Athletic Training and Sports Health Care

Original Research 

Musculoskeletal Characteristics of the Dominant Shoulder Complex in Intercollegiate Baseball and Softball Players by Position and Sport

Michelle Varnell, PhD, ATC; Karen A. Keenan, PhD, ATC; Deirdre McFate Rafferty, MS, ATC; Jennifer K. Csonka, MA, LAT; Karl A. Salesi, LPT, ATC, MA; Timothy C. Sell, PhD, PT

Abstract

Baseball and softball players demonstrate similar kinetics during throwing and sustain similar injuries at the shoulder; however, it remains unknown whether baseball and softball athletes possess similar musculoskeletal characteristics of the shoulder complex. The purpose of this study was to assess musculoskeletal characteristics of the dominant shoulder complex in baseball and softball athletes by position and sport. Forty healthy National Collegiate Athletic Association Division 1 athletes (21 softball and 19 baseball players) underwent posture, flexibility, range of motion, and strength assessment. Baseball position players demonstrated significantly greater pectoralis major (P < .001) and upper trapezius (P < .001) muscle strength compared to baseball pitchers. The findings of this study suggest that there are both unique and comparable musculoskeletal characteristics in the dominant shoulder between baseball and softball position players. These data will provide useful information regarding baseline musculoskeletal characteristics of the dominant shoulder complex in healthy softball and baseball players. [Athletic Training & Sports Health Care. 2016;8(2):70–81.]

Abstract

Baseball and softball players demonstrate similar kinetics during throwing and sustain similar injuries at the shoulder; however, it remains unknown whether baseball and softball athletes possess similar musculoskeletal characteristics of the shoulder complex. The purpose of this study was to assess musculoskeletal characteristics of the dominant shoulder complex in baseball and softball athletes by position and sport. Forty healthy National Collegiate Athletic Association Division 1 athletes (21 softball and 19 baseball players) underwent posture, flexibility, range of motion, and strength assessment. Baseball position players demonstrated significantly greater pectoralis major (P < .001) and upper trapezius (P < .001) muscle strength compared to baseball pitchers. The findings of this study suggest that there are both unique and comparable musculoskeletal characteristics in the dominant shoulder between baseball and softball position players. These data will provide useful information regarding baseline musculoskeletal characteristics of the dominant shoulder complex in healthy softball and baseball players. [Athletic Training & Sports Health Care. 2016;8(2):70–81.]

Across both sexes and all ages, softball and baseball are implanted firmly into the culture of American pastimes. Based on a recent survey from The National Federation of State High School Associations, more than 850,000 high school athletes participated in either baseball or softball during the 2011 to 2012 season.1 The National Collegiate Athletic Association reported 17,726 and 30,365 athletes participated in collegiate softball and baseball, respectively.2 The Amateur Softball Association, one of the largest national sport organizations, provides recreational and competitive softball programs for more than 3 million youth and adult athletes.3,4 The Little League organization is the largest youth sports organization with more than 2.6 million participants in more than 75 countries.5,6

Although the rules for each sport are similar and both sports require repetitive throwing, catching, and batting, there are several differences of note between baseball and softball: games last nine innings for baseball compared to seven innings for softball and ball weights are different (5 oz for baseball compared to 6.25 to 7 oz for softball). In addition, differences in field size can affect both the pitchers and position players: the standard length between bases is 90 ft in baseball compared to 60 ft in softball and the pitching distance is 60 ft 6 inches in baseball compared to 43 ft in softball. Management of pitchers and pitch count is also extremely different between sports. It is not clear why, but historically softball teams only have one or two pitchers, and these pitchers often will pitch full double-headers or entire tournaments, which could entail up to ten games over 3 days. Baseball teams often will roster and train upwards of ten pitchers, which allows each pitcher to only pitch a few innings per game and allows for several days' rest between outings.7 Currently, Little League Softball and youth baseball organizations are the only organizations enforcing pitching limitations. At the high school and collegiate levels, there are no formal regulations from governing bodies requiring the tracking of pitch counts or recovery time for either baseball or softball pitchers. Despite this, pitch counts typically are enforced voluntarily by baseball coaches at the high school and collegiate levels. Pitch count limits may be implemented more easily on a baseball team because a greater number of pitchers are carried on a single team as compared to softball, allowing for rest time and restriction of pitch numbers. Whether any of these differences between sports affect risk of injury or require unique musculoskeletal demands has not been established.

