DOI: https://doi.org/10.21203/rs.3.rs-2086548/v1
Subscapularis muscle strains can develop in professional baseball players, rendering the players unable to continue playing for a certain period. However, the characteristics of this injury are not well known. The purpose of the present study was to investigate the details of the injury and the post injury course of subscapularis muscle strain in professional baseball players.
Of 191 players (83 fielders and 108 pitchers) who were members of a single Japanese professional baseball team between January 2013 and August 2022, 8 players (4.2%) had subscapularis muscle strain and were enrolled in this study. The diagnosis of muscle strain was made on the basis of shoulder pain and magnetic resonance imaging findings. The incidence of subscapularis muscle strain, the details of the site of the injury, and the time to return to play were examined.
Subscapularis muscle strain occurred in 3 (3.6%) of 83 fielders and 5 (4.6%) of 108 pitchers, with no significant difference between fielders and pitchers. All players had injuries on the dominant side. Most injuries were located at the myotendinous junction and in the inferior half of the subscapularis muscle. The mean time to return to play was 55.3 ± 40.0 (range, 7-120) days. At a mean 21.0 months after the injury, there were no re-injured players.
Baseball players who have shoulder pain should be regarded as having possible subscapularis muscle strain, and proactive examination should be considered.
Not applicable
Muscle strain is one of the major sports injuries for professional baseball players.1–3 Many professional players are removed from the roster for official games as a result of muscle strains, which can place a significant burden not just on the players themselves, but also on their teams. The majority of muscle strains occurring as a result of sports activities are generally injuries to the leg muscles, such as hamstring strain.4,5 In baseball, the shoulder is subjected to a great deal of stress from pitching and other activities, and strains of the rotator cuff muscles can occur, as well as of the leg muscles.6 In Major League Baseball, the most common cause for players to miss official games is reported to be rotator cuff strain or tear, following hamstring strain. However, in that report, the details of rotator cuff strain were not unclear.
The subscapularis muscle, one of the rotator cuff muscles, stops the maximum external rotation of the shoulder during the late cocking phase of pitching and it is most active during the acceleration phase, contributing to internal rotation of the shoulder.7,8 During the late cocking to acceleration phase, the subscapularis muscle is most stretched,9 which may cause strain of the muscle due to eccentric contraction. However, there have been few reports of subscapularis muscle strain in professional baseball players and almost none in other sports.10–12 Several issues regarding subscapularis muscle strain remain unclear, such as the details of the injured site and the post injury course, as well as the difference between fielders and pitchers.
The present study investigated the incidence and site of subscapularis muscle strain and the time to return to play in a single professional baseball team.
This was a retrospective study. Of 191 players (83 fielders and 108 pitchers) who were members of a single Japanese professional baseball team between January 2013 and August 2022 and included full and partial participation, there were 8 players (3 fielders and 5 pitchers) with subscapularis muscle strain during the 10-year period, thus affecting 4.2% of all players. Those who developed subscapularis muscle strain were enrolled in this study. There were no players who were affiliated with the team for less than one season. The details of the mechanism of injury, such as hitting, throwing, and pitching, the injured side, the site of the injury, and the time to return to play, were investigated in these cases. All medical records were from the baseball team, and there were no uncounted players.
Diagnosis of Subscapularis Muscle Strain
In the present study, a diagnosis of subscapularis muscle strain was made on the basis of medical history and local shoulder pain, as well as the magnetic resonance imaging (MRI) findings with fat suppression showing high signal intensity in the same area in all cases.10 MRI was performed when the player could not play for more than one day due to the shoulder pain. During the observational period, 44 players had the pain in the shoulder, so MRI was performed in all 44 players; 14 of the 44 players had periarticular shoulder muscle strain, and 8 of those 14 players were diagnosed with subscapularis muscle strain (Figure 1). Although MRI findings were assessed by experienced orthopedic surgeons and radiologists, the reading results shown in the present study were basically those of the radiologist.
