Q1 and Q2, (b) Q1 and Q3, (c) Q1 and Q4, (d) Q2 and Q3, (e) Q2 and Q4, (f) Q3 and Q4.
Shoulder external rotation at BR demonstrated significantly different shoulder distractive force and superior force between quartiles (Table 3). Decreased shoulder distraction force was noted for Q1 vs. Q2, Q3 and Q4 (108.8 ± 16.1 vs. 117.4 ± 13.7, 116.0 ± 14.9, 119.7 ± 17.6%BW, respectively). Quartiles significantly differed among one another for shoulder superior force values (12.6 ± 8.6 vs. 16.5 ± 7.3 vs. 18.9 ± 7.8 vs. 22.8 ± 8.2%BW, respectively), except for Q3 vs. Q4.
Table 3.
Pitchers subdivided by shoulder abduction quartiles at ball release.
| Q1 (n = 81) | Q2 (n = 80) | Q3 (n = 81) | Q4 (n = 80) | Significance | |
|---|---|---|---|---|---|
| Kinematics | |||||
| Shoulder abduction at BR (deg) | 80.1 ± 3.7 | 87.8 ± 1.6 | 93.4 ± 1.5 | 101.2 ± 3.9 | a,b,c,d,e,f |
| Ball velocity (m/s) | 38.1 ± 2.1 | 38.4 ± 1.9 | 38.4 ± 1.5 | 38.5 ± 1.6 | |
| Peak kinetics | |||||
| Elbow varus torque (%BW × BH) | 4.9 ± 0.8 | 5.0 ± 0.7 | 4.8 ± 0.8 | 4.9 ± 0.8 | |
| Shoulder distractive force (%BW) | 108.8 ± 16.1 | 117.4 ± 13.7 | 116.0 ± 14.9 | 119.7 ± 17.6 | a,b,c |
| Shoulder anterior force (%BW) | 42.8 ± 6.4 | 43.0 ± 8.3 | 42.6 ± 7.2 | 40.9 ± 7.6 | |
| Shoulder superior force (%BW) | 12.6 ± 8.6 | 16.5 ± 7.3 | 18.9 ± 7.8 | 22.8 ± 8.2 | a,b,c,e,f |
| Shoulder horizontal adduction torque (%BW × BH) | 5.6 ± 1.0 | 5.6 ± 1.1 | 5.8 ± 0.9 | 5.5 ± 1.1 | |
| Shoulder internal rotation torque (%BW × BH) | 5.1 ± 0.8 | 5.0 ± 0.7 | 5.1 ± 0.8 | 5.0 ± 0.8 |
BR: ball release; deg: degrees; BH: body height; BW: body weight.
Values are presented as mean ± standard deviation; significant differences (p < 0.01) between (a) Q1 and Q2, (b) Q1 and Q3, (c) Q1 and Q4, (d) Q2 and Q3, (e) Q2 and Q4, (f) Q3 and Q4.
Only shoulder anterior force was noted to be significantly decreased for pitchers with decreased shoulder external rotation at FC for Q1, Q2, Q3 vs. Q4 (40.7 ± 7.3, 41.5 ± 7.9, 40.9 ± 6.5 vs. 45.6 ± 7.0%BW, respectively; Table 4).
Table 4.
Pitchers subdivided by shoulder external rotation quartiles at foot contact.
| Q1 (n = 81) | Q2 (n = 80) | Q3 (n = 81) | Q4 (n = 80) | Significance | |
|---|---|---|---|---|---|
| Kinematics | |||||
| Shoulder external rotation at FC (deg) | 0.8 ± 10.6 | 21.4 ± 4.8 | 37.8 ± 5.3 | 63.6 ± 12.3 | a,b,c,d,e,f |
| Ball velocity (m/s) | 37.9 ± 2.0 | 38.3 ± 1.8 | 38.4 ± 1.6 | 38.7 ± 1.7 | |
| Peak kinetics | |||||
| Elbow varus torque (%BW × BH) | 4.9 ± 0.9 | 5.0 ± 0.8 | 4.8 ± 0.7 | 5.1 ± 0.7 | |
| Shoulder distractive force (%BW) | 117.6 ± 17.4 | 114.5 ± 14.6 | 112.1 ± 17.2 | 117.4 ± 14.3 | |
| Shoulder anterior force (%BW) | 40.7 ± 7.3 | 41.5 ± 7.9 | 40.9 ± 6.5 | 45.6 ± 7.0 | c,e,f |
| Shoulder superior force (%BW) | 15.0 ± 9.8 | 17.5 ± 8.4 | 18.1 ± 7.1 | 19.9 ± 8.9 | |
| Shoulder horizontal adduction torque (%BW × BH) | 5.5 ± 1.0 | 5.7 ± 1.1 | 5.5 ± 1.0 | 5.8 ± 1.0 | |
| Shoulder internal rotation torque (%BW × BH) | 5.0 ± 0.9 | 5.1 ± 0.8 | 4.9 ± 0.7 | 5.1 ± 0.7 |
BH: body height; BW: body weight.