With the wide popularity and potential differences in demand between these sports, there are clinical implications that need to be considered. The principle concern with overhead athletes is identification of the adverse effects on musculoskeletal characteristics due to repetitive overhead motion and development of strategies to reduce injury risk. Overhead sports (eg, baseball, softball, swimming, and tennis) are often grouped together by clinicians. It is unknown whether this grouping is justified for treatment and preventative strategies without the support of research.

There is limited research evaluating differences and/or similarities in modifiable musculoskeletal characteristics in specific overhead athletic populations. Several studies have identified that similar joint loading, such as joint distraction force, is experienced at the shoulder during baseball and softball pitching.7–9 However, the modifiable musculoskeletal characteristics (eg, strength and range of motion [ROM]) contributing to these forces are potentially different between the two sports. The baseball pitch is similar to the mechanics of the overhead throw; however, it necessitates greater ROM and potentially greater strength for increased velocity to be achieved. Also, the windmill pitch, which primarily occurs in the frontal plane, may require more upward rotation of the scapula and may not be as reliant on external ROM for acceleration compared to the baseball pitch. Position players use similar throwing mechanics across sports; therefore, it would be expected for each group to demonstrate similar normalized strength.

However, differences in musculoskeletal characteristics of the shoulder have been found between baseball and softball position players. Hibberd et al.10 found that collegiate baseball players demonstrated greater humeral retroversion difference, internal rotation deficit, and total ROM differences compared to both softball players and non-throwing athletes. The study by Hibberd et al.10 did not compare strength or posture of the throwing shoulder between sports. Describing musculoskeletal characteristics such as shoulder complex and scapulothoracic strength, posture, and ROM that are unique to specific overhead sports and positions will provide a foundation for identifying potential risk factors for injury and enable clinicians to develop specific injury prevention and rehabilitative protocols based on sport and/or position.

Despite the similarities between baseball and softball, each has unique equipment, demands, and team composition. However, it remains unknown if baseball and softball athletes possess similar musculoskeletal characteristics of the shoulder complex; therefore, the purpose of this study was to assess musculoskeletal characteristics of the shoulder complex in baseball and softball athletes by sport and position. It was hypothesized that baseball pitchers would demonstrate decreased glenohumeral internal rotation ROM and increased internal rotation strength, while demonstrating similar posture, external rotation ROM posterior shoulder tightness, external rotation, and scapular stabilizers and pectoralis major muscle strength compared to softball pitchers. It was hypothesized that no differences would be present in posture, flexibility, or strength between baseball and softball position players. For the comparison within sports, it was hypothesized that baseball pitchers would demonstrate decreased glenohumeral internal ROM and increased internal rotation strength, while demonstrating similar posture, external rotation ROM, posterior shoulder tightness, external rotation, and scapular stabilizers and pectoralis major muscle strength, and a decreased external rotation/internal rotation ratio compared to baseball position players. It was hypothesized that softball position players would have greater internal rotation strength and a decreased external rotation/internal rotation ratio, while demonstrating similar posture, flexibility, external rotation, and scapular stabilizers and pectoralis major muscle strength compared to softball pitchers.

This article provides an understanding of sport- and sex-specific musculoskeletal characteristics (eg, flexibility, ROM, upper quadrant posture, and strength) in healthy baseball and softball populations. The findings of this study will provide normative data and elucidate the similarities and differences in musculoskeletal characteristics of the shoulder between two overhead sports, which can be used by clinicians, coaches, and strength and conditioning specialists to develop and implement rehabilitation, training, and injury prevention programs.

Methods

A cross-sectional study design was used. Ethical approval for this study was granted by an Institutional Review Board. Data were collected in a single session, lasting approximately 90 minutes, prior to the competitive season. The examiners assessing all variables were certified athletic trainers or physical therapists, proficient in postural assessment, ROM, flexibility, and manual muscle testing. Intra-tester and inter-tester reliability between test sessions using a two-way random effects model was established using a small sample of recreationally active individuals. Reliability, standard error of the mean (SEM), and minimal detectable change (MDC) for each variable are provided in Table 1. SEM was calculated as the square root of the mean square error and MDC was calculated using the formula MDC = SEM × 1.96 × √2.11


Intra- and Inter-tester Reliability and Standard Error of Measure for Musculoskeletal Characteristics of the Shoulder

Table 1:

Intra- and Inter-tester Reliability and Standard Error of Measure for Musculoskeletal Characteristics of the Shoulder

Participants

A convenience sample from a single baseball and softball program was used, from which a total of 40 National Collegiate Athletic Association Division 1 athletes volunteered to participate in the study: 19 baseball players (9 position players and 10 pitchers) and 21 softball players (19 position players and 2 pitchers) (Table 2). Inclusion criteria required participants to be between the ages of 18 and 35 years, currently rostered as an intercollegiate baseball or softball player, and cleared by medical personnel for full physical activity. Paticipants were excluded if they had sustained an upper extremity injury within the 4 weeks prior to data collection.