Classification of the Muscle Injury Site on MRI
The site of the subscapularis muscle injury was assessed using MRI of the axial, coronal, and sagittal planes with respect to the glenoid fossa. MRI was conducted using a 3.0-T scanner (MAGNETOM Lumina; Siemens Health Care, Erlangen, Germany). Settings for axial and coronal spectral adiabatic pulse inversion recovery (SPAIR) imaging were as follows: repetition time, 4000 ms; echo time, 65 ms; echo train length, 13; slice thickness, 3.0 mm; field of view, 180×180 mm2; matrix size, 320×259; and number of excitations, 1. Settings for sagittal SPAIR imaging were as follows: repetition time, 5170 ms; echo time, 62 ms; echo train length, 14; slice thickness, 3.0 mm; field of view, 180×180 mm2; matrix size, 320×259; and number of excitations, 2. The subscapularis muscle originates from the subscapular fossa and inserts into the lesser tubercle of the humerus. The site of muscle injury was classified into the myotendinous junction or the muscle belly, as well as whether it was in the superior or inferior half of the muscle (Figure 2).
On axial MRI with fat suppression, the site of muscle injury was classified as the myotendinous junction and the muscle belly.
Injured Side
The injured side was assessed in the case of injuries caused by batting, throwing, or pitching. In the present study, it was more straightforward to use the dominant batting side or the dominant arm as a reference point. The dominant batting side in this study was defined as the side of the batting box used.
Return to Play
For the present study, return to play was defined as participation in an official game. The time to return to play was defined as the period from injury to return to official games, even in the case of players who were able to play for a while after their injury. If the season ended before a player returned to official games, the time to return to play could not be measured, and the player was therefore excluded from this evaluation.
Statistical Analysis
The frequency of subscapularis muscle strain and the time to return to play were compared between fielders and pitchers using Fisher’s exact probability test and Student’s t-test, respectively. The level of significance was set at p<0.05. The statistical analyses were performed with BellCurve for Excel (Social Survey Research Information Co., Ltd., Tokyo, Japan).
This study was approved by our institutional review board (file no. blind for review).
The details of the 8 players are shown in Table 1. The mean age of the injured athletes was 25.0 years, and their mean body mass index was 25.5 kg/m2. The rate of occurrence of subscapularis muscle strain was 3.6% in fielders and 7.4% in pitchers, with no significant difference.
Regarding the mechanism of injury, all injuries in pitchers were caused by pitching, whereas the causes of injury in fielders included both throwing and batting. As to the injured side, all players were injured on the dominant side. In the injury site classification, all but 1 injury was located at the myotendinous junction and in the inferior half (Fig. 3). All cases were treated conservatively. None of the cases have received corticosteroid or platelet-rich plasma injections.
One of 8 cases was excluded from the measurement of time to return to play because the season ended before the player returned to official games. The overall mean time from the injury to return to play in a game was 55.3 ± 40.0 days, and the mean was 20.3 ± 13.5 days in fielders and 81.5 ± 30.6 days in pitchers, with a significant difference between them (p = 0.025). At a mean time of 21.0 months after the injury, there were no re-injured players.
On coronal and axial MRI with fat suppression, an area of high signal intensity is visible at the myotendinous junction and in the inferior half of the muscle (white arrow).
In the present study, 4.2% of players of a single Japanese professional baseball team developed subscapularis muscle strain. Most injury sites were at the myotendinous junction and in the inferior half of the muscle. A mean time of 55.3 days was needed to return to play in an official game.
A retrospective study evaluated MRIs of the shoulders of 133 professional baseball players and found signal changes in the subscapularis muscle in 11 cases. Ten of these cases were reported to be diagnosed with throwing-related subscapularis muscle strain.10 In the current study, of the 44 professional baseball players with shoulder pain that made it difficult to continue playing, eight were diagnosed with subscapularis muscle strain. Although indications for MRI imaging vary by institution, a subset of professional baseball players who have shoulder pain and require MRI have subscapularis muscle strain. Baseball players who complain of shoulder pain should be considered for proactive examination for suspected subscapularis muscle strain.
