Abstract
[Purpose] This study aimed to examine the association among zero-position external rotation muscle strength, shoulder horizontal abduction angle, and humeral head alignment. We hypothesized that individuals who cannot exert zero-position external rotation strength may adopt a horizontal abduction position and have difficulty maintaining a centripetal position for humeral head alignment. [Participants and Methods] Eighteen professional baseball players were included in the study. A handheld dynamometer was used to measure zero-position external rotation muscle strength. The isometric shoulder zero-position external rotation muscle strength was measured, and the weight ratio of the average value of the three trials was calculated. The shoulder horizontal abduction angle and anterior and posterior glenohumeral distances were measured using magnetic resonance imaging in the abduction external rotation position. The glenohumeral distance ratios were also calculated. We investigated the relationship between zero-position external rotation strength and shoulder horizontal abduction angle, anterior and, posterior glenohumeral distances, and glenohumeral distance ratios. [Results] Zero-position external rotation strength, shoulder horizontal abduction angle, and glenohumeral distance ratio were significantly negatively correlated. [Conclusion] Athletes who cannot exert zero-position external rotation strength may have a large horizontal shoulder abduction and glenohumeral distance ratio on magnetic resonance imaging.
Keywords: Zero position external rotation strength, Shoulder horizontal abduction angle, Humerus head alignment
INTRODUCTION
Posterosuperior impingement (PSI) is a typical throwing shoulder disorder. PSI occurs when the humerus deviates from the scapular plane from the late cocking phase (from the lead foot that makes contact with the ground to the throwing shoulder that reaches maximum external rotation) to the acceleration phase (begins at shoulder maximum external rotation of the throwing side and ends at ball release)1). During this action, horizontal shoulder hyperabduction and excessive external rotation (ER) of the glenohumeral joint are forced, causing glenoid labrum and rotator cuff impingement2,3,4). Magnetic resonance imaging (MRI) in the shoulder abduction and ER (ABER) positions is used to diagnose PSI5). Previous reports have evaluated anatomical changes such as soft tissue damage and cystic bone changes in professional baseball players6,7,8). Horizontal shoulder abduction and shoulder ER angles increased in the ABER position, which aggravated the contact pressure in the posterosuperior glenoid fossa9, 10). Therefore, an increase in the shoulder horizontal abduction angle (HA) during the pitching motion is a risk factor for throwing disorders. The ABER simulates the limb position at which PSI occurs. Evaluating MRI for the glenohumeral joint position and humeral head alignment in the ABER position will help understand the risk factors for shoulder throwing disorders.
The zero position is defined as the limb position in which the scapular spine and the humerus are linearly aligned within the scapular plane. This position is reported to occur at approximately 135–155 degrees of scaption11).
The joint should keep a zero position to avoid excessive stress to the glenohumeral joint during the acceleration phase12). Hence, the humerus must remain positioned in the scapular plane. Additionally, the physical function for maintaining the zero position is crucial. Prospective studies on shoulder ER muscle strength and the incidence of disability have reported prone shoulder ER muscle strength as a risk factor for throwing disorders13,14,15). However, these muscle strengths were measured with the shoulder abducted at 90°and not at the zero position, which is close to the pitching motion. We used zero-position ER (Zero ER) shoulder strength to evaluate and treat professional baseball players with pitching disorders16, 17). Zero ER strength is the isometric shoulder external rotation strength at 135° of scaption and 90° of elbow flexion. Reportedly, such patients are unable to fully exert their Zero ER strength or maintain the starting limb position, causing compensatory horizontal shoulder abduction16) (Fig. 1). Additionally, the Zero ER strength was positively correlated with the thoracic anterior tilt angle during the pitching motion and negatively correlated with the mechanical load generated on the shoulder joint17). Athletes who exert Zero ER strength may be able to reduce the mechanical load by allowing their thorax to participate in the pitching motion17). Exerting Zero ER strength is a crucial factor for performing pitching motions while maintaining the humerus in the scapular plane without increasing the mechanical load. Large migrations of the humeral head do not occur during internal and ER in the zero position in healthy adults18). Creating a physical condition that allows players to fully exert Zero ER strength in a static state is crucial for preventing excessive horizontal shoulder abduction and humeral head displacement during the acceleration phase. However, the association between Zero ER strength, shoulder HA, and humeral head alignment on MRI remains unclear. This study aimed to examine these associations using MRI images of professional baseball players in the ABER position. We hypothesized that professional baseball players with weak Zero ER strength present with displacement of humeral head in the glenoid fossa during shoulder horizontal abduction movements.