Athlete Demographics by Sport and Position

Table 2:

Athlete Demographics by Sport and Position

Postural Measurements

A lateral photograph was used to assess forward head posture and forward shoulder posture12–14 (Figure 1) using a digital camera, which was placed 2 meters lateral to and at shoulder height of each participant. Participants were instructed to assume a natural double leg stance and look straight ahead. The approximate center of the humeral head was identified and marked with a surgical pen, and the spinous process of the 7th cervical vertebra (C7) was marked with a spherical reflective marker. Both forward head posture and forward shoulder posture were measured using ImageJ (Wayne Rasband; National Institute of Health, Bethesda, MD). The degree of forward head posture was measured using the angle created by the intersection of the horizontal line through C7 and the line connecting the tragus of the ear to C7. Similarly, forward shoulder posture was measured using the angle created by a line connecting C7 to the point on the center of the humeral head and the horizontal line through C7.


The arc indicates the angle measured to assess the degree of forward head posture and forward shoulder posture.

Figure 1.

The arc indicates the angle measured to assess the degree of forward head posture and forward shoulder posture.

Flexibility and ROM

Passive ROM at the shoulder was evaluated using methods established previously15,16 and recorded to the nearest degree. Passive glenohumeral internal rotation and external rotation ROM were assessed based on the recommendations of Norkin and White,16 with the participant supine, the scapula stabilized to isolate glenohumeral motion, and the shoulder and elbow at 90°of abduction and flexion, respectively. Posterior shoulder tightness was assessed with the participant supine and the shoulder at 90° of flexion, the elbow resting in full flexion, and the scapula stabilized against the thorax to isolate posterior shoulder laxity.15 All flexibility measures were performed with two evaluators, one of whom was a clinician (certified athletic trainer or physical therapist) and passively moved the limb while the second evaluator measured the joint position. A total of three measured trials were performed and the average of the three trials was used for analysis.

Strength Testing

Isometric strength testing was assessed using a handheld dynamometer (Lafayette Instrument Co., Lafayette, IN) based on previously established grade 5 manual muscle testing procedures.17,18 Participants were asked to perform a make test, in which as much force as possible was exerted against the handheld dynamometer for 5 seconds, with the tester not allowing limb movement. Participants were allowed one practice trial to ensure the task was performed correctly. A total of three measured trials were performed, with a rest period of 1 minute between trials to offset fatigue. Peak force was measured to the nearest 0.1 kg and the three trials were averaged. The average of each variable was normalized to bodyweight and reported as a percentage of bodyweight (% BW = [average (kg)/bodyweight (kg)]×100), allowing for better comparison between participants. The normalized averages were used for analysis. External rotation/internal rotation ratios were calculated for each participant.

Shoulder Girdle Musculature

Shoulder internal rotation strength and external rotation strength were assessed while prone, with 90° of shoulder abduction and elbow flexion.17,18 The arm was held at mid-range for both tests, with instruction to produce an upward rotational force.

The pectoralis major muscle was assessed in a supine manual muscle testing position with the intention of isolating the lower fibers. Assessment of the lower fibers may be more relevant functionally to throwing in position players and pitching in baseball. The arm was positioned at 120° abduction, and a force was exerted by the participant in the direction of the contralateral hip while resistance was applied by the examiner on the anterior arm proximal to the elbow. During the acceleration phase of throwing, the arm moves in a diagonal pattern, bringing the arm down from a flexed shoulder/elevated humeral position and across the body. This motion uses the lower fibers of the pectoralis major muscle more than the sternal portion, which would be used more in bringing the arm up and across the body (shoulder flexion/humeral elevation); therefore, the pectoralis major muscle was assessed rather than the pectoralis minor muscle.

The full can method for assessing the supraspinatus muscle as described by Hislop and Montgomery17 was used for this study.17,19 In the current study, the full can test was performed with the participant sitting in a chair, with the arm positioned at 90° abduction in the scapular plane (40° anterior to the coronal plane) and the elbow extended. The examiner provided resistance at the distal humerus while the participant exerted an upward force.

Scapular Stabilizers

The seated flexion method for assessing serratus anterior muscles, as described by Kendall et al.18 and recommended by Hislop and Montgomery,17 was used for this study to better isolate the muscle.17,18,20 Assessment of the serratus anterior muscle was performed while seated in a chair with the shoulder at 130° of flexion.21 The examiner placed resistance over the forearm while stabilizing the scapula and the participant was instructed to produce an upward force.

