Authors: Andrew M Busch EdDa, Tyler L Mansfielda, Morgan L Barnarda, Elizabeth L Mayioa

aDepartment of Health and Human Kinetics, Ohio Wesleyan University

Corresponding Author:
Andrew M Busch
107C Edwards Gymnasium
61 S. Sandusky St.
Delaware OH 43220
Phone: (740) 368-3864
[email protected]

Conflict of Interest and Source of Funding: There are no conflicts of interest to report and no funding was received for this study.

Andrew Busch is an assistant professor at Ohio Wesleyan University and is an alumni of the United States Sports Academy.

Gender Differences in Shoulder Strength, Range of Motion, and Functional Movement across a Division III Collegiate Swim Season.

ABSTRACT

Background: Musculoskeletal variables used to identify at-risk swimmers for shoulder injuries are inconsistent, possibly resulting from fluctuations in training volume across a competitive season or gender differences in training adaptations.

Purpose: The purpose of this study was to examine gender differences in shoulder strength, range of motion, and functional movement across a competitive collegiate swim season.

Methods: Twenty-nine healthy swimmers underwent preseason and postseason testing for glenohumeral internal rotation (IR) and external rotation (ER) range of motion (ROM); maximal voluntary isometric contraction (MVIC) strength for IR and ER; and Functional Movement Screen(FMSTM) shoulder mobility scores. Glenohumeral ROM was performed using a digital inclinometer; strength tests were performed using a handheld dynamometer and normalized for body weight.

Results:  Glenohumeral ROM significantly decreased in both arms of both genders postseason. Gender differences showed males exhibited less IR in dominant (p = .002) and nondominant (p = .001) arms preseason, and only nondominant arm (p = .004) postseason. Relative MVIC strength did not change in males postseason, whereas females showed significant improvement in IR strength for both dominant (p = .008) and nondominant (p = .005) arms, and ER strength for the nondominant arm (p = .001). Gender differences revealed males had greater relative strength for IR and ER preseason, with no differences noted between genders postseason. No differences were observed in FMSTM shoulder mobility scores between genders or across the season.

Conclusion: Decreases in glenohumeral ROM were observed in both arms of both genders in the postseason. Females noted significant increases in relative MVIC strength while males showed no differences in relative strength measures. There were no differences noted in FMSTM shoulder mobility scores based on gender.

Applications in Sport: Male and female swimmers may respond differently to the demands of a collegiate swim season when comparing shoulder strength values. Postseason glenohumeral ROM measures all resulted in marked decreases in both arms for both genders.

Keywords:  Swimming, performance, Functional Movement ScreenTM, dynamometer, isometric contraction

INTRODUCTION

The repetitive nature of swimming and the physical demands competitive programs require have demonstrated an increased risk of overuse injuries in the shoulders of swimming athletes (4,12,16,17,20,27,29). The reported injury rate for all injuries in collegiate swimming expressed as the number of injuries per athlete exposure (AE), where one AE is defined as a single NCAA-sanctioned practice or competition, is 1.48/1000 AE’s for males, and 1.63/1000 AE’s for females (16). Shoulder injuries comprise the greatest amount of all injuries reported by collegiate swimmers, with males reporting a rate of 0.51/1000 AE’s, and females report a rate of 0.60/1000 AE’s (16). Anywhere between 40-91% of swimmers will experience shoulder pain or discomfort during a given season (4,16,27,29).

Among the myriad variables involved in shoulder injuries, the relationship between shoulder pain and overuse injuries with imbalances in shoulder strength and range of motion (ROM) are often thought to contribute to such injuries (3,12,24,29). A systematic review by Hill (12) examined 29 articles investigating different risk factors for shoulder pain and injury in swimmers, and not one single risk factor consistently correlated with shoulder pain. The articles were organized into four categories of risk factors including: 1) shoulder joint anatomy and strength [internal rotation (IR) / external rotation (ER) ROM); IR:ER strength ratio, joint laxity/instability, shoulder flexibility, and glenohumeral translation]; 2) activity history [training load, volume and intensity, stroke distance and specialty, years of experience, breathing side, training equipment, cross training and stretching]; 3) demographics [sex, age, competition level]; 4) musculoskeletal factors [shoulder kinematics and dyskinesis, core stability, and pectoral length]. Of all risk factors included, only a moderate level of certainty associated shoulder pain with IR/ER ROM, joint laxity, instability, and previous injury.

Other studies investigating shoulder pain in collegiate and youth swimmers have also noted inconsistent relationships between shoulder flexibility and IR:ER strength ratios with shoulder pain (4,17,26). In addition, greater imbalances are often noted in the IR:ER strength ratios of swimmers compared to non-swimming controls (18,19,24). Strength ratios have been found to change across a single season in youth and high school swimmers (3,24), and across several years in elite adolescent swimmers (10). The use of electromyography has helped demonstrate why such proportionally larger increases in IR strength occur compared to ER strength, as the internal rotators provide a major role the ‘pull’ phase of swimming strokes compared to a minor role the external rotators deliver (5). Such changes in IR/ER strength ratios may be a factor for predisposing swimmers to future shoulder injuries.

Competitive swimming has essentially become a year-round sport where swimmers are expected to maintain somewhat constant levels of practice in the water and on dry-land throughout the off-season (12). Several gender differences have been noted in isometric shoulder strength improvements across three years in elite adolescent swimmers (10), yet two separate studies investigating collegiate swimmers found no significant gender differences after completing six weeks of shoulder strengthening exercises targeting the scapular stabilizers (11,28). It is quite possible that musculoskeletal variables often tested in swimmers (shoulder strength, ROM, joint laxity, etc.) change throughout the calendar year based on fluctuations in training volumes, especially at the collegiate level where volume significantly increases compared with high school athletes. The differences between genders may therefore contribute to inconsistencies in the literature describing shoulder pain in elite competitive swimmers.