Fig. 1.
Typical compensatory motion of zero-position external rotation test.
a: Starting position. b: Finishing position. Compensatory horizontal shoulder abduction occurs in athletes who cannot exert zero-position external rotational strength.
PARTICIPANTS AND METHODS
This retrospective study included 23 professional baseball players who underwent medical checkups at our institute from November 2017 to December 2018. The study excluded those in which both the Zero ER strength measurement and MRI could not be performed and those who underwent medical checkups in 2017 and 2018. Demographic and anthropometric data were collected from all participants. The Showa University Research Ethics Review Board approved this study (approval number: 2023-051-A). An opt-out document was made available to all participants on the website to ensure that they had the opportunity to refuse the research without any consequences.
A handheld dynamometer Mobie (SAKAI Medical Co., Tokyo, Japan), was used to measure Zero ER strength in the standing position, with the shoulder joint in the zero position, where the long axis of the humerus and scapular spine were aligned in a straight line on the scapular plane11), elbow joint at 90° flexion, forearm in the neutral position, and resistance position in the distal forearm (Fig. 2). Isometric shoulder ER muscle strength in the limb position was measured. The average values of the three trials and body weight ratios were calculated.
Fig. 2.
Zero-position external rotation strength measurement method.
a: starting position in the coronal plane; b: starting position in the horizontal plane.
An MR device, Signa EXCITE 1.5T (GE Healthcare Japan Co., Tokyo, Japan), was used to acquire MRI images. Oblique coronal images were obtained with the limb in the supine position and the shoulder joint in the ABER position. Imaging conditions were T2-weighted images, repetition time: 3,620 ms, echo time: 100.85 ms, field of view: 50 × 22 cm and 192 × 288 matrix, and slice thickness: 3 mm. A slice in which the glenoid fossa was visualized at its maximum transverse diameter on a T2-weighted image was used to measure the shoulder HA. Synapse (Fuji Film Co., Tokyo, Japan), an image analysis software, was used to measure the shoulder HA. The long axis of the humerus (line A), the line segment connecting the anterior edge (B) and posterior edge (C) of the glenoid fossa (line segment BC), and a line perpendicular to line segment BC (line D) were drawn. The shoulder HA is the angle formed by straight lines A and D (Fig. 3a).
Fig. 3.
Magnetic resonance imaging measurements of shoulder parameters in the study participants.
a: Horizontal abduction angle measurement, b: anterior glenohumeral distance (AGHD) and posterior glenohumeral distance (PGHD) measurement
Line A, humeral long axis; line BC, glenoid line; line D, perpendicular to line BC; line BE, AGHD; line CF, PGHD.
MRI was used to measure the humeral head alignment and shoulder HAs. First, a line segment (BC) connecting the anterior and posterior edges of the glenoid was drawn, and a perpendicular line-to-line segment BC was drawn from the anterior (B) and posterior (C) edges of the glenoid toward the humeral head. Intersections E and F are the intersections with the humeral head, respectively. The anterior glenohumeral distance (AGHD) was defined as the distance between the anterior edge of the glenoid fossa and intersection point E, whereas the posterior glenohumeral distance (PGHD) was defined as the distance between the posterior edge of the glenoid fossa and intersection point F19) (Fig. 3b). The AGHD-to-PGHD ratio was calculated using the glenohumeral distance (GHD) ratio. The closer the distances of AGHD and PGHD are to being equal, the closer the GHD ratio is to 1.0, indicating an equal distance between the glenoid fossa and the humeral head in the front and back.
R4.2.1 (CRAN, The R Foundation for Statistical Computing, Vienna, Austria) software was used for statistical analysis. First, we used the formula of Shrout and Fleiss20) to calculate the intraclass correlation coefficient (ICC: [1, 1], ICC: [1,3]) and standard error of measurement (SEM) to investigate the reliability of muscle strength measurement using a handheld dynamometer and MRI image measurement20, 21).