The upper trapezius muscle was assessed while the participant was seated in a low back chair with the handheld dynamometer placed over the acromion and tensioned with a therapy gait belt. The participant was instructed to push upward into the dynamometer as if performing a shoulder shrug. The evaluator controlled for shearing of the dynamometer and ensured that trunk lateral flexion was minimal.17

Lower trapezius, middle trapezius, and rhomboid muscles were all assessed in the prone position with the elbow extended and the arm horizontal to the floor. For assessment of the lower trapezius muscle, the test arm was positioned in the plane of the scapula (130°).17,18,21 The middle trapezius muscle was assessed with the arm at 90° of shoulder abduction and 90° of shoulder external rotation. For assessment of the rhomboid major muscle, the arm was positioned at 90° shoulder abduction and 90° internal rotation. An upward force was produced for both trapezius and rhomboid muscle tests.17,18,22 Testing positions were similar for the middle trapezius and rhomboid muscles. Although neither position isolates either muscle, the internal rotation of the arm for the rhomboid test elevates the scapula and, given the orientation of the muscle fibers, better isolates the rhomboids and decreases the activity of the middle trapezius muscle.22

Data Analysis

Data analysis was completed using SPSS for Windows software (version 20.0; SPSS, Inc., Chicago, IL). Descriptive statistics were calculated for each of the measures and all variables were assessed for normality prior to statistical testing. Because normality was violated, Mann–Whitney U tests were used to assess dominant shoulder posture, flexibility, and strength between sports and positions. The criterion for significance was established at P = .05 a priori. Due to the large number of variables being assessed, a Bonferroni correction was used to protect against type 1 error. A correction was applied within each variable family: posture/shoulder flexibility (5 variables), shoulder strength (5 variables), and scapular stabilizer strength (5 variables). With the applied corrections, significance was set at P = .01 a priori. Due to the small sample size, post hoc power was also calculated for each analysis (Table 3) using G*Power 3.1.6 (Universität Kiel, Kiel, Germany). Adequate power was considered achieved at the level of beta = 0.80 as reasonable protection against type II error.23


Post hoc Power for Statistical Comparisons Between Groups

Table 3:

Post hoc Power for Statistical Comparisons Between Groups

Results

Median and interquartile ranges for all variables within each group are provided in Table 4. Statistical outcomes are presented in Table 5. Due to obstruction of the ear in the lateral photograph, a forward head posture measure was lost for one softball position player. Additional data loss was the result of instrumentation error while measuring the pectoralis major muscle strength (1 baseball position player, 4 baseball pitchers), supraspinatus muscle strength (1 baseball pitcher), and upper trapezius muscle strength (1 baseball position player, 2 baseball pitchers).


Comparison of Musculoskeletal Variables of the Dominate Shoulder Complex Between Baseball and Softball Players

Table 4:

Comparison of Musculoskeletal Variables of the Dominate Shoulder Complex Between Baseball and Softball Players


P Values From Mann–Whitney U Test for Between Sport and Position Comparisons

Table 5:

P Values From Mann–Whitney U Test for Between Sport and Position Comparisons

Musculoskeletal Characteristics Between Playing Position by Sports

Analysis within the baseball team was conducted by comparing dominant shoulder characteristics between pitchers and position players. No statistically significant differences in posture, shoulder flexibility, shoulder strength, and scapular stabilizer strength were identified between baseball position players and baseball pitchers. Post hoc power analysis showed that adequate power was achieved for external rotation ROM and pectoralis major muscle strength (Table 4). Within the sport of softball, statistical analysis was not performed due to the low number of softball pitchers enrolled in the study; descriptive findings are presented in Table 3.

Musculoskeletal Characteristics Between Sports by Position

Analysis between sports was conducted by comparing dominant shoulder characteristics between baseball position players and softball position players. No statistically significant differences in posture and shoulder flexibility were identified. In regard to strength, baseball position players demonstrated significantly greater pectoralis major (P < .001) and upper trapezius (P < .001) muscle strength compared to softball position players. Post hoc power analysis showed that adequate power was achieved for forward head posture, internal rotation ROM, and serratus anterior, pectoralis major, and upper trapezius muscle strength (Table 4). Statistical analysis of dominant shoulder characteristics between baseball pitchers and softball pitchers was not performed due to the low number of softball pitchers enrolled in the study; descriptive findings are presented in Table 3.