Values are presented as mean ± standard deviation; significant differences (p < 0.05) between (a) Q1 and Q2, (b) Q1 and Q3, (c) Q1 and Q4, (d) Q2 and Q3, (e) Q2 and Q4, (f) Q3 and Q4.
Pitchers who achieved greater MER had significantly greater ball velocity compared to pitchers who had the least external rotation (Table 5). Q1 had significantly slower ball velocity compared to Q3 and Q4 (37.7 ± 1.9 vs. 38.5 ± 1.9, 38.7 ± 1.7 m/s). Decreased shoulder distraction force was noted for Q1 vs. Q3 and Q4 (110.0 ± 16.8 vs. 115.7 ± 14.5, 121.2 ± 16.8%BW, respectively) as well as for Q2 vs. Q4. Decreased shoulder superior force was noted for Q1 vs. Q4 (15.4 ± 8.2 vs. 21.2 ± 8.2%BW, respectively). Decreased shoulder superior force was also noted for Q2 vs. Q3 and Q3 vs. Q4.
Table 5.
Pitchers subdivided by maximum shoulder external rotation quartiles.
| Q1 (n = 81) | Q2 (n = 80) | Q3 (n = 81) | Q4 (n = 80) | Significance | |
|---|---|---|---|---|---|
| Kinematics | |||||
| Maximum shoulder external rotation, (deg) | 153.0 ± 5.4 | 162.2 ± 1.9 | 168.0 ± 1.6 | 177.2 ± 4.7 | a,b,c,d,e,f |
| Ball velocity (m/s) | 37.7 ± 1.9 | 38.4 ± 1.6 | 38.5 ± 1.9 | 38.7 ± 1.7 | b,c |
| Peak kinetics | |||||
| Elbow varus torque (%BW × BH) | 5.0 ± 0.9 | 5.0 ± 0.8 | 4.9 ± 0.8 | 4.9 ± 0.7 | |
| Shoulder distractive force (%BW) | 110.0 ± 16.8 | 114.7 ± 14.1 | 115.7 ± 14.5 | 121.2 ± 16.8 | b,c,e |
| Shoulder anterior force (%BW) | 43.5 ± 7.0 | 43.5 ± 7.9 | 42.2 ± 7.3 | 39.8 ± 7.2 | |
| Shoulder superior force (%BW) | 15.4 ± 8.2 | 16.7 ± 9.5 | 17.6 ± 8.2 | 21.2 ± 8.2 | c,e,f |
| Shoulder horizontal adduction torque (%BW × B) | 5.6 ± 0.8 | 5.7 ± 1.0 | 5.7 ± 1.1 | 5.5 ± 1.1 | |
| Shoulder internal rotation torque (%BW × BH) | 5.1 ± 0.8 | 5.1 ± 0.7 | 5.0 ± 0.9 | 4.9 ± 0.7 |
deg: degrees; BH: body height; BW: body weight.
Values are presented as mean ± standard deviation; significant differences (p < 0.01) between (a) Q1 and Q2, (b) Q1 and Q3, (c) Q1 and Q4, (d) Q2 and Q3, (e) Q2 and Q4, (f) Q3 and Q4.
Kinetic and ball velocity values that derived significance from quartile subdivisions for shoulder abduction and shoulder external rotation were analyzed with regression correlation coefficients (Table 6). For every 10° increase in shoulder abduction at FC, shoulder superior force increased by 2.6%BW. For every 10° increase in shoulder abduction at BR, shoulder superior force increased by 3.7%BW while shoulder distraction force increased by 11.7% BW. For every 10° increase in shoulder external rotation at FC, shoulder anterior force increased by 0.7%BW. For every 10° increase in MER, shoulder superior force increased by 2.3%BW, shoulder distraction force increased by 5.9%BW, and ball velocity increased by 0.6 m/s (1.3 mile/h).
Table 6.
Correlation coefficients for shoulder abduction and external rotation at key timepoints.
| Shoulder superior force |
Shoulder distraction force |
Shoulder anterior force |
Ball velocity |
|||||
|---|---|---|---|---|---|---|---|---|
| B | β | B | β | B | β | B | β | |
| Shoulder abduction | ||||||||
| Foot contact | 0.26 | 0.34* | – | – | – | – | – | – |
| Maximum external rotation | 0.01 | 0.01 | – | – | – | – | – | – |
| Ball release | 0.37 | 0.35* | 1.17 | 0.45* | – | – | – | – |
| Shoulder external rotation | ||||||||
| Foot contact | – | – | – | – | 0.07 | 0.25* | – | – |
| Maximum external rotation | 0.23 | 0.26* | 0.59 | 0.27* | – | – | 0.06 | 0.27* |
Represents values that derived statistical significance with p value < 0.01.