In the present study, most injury sites were at the myotendinous junction and in the inferior half of the subscapularis muscle. Polster et al. reported, similarly to the present study, that the site of injury in all 10 cases of subscapularis muscle strain was the myotendinous junction and the inferior half of the muscle.10 In addition, in previous case reports of subscapularis muscle strain, all cases occurred at the myotendinous junction and in the inferior half of the muscle.11 The superior and inferior portions of the subscapularis muscle are independently innervated and activated. It has been reported that the inferior portion of the subscapularis has significantly higher electromyographic activity at 90 degrees of shoulder abduction than the superior portion.13 During the late cocking to acceleration phase, the shoulder is abducted to about 90 degrees. Moreover, in this shoulder position, the fibers of the inferior half of the subscapularis muscle run across the glenohumeral joint, and the area is reported to be subject to the strongest tension.10,14,15 These are possible reasons why the lower half of the subscapularis muscle is susceptible to the injury.
Regarding the injured side, all cases of subscapularis muscle strain occurred on the dominant side in hitting, as well as in pitching and throwing, in the present study. All cases of throwing-related subscapularis muscle strain were reported to occur on the dominant side.10,11 In the report of a case who sustained a subscapularis muscle strain at the time of batting, the dominant side was injured, as in the present study.16 Although it depends on the batting form, the shoulder on the dominant side is abducted at about 90 degrees at the beginning of batting, so it can be assumed that the injury was caused by the same mechanism as with pitching. The details of the mechanism of subscapularis muscle strain are still not fully known, and further studies are needed in this area.
It has been reported that the mean number of days of baseball activities missed was 27 days (range, 11–16 days), and no significant difference was found in the number of days depending on the severity of the injury. In the present study, the mean time from injury to return to play was 55 days (range, 7-120 days), which is longer than in the previous report. This may be due in part to the fact that the definition of return to play differs between studies, and that each team has its own criteria for return to play. In the present study, return to play was significantly faster for fielders than for pitchers. Fielders may return to play earlier because they can participate at least partially in a game through pinch batting, pinch running, or defense only, even if they have not returned to their full performance level.
The present study has some limitations. First, it included a small number of cases limited to a single Japanese professional baseball team. Since only one team was surveyed in this study, the results may have been influenced by the circumstances and policies of this team. Second, all cases of subscapularis muscle strain could not be examined. Only the injuries that occurred during the season were examined. Furthermore, it is possible that a minor subscapularis muscle strain that did not make playing impossible for more than 1 day may have been missed. Third, it was not possible to assess the severity of the subscapularis muscle strain in this study. Therefore, there is a large variation in the time from injury to return to play. Fourth, as an indicator of return to play, the date of available to play should be evaluated, but the record was not available, and the date of return to play in an official game was evaluated in the present study. The date of return to play in a game could have been influenced by the importance of the player to the team. We intend to investigate the occurrence and recurrence of subscapularis muscle strain to gain a fuller understanding of this injury and to investigate preventive measures.
In a single Japanese professional baseball team, there were 8 cases of subscapularis muscle strain in 8 players (4.2% of all players) during a 10-year period. Baseball players who have shoulder pain should be regarded as having possible subscapularis muscle strain, and proactive examination should be considered.
MRI: magnetic resonance imaging
SPAIR: spectral adiabatic pulse inversion recovery
Ethics approval and consent to participate
This retrospective study was conducted according to the Declaration of Helsinki and approved by the institutional review board of the Institute for Integrated Sports Medicine, Kitasato University, Kitasato Institute Hospital (file no. 17030). Informed consent was waived by the ethics committee, because of the retrospective nature of the study.
Consent for publication
Not Applicable
Availability of data and materials
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.
Competing interests
The authors declare that they have no competing interests.
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Authors’ contributions
SK, MN, and HK conceived and designed the study. SK and MN wrote the paper. SK and HK performed the data analysis. All authors read and approved the final manuscript.
Acknowledgements
Not applicable
Table 1 is available in the Supplementary Files section.