Discussion

Previous research has evaluated different characteristics within baseball and softball individually and has acknowledged both the similarities and differences between the sports.8,24 However, only limited research has been conducted assessing musculoskeletal characteristics of baseball and softball players concurrently.10 Assessment of multiple overhead sports within a single study allows for better comparison of musculoskeletal characteristics through standardization of assessment methods and study design. This study aimed to concurrently evaluate the musculoskeletal characteristics within and between the sports of softball and baseball to identify those unique to each sport and position and provide insight for clinicians about the similarities and differences in musculoskeletal characteristics of the shoulder between two overhead sports. Further, this study aimed to provide normative data about the musculoskeletal characteristics of the shoulder complex in softball athletes, which is lacking in previous research. The hypothesis that baseball players and softball players would demonstrate unique musculoskeletal characteristics was partially supported by the findings of this study, because both differences and similarities were found between sports and positions.

Musculoskeletal Characteristics Between Playing Position by Sport

No significant differences in shoulder musculoskeletal characteristics were found between baseball pitchers and position players. It was hypothesized that baseball pitchers would demonstrate decreased glenohumeral internal ROM and increased internal rotation strength, while demonstrating similar posture, external rotation ROM, posterior shoulder tightness, external rotation, and scapular stabilizers and pectoralis major muscle strength compared to baseball position players. This hypothesis was based on the unique musculoskeletal characteristics of pitchers, who are required to throw the ball at higher velocities and greater volume compared to the position players. Pitchers have been observed to have position-specific adaptations such as stronger internal rotation strength25 and a decrease in internal rotation ROM19,26 and increase in external rotation ROM27 in their pitching shoulder. Pitchers require an increase in external rotation ROM to allow for more time to transfer energy and increased internal rotation strength to accelerate the arm to increase the transfer of force to the ball, resulting in higher velocity and greater volume compared to position players.

Despite the fact that no statistically significant differences were found in ROM measures, the findings of the current study were consistent with previous research, which identified that pitchers had similar internal rotation ROM and greater external rotation ROM compared to position players.28–30 In the current study, on average the pitchers had an 11° increase in external rotation ROM compared to position players; this may be clinically meaningful and is consistent with the findings of Sauers et al., who did find a statistical significance (5°) in external rotation ROM between pitchers and position players.29 This increase in external rotation ROM would allow a greater arc of motion to accelerate through to generate a greater force for pitching the ball. A direct comparison of the degree of internal rotation or external rotation ROM to other studies is difficult due to differences in methods used. For instance, studies that measured internal rotation and external rotation ROM without stabilizing the scapula reported a greater degree of motion measured; however, the general conclusions remain consistent between studies.28,31

Posture is an important component of shoulder health. Alterations in posture can be indicative of altered scapular position and muscular strength and flexibility imbalances, which could contribute to the development of shoulder pathologies such as subacromial impingement syndrome.32–35 For the purposes of this study, it was of interest to see if a difference in posture was present between sports or positions; however, the findings in the current study indicate that there were no differences present in posture between baseball pitchers and position players. It is possible that the lack of significant differences in posture may be due to the inclusion of participants cleared for full, unrestricted activity with no history of shoulder injury in the 4 weeks prior to testing.

Although there was no difference in internal rotation strength between positions, the difference present in pectoralis major muscle strength between positions may be clinically meaningful. The pitchers on average demonstrated 11% BW greater pectoralis major muscle strength compared to the position players. Although these findings were not hypothesized, they are not completely surprising. The pectoralis major muscle, which is a humeral adductor, has been shown to be highly active during the acceleration phase of overhead movements.36,37 An increase in pectoralis major muscle strength could contribute to an increase in force production during the acceleration, which is specific to the demands of pitching.

It was hypothesized that softball position players would have greater internal rotation strength and a decreased external rotation/internal rotation ratio, while demonstrating similar posture, flexibility, external rotation, and scapular stabilizers and pectoralis major muscle strength compared to softball pitchers. These hypotheses were formulated based on the differences in traditional throwing mechanics softball position players would use compared to the windmill pitch softball pitchers use. Due to the position of the arm during the acceleration phase, the position players would need stronger shoulder internal rotators using the traditional throwing motion, whereas the windmill pitchers would need stronger external rotators. Previous research has reported that the shoulder external rotators are highly active during the acceleration phase of the windmill pitch.38 Due to the low participant numbers for the softball pitching group, statistical analysis was not used.