Next, we examined these correlations. Pearson’s product-moment correlation coefficient (correlation coefficient: r) was used if normal distribution was assumed after the Shapiro–Wilk test. Spearman’s rank correlation coefficient (correlation coefficient: ρ) was utilized if normality was not assumed. Moreover, professional baseball players were allocated into two groups (high and low) based on the mean value of the Zero ER strength-to-body weight ratio. Zero ER strength, shoulder HA, AGHD, PGHD, and the GHD ratio were compared between the two groups. The Wilcoxon rank-sum test was used for group comparisons, with a significance level set at p<0.05.
RESULTS
Eighteen participants, including 13 right-handed and 5 left-handed throwers, met the inclusion. All the athletes were pitchers. The mean ± standard deviations for their age, height, and weight were 26.3 ± 3.8 years, 180.3 ± 4.7 cm, and 85.6 ± 5.1 kg, respectively. The ICC (1, 1) of the Zero ER strength-body weight ratio was 0.66 (95% confidence interval [CI]: −0.196–0.957), with SEM of 1.57, and ICC (1, 3) of 0.91 (95% CI: 0.529–0.991), with an SEM was 0.02. Furthermore, the ICC (1, 1) of the HA was 0.93 (95% CI: 0.833–0.974) with an SEM of 1.89. The ICC (1, 1) for AGHD was 0.94 (95% CI: 0.866–0.979), with an SEM of 0.53. The ICC (1, 1) of PGHD was 0.94 (95% CI: 0.850–0.976), with an SEM of 0.63. The ICC (1,1) demonstrated a wide confidence interval, with a negative lower bound, indicating that the reliability based on a single measurement was insufficient. Therefore, the ICC (1,3), which reflects the average of three raters, was calculated. The result showed a high level of reliability (ICC=0.91) with a stable 95% confidence interval [0.529, 0.991], suggesting that consistency across raters was adequately ensured.
Zero ER strength was 48.7 ± 10.3 N, Zero ER strength-body weight ratio was 0.57 ± 0.1 N/BW, shoulder HA was 6.8° ± 7.0°, AGHD was 7.7 ± 2.4 mm, PGHD was 7.4 ± 2.8 mm, and GHD ratio was 1.2 ± 0.6 (Table 1).
Table 1. Shoulder measurement parameters per magnetic resonance imaging result in professional baseball players.
| Professional baseball players | |
| (n=18) | |
| Zero ER strength (N) | 48.7 ± 10.3 (36.2 to 75.1) |
| Zero ER strength (N/BW) | 0.57 ± 0.11 (0.4 to 0.9) |
| Horizontal abduction angle (°) | 6.8 ± 7.0 (−4.0 to 23.0) |
| AGHD (mm) | 7.7 ± 2.4 (3.6 to 12.2) |
| PGHD (mm) | 7.4 ± 2.8 (3.6 to 13.6) |
| GHD ratio | 1.2 ± 0.6 (0.6 to 3.2) |
Mean ± standard deviation (minimum–maximum).
Zero-position ER: zero-position external rotation; BW: body weight; AGHD: anterior glenohumeral distance; PGHD: posterior glenohumeral distance; GHD: glenohumeral distance.
A significantly negative correlation was found between Zero ER strength-body weight ratio and shoulder HA (r=−0.61, p=0.006) as well as with GHD ratio (ρ=−0.58, p=0.01). Other items showed no significant correlations (Fig. 4). Based on the mean value of the Zero ER strength-to-body weight ratio, the participants were assigned into a high group (n=7) and a low group (n=11). The between-group comparison revealed that the low group showed significantly greater shoulder horizontal abduction angle (p=0.018) and GHD ratio (p=0.018, Table 2).
Fig. 4.
Correlation coefficient.
BW: body weight; AGHD: anterior glenohumeral distance; PGHD: posterior glenohumeral distance; GHD: glenohumeral distance.