Discussion
Professional baseball pitchers with increased shoulder abduction at FC, MER, and BR had increased shoulder superior force with no significant difference in ball velocity. Increased shoulder abduction at BR specifically correlated with increased shoulder superior and distraction forces. Pitchers with greatest MER had faster ball velocity and increased shoulder distraction and shoulder superior forces.
Pitchers with increased shoulder abduction throughout the pitch motion as well as those with increased MER had greater shoulder superior forces. The most striking result was observed at BR; for every 10° increase in shoulder abduction, shoulder superior force increased by 3.7%BW (30 N in an 80 kg pitcher), which translates to approximately a 20% increase in the average peak superior force of all pitchers. This motion of shoulder abduction is primarily achieved by the middle deltoid muscle during the early cocking phase, with the supraspinatus providing minor adjustments to the position of the humeral head in the glenoid. 19 Interestingly, Itoi et al. 20 demonstrated cadaveric shoulder joints with coracohumeral ligament (CHL) deficiencies had significantly increased translation when load was applied in the superior-inferior direction at 53° of external rotation. Given this translatability to the pitching motion at moderate to extreme levels of external rotation, increased peak shoulder superior forces can hypothetically strain the CHL and rotator internal capsule, placing these structures at risk of injury by way of repetitive microtears. The CHL has been well established to limit the external rotation of the arm, with maximum tautness observed at MER, preventing inferior humeral head translation.20–25 Therefore, evaluation of pitchers who demonstrate repetitively high MER and shoulder abduction for capsular or CHL tears may be worth further investigation. It should be noted these comparisons are not directly translatable given the cadaveric nature of the aforementioned study; muscular recruitment as well as the acromion structure, which act as secondary stabilizers to superior load attenuation, were not present. 20
Normally, the supraspinatus provides a superior force of compression and preserves congruence between the humeral head and glenoid. However, at MER this structure is rotated back and instead, compensated by the subscapularis.26–28 The subscapularis provides the upper part of the anterior wall of the humeral head with both compression and support, protecting the joint against superior translation of the humeral head. By attenuating the bulk of the superior force encountered at MER, the subscapularis can also be at theoretical risk of injury with increasing MER and shoulder abduction during the pitching motion via chronic overuse. Cadaveric models with excessive superior and posterior translation have demonstrated tight posteroinferior capsules, a common finding in patients with Type II Superior Labrum Anterior and Posterior (SLAP) lesions, another potential shoulder injury these pitchers may incur.27–29
Increased distraction force was observed in pitchers with higher MER and shoulder abduction at BR. For every 10° increase in shoulder abduction at BR from the population average, shoulder distraction force increased by 11.7% BW, or 10% of the peak distractive force achieved by all pitchers. The means by which distractive forces are resisted include contraction of the rotator cuff muscles as well as the long head of the biceps, which induce a transverse shear force at the glenohumeral joint by compressing the humerus in the glenoid. This compressive-like force on the origin of the bicep tendon may in fact be exacerbated as the shoulder increasingly abducts.28,30 This in turn can contribute to risk of shoulder injury during pitching, including the development of proximal humeral epiphysiolysis, labral tears, bicipital tendinitis, and rotator cuff lesions.2,9,31,32
The degree of shoulder rotation at FC continues to be a highly discussed topic in the baseball coaching community. It is generally thought that being ‘early’ (i.e. too much shoulder rotation) or ‘late’ (i.e. either no shoulder external rotation or internal rotation) at FC can contribute to increased kinetics at the shoulder and elbow as well as cause detriments to ball velocity. The ‘Inverted W’ (where a pitcher’s shoulder is in a position of internal rotation at FC) has anecdotally been suggested to cause injury in pitchers of all levels. However, research has not been able to back up these claims. In the current study, no relationship between shoulder ER at FC and shoulder internal rotation torque or elbow varus torque was appreciated. It is possible that timing of shoulder rotations and rotational velocities have an impact on throwing arm kinetics and injury; however, these temporal parameters were not analyzed in the current study. Further investigation is warranted to evaluate the timing of arm rotation, the Inverted W position, and its relationship to throwing arm kinetics.