The softball pitchers on average demonstrated 15° more posterior shoulder tightness and 20% BW greater upper trapezius muscle strength compared to the position players, which could be clinically meaningful and warrants further investigation. Posterior shoulder tightness and upper trapezius muscle strength demonstrated large effect sizes (d = 1.73 and 1.3, respectively). If adequate power had been achieved, the differences between groups could have been statistically significant. Upper trapezius and serratus anterior muscle strength were both outside the reported SEM, indicating that differences were outside measurement error. If these findings were to hold true in a larger sample, it could mean that greater strength in the upper trapezius and serratus anterior muscles is a unique adaptation necessary for the mechanics of the windmill pitch. The windmill pitch is performed primarily in the frontal plane; this abducted position of the arm from windup through the top of the back swing would require upward rotation of the scapula, thus challenging the upper trapezius and serratus anterior muscles, which are primary scapular upward rotators. Increased strength of these muscles would be necessary to maintain optimal glenohumeral alignment.

This is the first study to present descriptive data of softball pitchers compared to softball position players; therefore, comparison to previous literature is not possible. The current study provides preliminary baseline data for healthy softball players that may be of use to clinicians working with softball athletes and researchers in the development of future softball studies.

Musculoskeletal Characteristics Between Sports by Position

It was hypothesized that baseball pitchers would demonstrate decreased glenohumeral internal ROM and increased internal rotation strength, while demonstrating similar posture, external rotation ROM, posterior shoulder tightness, external rotation, and scapular stabilizers and pectoralis major muscle strength compared to softball pitchers. Again, due to the low number of softball pitchers, statistical analysis was not used. The baseball pitchers on average demonstrated 11° greater external rotation ROM, 20° less posterior shoulder tightness, decreased external rotation/internal rotation ratio, and 10% BW less upper trapezius muscle strength compared to the softball pitchers, which could be clinically meaningful and warrants further investigation. External rotation ROM, posterior shoulder tightness, and external rotation/internal rotation strength ratios all demonstrated large effect sizes (d = 1.44, 2.85, and 1.29, respectively), which indicates that if adequate power had been achieved the differences between groups could be statistically significant. If these findings were to hold true in a larger sample, they could identify unique musculoskeletal characteristics between the baseball and softball pitchers.

The hypothesis that no differences would be present in posture, flexibility, or strength between baseball position players and softball position players was partially supported. No differences were present in posture between baseball pitchers and softball position players, with adequate power achieved for forward head posture. Raine and Twomey14 also found no differences in posture when comparing asymptomatic males and females.

No differences in glenohumeral flexibility were present. The analysis of position players between sports identified that baseball players demonstrated significantly greater pectoralis major muscle strength (baseball: 41%; softball: 23.6% BW) and upper trapezius muscle strength (baseball: 59.4%; softball: 38.1% BW). Both of these variables achieved adequate power and group differences were outside the reported SEM and MDC, indicating that the differences were outside measurement error. Differences of 17.4% BW for the pectoralis major and 21.3% BW for the upper trapezius muscles are clinically meaningful differences between groups. The pectoralis major muscle, which was weaker in softball players, is primarily active during the acceleration phase of throwing.

The decreased strength demonstrated in the softball position players compared to the baseball position players may be a function of shorter throwing distance necessitated in softball. Upper trapezius muscle strength was also significantly weaker in the softball position players compared to the baseball position players, which was opposite of the trend noted in the comparison of pitchers between sports. This is of note given the similarities across all other variables. Position players, with the exception of throwing distance, use similar throwing mechanics across sports; therefore, it would be expected for each group to demonstrate similar upper trapezius muscle strength. Weakness of the upper trapezius muscle may decrease the amount of scapular elevation and upward rotation during humeral elevation, potentially placing the shoulder at risk for subacromial impingement.

No significant differences in ROM were found between baseball and softball position players. Hibberd et al.10 found significant differences in bilateral deficits of glenohumeral rotation, with the baseball players demonstrating increased glenohumeral internal rotation deficit, and total ROM differences compared to softball players. This study did not evaluate deficits, based on bilateral comparisons, and the direct measures of the dominant shoulder flexibility did not show any significant differences. However, the findings of Hibberd et al.10 indicate that there may be other unique musculoskeletal adaptations that were not evaluated in this study.

Limitations

A recognized limitation of this study was the small sample size. For variables that did not achieve adequate power, it cannot be concluded that the characteristics that were not statistically different are actually different and not a result of type II error. Future studies should aim to assess a larger sample group across multiple teams to gain better generalizability to the softball and baseball population. In addition, future research should prospectively evaluate these shoulder complex characteristics relative to injury risk in both populations. Although it may be expected that both populations should have similar ROM and strength, there could be unique risk factors for injury in each population that are yet unknown. This study did not use a non-overhead athletic (control) group; therefore, it cannot definitively be concluded that the differences between baseball and softball players are based on sport or simply a difference between genders. Despite this, because each sport is gender-specific at the collegiate level, the presence of difference between groups is still clinically meaningful for clinicians working with these sports.