Table 2. Comparison of Zero ER strength between the high and low groups.
| Zero ER strength low group | Zero ER strength high group | p-value | |
| (n=11) | (n=7) | ||
| Horizontal abduction angle (°) | 9.9 ± 6.8 | 2.0 ± 4.5 | 0.018* |
| AGHD (mm) | 8.0 ± 2.5 | 7.4 ± 2.4 | 0.78 |
| PGHD (mm) | 6.6 ± 2.5 | 8.8 ± 3.0 | 0.17 |
| GHD ratio | 1.3 ± 0.7 | 0.9 ± 0.2 | 0.018* |
*p<0.05.
Zero ER: zero-position external rotation; BW: body weight; AGHD: anterior glenohumeral distance; PGHD: posterior glenohumeral distance; GHD: glenohumeral distance.
DISCUSSION
This study aimed to investigate the relationship between shoulder external rotation strength in the zero position and glenohumeral joint alignment on MRI in professional baseball players. The results demonstrated negative correlations between Zero external rotation strength and both the shoulder horizontal abduction angle and GHD ratio. These findings suggest that reduced external rotation strength in the zero position could be associated with greater horizontal abduction angles and poorer humeral head alignment.
Cases of pitching disorders in professional baseball players tend to demonstrate compensatory movements, such as elbow drop22, 23), early trunk rotation23,24,25,26), and shoulder horizontal abduction, when exerting Zero ER strength16).
The area and pressure of contact at the greater tuberosity and posterior glenoid rim increased when baseball players pitched with elbow drop9). Such pitching forces the glenohumeral joint repeatedly into hyperangulation2,3,4). Thus, athletes who cannot exert Zero ER strength may be forced to move the glenohumeral joint in the horizontal abduction direction during pitching. This loosened the anterior joint capsule, making it easier for the players to assume horizontal abduction in a static state. This is supported by the negative correlation between Zero ER strength and humeral head alignment observed in our study.
No correlation was found between the Zero ER strength and AGHD and PGHD; however, a negative correlation was observed between the Zero ER strength and the GHD ratio. Our results indicate that AGHD may be greater than PGHD in athletes with weak Zero ER strength. The distance between the AGHD and PGHD will be approximately the same if the anteroposterior alignment of the glenohumeral joint is well maintained, and the GHD ratio will approach 1.0. Conversely, the AGHD-to-PGHD ratio increases when the GHD ratio is large, which is considered to indicate a widened distance between the anterior glenoid and humeral head. Other studies revealed that repeated pitching motions loosen the anterior joint capsule27, 28). Humeral head alignment tends to shift from a static state due to decreased rotator cuff function5, 29,30,31,32) or a loosened anterior joint capsule10, 22, 27, 28, 33) in athletes who cannot exert Zero ER strength. Changes in humeral head alignment, as a risk factor for PSI, are thought to increase the contact pressure between the rotator cuff and posterior glenoid labrum34). Repeated pitching motions with weak Zero ER strength resulted in horizontal abduction of the glenohumeral joint limb and poor humeral head alignment. Therefore, zero ER strength is thought to be important for maintaining the glenohumeral joint in the scapular plane and the humeral head in the centripetal position. Muscle activity around the scapula is important for exerting external rotational strength during shoulder joint elevation. In the future, it will be necessary to consider the functions of the scapula35).
This study has a few limitations as it was based on retrospective data. And only a static evaluation was conducted. The relationship with actual pitching motion was not considered. However, with effect sizes of 0.58 and 0.61, a significance level of α=0.05, and a sample size of 18 participants, a post-hoc power analysis revealed statistical powers (1-β) of 0.80 and 0.86, respectively. Although the sample size was relatively small (n=18), it was considered to have sufficient statistical power. This study provided new insights into clarifying the relationship between Zero ER shoulder strength, glenohumeral joint position on MRI, and humeral head alignment. These findings might offer useful perspectives for improving treatment of pitching-related disorders. Future research should examine how this relationship evidently works during actual pitching motion.
Conference presentation
Portions of this study were presented at a conference (Japanese Society of shoulder Joint, 2024; 48(3)).
Funding
This study did not receive any funding.
Conflict of interest
There are no conflicts of interest to disclose.
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