Prior studies have speculated a position of 90° of shoulder abduction is optimal in maximizing the functional stability of the shoulder in the plane of the scapula and in turn, ball velocity.1,5 However, our results showed quartiles near this angle of shoulder abduction at FC, MER, and BR had no significant difference in ball velocity compared to other quartiles, a finding corroborated in youth and adult populations.4,5 MER was found to demonstrate a positive relationship with ball velocity, a finding previously supported.12,13,33,34 In the current study, we found for every 10° increase in MER, ball velocity increased by 0.6 m/s (1.3 mile/h). With increasing layback during the arm cocking phase, greater stored elastic energy and stretch is generated, ultimately utilized to increase the accelerating force applied to the ball for the greatest distance during the subsequent acceleration phase. 35
Prior studies have suggested elbow varus torque is minimized at specific ranges of shoulder abduction at FC 6 and BR5,7,33 in adult pitchers, but no significance was derived at either timepoint with elbow varus torque in our study. Matsuo et al. 5 suggested 100° of shoulder abduction and 10° of contralateral trunk tilt at BR to minimize elbow varus torque. While Werner et al. 6 did not specify a range, rather suggested more generally, minimal shoulder abduction at FC. When evaluating kinetics beyond elbow varus torque, Tanaka et al. 4 demonstrated in Japanese adolescent pitchers the superior-inferior shear force exerted on the shoulder was minimized at 80.6° of shoulder abduction at BR with no change in ball velocity. This angle of shoulder abduction agrees with our findings, corresponding with the lowest quartile also had the smallest shoulder superior force. It should be noted the aforementioned study assessed forces at BR, while the current study calculated the peak kinetics during the pitching motion. We believe investigating the peak kinetics, rather than the force endured by the shoulder at FC, MER, or BR, has more clinical relevance for injury risk given peak values do not always coincide with the predefined timepoints. 36
Limitations of this study include not differentiating pitchers on the basis of shoulder abduction with other kinematics concomitantly. Prior studies have shown multivariate relationships between elbow varus torque and shoulder abduction with trunk tilt, arm slot, and external rotation.5,7,33 Ultimately, these studies establish kinetic–kinematic relationships may in fact exist through a combination of factors, where at a certain degree of shoulder external rotation combined with elevated abduction for example, may in fact be the root cause of peak kinetic values being observed. Additional evaluation of multivariate models for shoulder abduction is likely warranted. Hsu et al. 37 noted increased scapular upward rotation as a compensatory strategy to avoid symptomatic subacromial impingement in amateur baseball pitchers. Because of the inherent scapular motion, it is not possible to measure scapular movement with adhesive markers and motion capture; therefore, it is not clear whether this compensation was occurring in throwers who had abduction angles greater than 80°. Even more, a varied scapula position can affect the rotator cuff muscle function. Suboptimal marker placement, non-precise joint center estimations, and evaluation of inter-player comparisons rather than intra-player correlations are additional shortcomings of this study. Though injury risk has been implied in pitchers with increased shoulder abduction and external rotation positions, we have not reported actual injury incidence; thus, increased injury risk should not be presumed as a definitive conclusion. Future studies may consider analyzing injury incidence in pitchers with increased shoulder angles to observe more direct comparisons, though additional variables like pitch count, that may contribute to repeated microtrauma to shoulder joint soft-tissue structures, likely play an important conjunctive role and should also be considered. Lastly, though we may make these suggestions for professional pitchers, it may be difficult to implement such recommendations in a population that may not be amendable to adapting their pitching style. Some part of education in coaching and training staff is likely warranted to most effectively implement change.
Conclusion
Professional baseball pitchers with increased shoulder abduction at BR and external rotation at MER were associated with increased superior translation and distraction forces in the shoulder, potentially placing them at increased risk of shoulder injury. Pitchers can consider decreasing shoulder abduction at later stages of the pitching motion to approximately 80° to minimize shoulder superior force, with no impact on ball velocity.
Acknowledgments
The investigation was performed at Hospital for Special Surgery.
Footnotes
Declaration of Conflicting Interest: The author(s) declared the following potential conflicts of interest with respect to the research, authorship, and/or publication of this article: JD has received consulting fees from Arthrex, Linvatec, Merck Sharp & Dohme, Trice Medical, and Wright Medical Technology; royalties from Linvatec and Conmed; research support from Arthrex; and hospitality payments from Horizon Pharma.
Funding: The author(s) received no financial support for the research, authorship, and/or publication of this article.
Ethical Review: All data were de-identified, and an exemption as granted by Hospital for Special Surgery Institutional Review Board (Study# 2020-1957).
Patient Consent: Written informed consent was obtained from the participant(s) for their anonymized information to be used for research purposes.
Guarantor: JD.
Contributorship: JM, BD, and JD researched literature and conceived the study. BD was involved in protocol development, gaining ethical approval, and participant recruitment. NT was tasked with data analysis. MF assisted in data interpretation and crafting of the manuscript; JM wrote the first draft of the manuscript. All authors reviewed and edited the manuscript and approved the final version of the manuscript.
ORCID iD: Joseph E Manzi https://orcid.org/0000-0002-8825-0105
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