Implications for Clinical Practice

Little research with concurrent data collection is available comparing musculoskeletal characteristics of baseball and softball players. This study provides necessary descriptive data for musculoskeletal characteristics of the dominant shoulder in both pitchers and position players for baseball and softball athletes.

The findings of this study suggest that there are unique musculoskeletal characteristics in the dominant shoulder that may be affected by sport and position. The findings also indicate that within sports, baseball pitchers require increased external rotation ROM and pectoralis major muscle strength compared to baseball position players, which may result from increased throwing volume and contribute to increased throwing velocity. Softball pitchers demonstrated increased posterior shoulder tightness and require greater upper trapezius muscle strength compared to softball position players, which may be a result of repetitive throwing in the frontal plane. Between sports, softball pitchers demonstrated increased posterior shoulder tightness and greater upper trapezius muscle strength, while the baseball pitchers demonstrated increased external rotation ROM and decreased external rotation/internal rotation strength ratio. These findings highlight the potential differences in musculoskeletal demands/adaptations between the windmill pitch and baseball pitch. Baseball and softball position players use similar mechanics for throwing and demonstrated similar characteristics in regard to ROM and posture; however, the baseball position players did demonstrate increased pectoralis major and upper trapezius muscle strength, which may be a result of gender differences or differences in field size and throwing distance.

This study provides clinicians with quantitative normative data regarding the musculoskeletal characteristics of the dominant shoulder by sport (baseball or softball) and position (pitcher or position player). These data will provide clinicians with a means to evaluate and identify suboptimal characteristics and can guide rehabilitation and preventative treatment protocols based on the unique demands of each sport and position.

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Intra- and Inter-tester Reliability and Standard Error of Measure for Musculoskeletal Characteristics of the Shoulder

PARAMETERINTRA-TESTER RELIABILITYINTRA-TESTER RELIABILITY 95% CIINTRA-TESTER SEMINTRA-TESTER MDC (95%)INTER-TESTER RELIABILITYINTER-TESTER RRELIABILITY 95% CIINTER-TESTER SEMINTER-TESTER MDC (95%)


LOWER BOUNDUPPER BOUNDUPPER BOUNDLOWER BOUND
Posture
  FHP (degrees)0.9610.8260.991240.9500.7770.98925
  FSP (degrees)0.394−1.6840.8635130.8690.4200.97038
Flexibility
  IR (degrees)0.8400.2920.964380.723−0.2280.938411
  ER (degrees)0.9100.6020.9804120.8640.5290.976513
  PST (degrees)0.8860.4960.9744120.7910.0710.953721
Isometric muscular strength
  IR strength (% BW)0.9830.9230.996130.8530.3500.96727
  ER strength (% BW)0.9910.960.998120.9200.6450.98226
  Pectoralis major (% BW)0.9540.7970.990390.9890.9510.99825
  Supraspinatus (% BW)0.9900.9560.998120.9850.9350.99712
  Rhomboids (% BW)0.8530.3490.967260.8660.4040.97025
  Serratus anterior (% BW)0.8660.4060.970260.8620.3900.96926
  Upper trapezius0.9760.8940.9955130.9390.7300.986719
  Middle trapezius (% BW)0.8380.2830.964250.9730.8820.99415
  Lower trapezius (% BW)0.9060.5830.979140.9430.7490.98714

Athlete Demographics by Sport and Position

PARAMETEROVERALLPITCHERSPOSITION
Baseball
  N19109
  Age (years)19.7 ± 0.819.5 ± 0.719.9 ± 1.1
  Height (cm)185.8 ± 6.2188.2 ± 4.6183.2 ± 7.0
  Weight (kg)89.9 ± 8.291 ± 8.388.6 ± 8.5
Softball
  N21219
  Age (years)19.3 ± 1.320 ± 019.3 ± 1.3
  Height (cm)166.8 ± 5.1172.7 ± 3.6166.2 ± 4.9
  Weight (kg)68.2 ± 9.570 ± 7.668.0 ± 9.9

Post hoc Power for Statistical Comparisons Between Groups

PARAMETERBASEBALL POSITION VS BASEBALL PITCHERSBASEBALL POSITION VS SOFTBALL POSITION
Posture
  FHP (degrees)0.070.83a
  FSP (degrees)0.060.63
Flexibility
  IR (degrees)0.050.81a
  ER (degrees)0.81a0.51
  PST (degrees)0.550.55
Isometric muscular strength
  IR strength (% BW)0.560.71
  ER strength (% BW)0.510.59
  Pectoralis major (% BW)0.84a0.99a
  Supraspinatus (% BW)0.530.73
  Rhomboids (% BW)0.500.71
  Serratus anterior (% BW)0.690.98a
  Upper trapezius0.560.99a
  Middle trapezius (% BW)0.510.72
  Lower trapezius (% BW)0.050.79

Comparison of Musculoskeletal Variables of the Dominate Shoulder Complex Between Baseball and Softball Players

PARAMETERBB POSITIONBB PITCHERSSB POSITIONSB PITCHERS




NMEDIANIQR [25TH, 75TH]NMEDIANIQR [25TH, 75TH]NMEDIANIQR [25TH, 75TH]NMEAN ± SD
Posture
  FHP (degrees)949[47, 53]1048[46, 49]1845[43, 48]244 ± 8.45
  FSP (degrees)955[50, 62]1053[47, 58]1950[43, 59]252 ± 10.6
Flexibility
  IR (degrees)950[42, 55]1050[41, 55]1958[49, 62]258 ± 6
  ER (degrees)9100[99, 112]10111[101, 120]19102[93, 115]2100 ± 4
  PST (degrees)9109[103, 114]10112[106, 119]19107[103, 111]292 ± 7
Shoulder girdle muscular strength
  IR strength (% BW)916.0[12.8, 20.2]1017.7[12.1, 20.1]1913[11.0, 16.1]214.6 ± 6.2
  ER strength (% BW)917.4[14.0, 19.2]1016.9[14.0, 19.3]1916[12.8, 17.4]218.0 ± 4.2
  ER/IR ratio91.10[0.80, 1.25]101.00[0.95, 1.1]191.1[1.1, 1.2]21.3 ± 0.3
  Pectoralis major strength (% BW)a841.0[35.8, 44.0]630.3[23.9, 38.2]1923.6[20.8, 29.4]224.0 ± 2.3
  Supraspinatus strength (% BW)912.5[10.0, 14.4]911.8[10.4, 13.5]1910.6[8.5, 11.6]210.5 ± 0.3
Scapular stabilizer muscular strength
  Rhomboids strength (% BW)910.7[9.7, 13.4]1010.4[7.5, 14.2]199[7.8, 11.2]210.5 ± 4.1
  Serratus anterior strength (% BW)912.8[11.0, 13.4]1010.8[8.1, 13.1]1910.1[9.5, 11.9]212.1 ± 1.2
  Upper trapezius strength (% BW)a859.4[50.8, 71.6]851.1[27.1, 64.0]1938.1[34.8, 42.7]258.5 ± 18.5
  Middle trapezius strength (% BW)98.9[6.9, 10.2]108.5[6.9, 10.7]197.5[6.1, 8.3]28.2 ± 2.1
  Lower trapezius strength (% BW)99.5[9.5, 10.9]109.7[7.4, 11.7]197.2[5.6, 9.1]27.0 ± 1.9

P Values From Mann–Whitney U Test for Between Sport and Position Comparisons

PARAMETERBB POSITION TO SB POSITION P VALUEBB POSITION TO BB PITCHER P VALUE
Posture and flexibility
  FHP (degrees).278.023a
  FSP (degrees).356.061
  IR (degrees).968.156
  ER (degrees).113.923
  PST (degrees).315.699
Shoulder isometric strength
  IR strength (% BW).842.156a
  ER strength (% BW).78a.243
  ER/IR ratio.356.664
  Pectoralis major strength (% BW).059a< .001a,b
  Supraspinatus strength (% BW).546.028
Scapular stabilizer strength
  Rhomboids strength (% BW).497.085
  Serratus anterior strength (% BW).278.037a
  Upper trapezius strength (% BW).195< .001a,b
  Middle trapezius strength (% BW).842.129
  Lower trapezius strength (% BW).604.172
Authors

From the Department of Sport and Movement Science, Salem State University, Salem, Massachusetts (MV); the Department of Sports Medicine and Nutrition/Neuromuscular, University of Pittsburgh, Pittsburgh, Pennsylvania (KAK); the University of Pittsburgh Warrior Human Performance Laboratory, Hurlburt Field, Florida (DMR); the University of Pittsburgh, Pittsburgh, Pennsylvania (JKC, KAS); and the Department of Sports Medicine and Nutrition, University of Pittsburgh, Pittsburgh, Pennsylvania (TCS).

The authors have no financial or proprietary interest in the materials presented herein.

Correspondence: Michelle Varnell, PhD, ATC, 352 Lafayette St., Salem, MA 01970. E-mail: mvarnell@salemstate.edu

Received: October 23, 2014
Accepted: October 28, 2015

10.3928/19425864-20160204-04

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