Glenohumeral ROM and isometric strength testing of shoulder muscles, specifically those responsible for internal rotation and external rotation, creating a IR/ER ratio, are common methods used in evaluating the overall health of swimmers’ shoulders (4,17–19,24,26). The Functional Movement Screen (FMSTM) is also a commonly accepted evaluation tool to assess fundamental movement patterns (8), and has high inter-rater reliability when administered by experienced individuals (21). Previous research has indicated collegiate athletes of various sports with prior shoulder injuries or surgeries demonstrated worse overall performance in FMSTM shoulder mobility scores (6).Specific to collegiate swimmers, greater shoulder dysfunction has been positively related to the years of participation in competitive swimming (17) suggesting the FMSTM may identify global changes in upper extremity movement.

Due to the repetitive stress swimming places on the shoulders and inconsistent findings correlating pain with other musculoskeletal variables in competitive swimmers, a greater understanding is needed to delineate gender differences that may exist prior to, or as a result of a competitive season. Such information may assist coaches adapt training programs for males and females.

The purpose of this study was to examine glenohumeral ROM, shoulder strength, and functional movement across a collegiate swim season to determine if changes occur or differ between genders. It was hypothesized that both genders would notice a decrease in glenohumeral ROM, thereby negatively affecting post-season FMSTM shoulder mobility scores while increasing relative shoulder strength across a season.

Study Design

This study used an observational design in a controlled exercise laboratory to examine if gender differences exist in the shoulder strength, glenohumeral ROM, or functional movement of collegiate swimmers. The study also assessed how the measured factors influenced one another over the course of a season. Glenohumeral ROM measures were recorded using a digital inclinometer, MVIC measures were recorded using a handheld dynamometer, and the FMSTM shoulder mobility scores were all recorded preseason and postseason.

METHODS

Participants

Thirty-two healthy swimmers from a Division III NCAA university (males = 14, age =19.64 ± 1.15 years, height = 1.85 ± 0.06 m, mass = 82.42 ± 2.94 kg, females = 18, age = 19.20 ± 1.32 years, height = 1.70 ± 0.07 m, mass = 67.34 ± 2.20 kg) from a local university were recruited to participate. Subjects were included, if cleared to participate in team activities by a medical practitioner; and, if he or she completed all practice and competition events throughout the season. Participants were excluded if they reported any upper extremity injuries at the time of testing or if they were not able to participate in all practice and competitive events. A university institutional review board approved the study and written informed consent was obtained prior to testing.

Data Collection

Preseason data collection was conducted before the start of official team practices during the 2018-2019 winter season and postseason data was collected one week after conference championships concluded. All participants completed a demographic questionnaire indicating gender identification, age, athletic eligibility, dominant limb, and self-reported injury history. Height was recorded to the nearest cm, and mass to the nearest 0.1 kg using a standard stadiometer. Participants were individually screened in the FMSTM shoulder mobility, along with an impingement clearing test described by Cook et al. (7). Glenohumeral internal and external ROM was measured with participants laying supine on an examination table. One examiner stabilized the shoulder to prevent unwanted scapular motion with the shoulder abducted 90° and elbow flexed 90° while another examiner used a digital inclinometer (model ACU001: Lafayette Instrument Company, Lafayette, IN) to passively measure ROM. The average of three trials were recorded (Figure 1) (23). Manual muscle testing was performed with participants laying prone on an examination table to test maximal voluntary isometric contractions (MVIC) using a handheld dynamometer (model 01165: Lafayette Instrument Company, Lafayette, IN). Each trial was a ‘make test’ as described by Conable and Rosner (7), where participants were instructed to push as hard as possible for three seconds against a fixed dynamometer for shoulder IR and ER (Figure 2). For both IR and ER, the participants’ arm was positioned in 90° of shoulder abduction and elbow flexion. The dynamometer was placed at the distal radius. An average of three trials was recorded for each arm/position, and subsequently normalized to body mass (strength/body weight) for statistical analyses.

Figure 2

The examiner for the FMSTM data was certified in the FMSTM with over seven years of experience screening individuals. The examiners for glenohumeral ROM and MVIC were the same pre- and postseason, and all examiners demonstrated strong reliability, as intraclass correlation coefficient (ICC) scores during pilot testing of ten individuals ranged from (ICC(3, k) = 0.82 – 0.95) for all measures.

Statistical Analysis

Descriptive statistics were calculated for each measure. Chi-square analyses were performed to assess differences between gender in pre- and postseason FMSTM shoulder mobility scores. Each of the measures were normally distributed, as assessed by Shapiro-Wilk’s test. Paired-samples t tests were used to compare pre- and postseason measures in glenohumeral ROM, and MVIC. Independent-samples t tests were used to compare gender differences in glenohumeral ROM, and MVIC measures. Cohen’s d effect sizes were calculated as the difference between groups divided by the pooled standard deviation. Effect sizes were interpreted as trivial (<0.25), small, (0.25-0.5), medium (0.5-1.0), or large (>1.0) (25). Data analyses were conducted using the Statistical Package for the Social Sciences (SPSS version 26.0 for Mac; IMB Corp, Armonk, NY). Statistical significance was determined a priori at p < 0.05, with Bonferroni corrections applied for paired and independent samples t tests, generating significance at p < 0.0125.

RESULTS

Twenty-nine swimmers completed all practices and competitions. Distribution of the twenty-nine swimmers by NCAA eligibility were seniors (n=7), juniors (n=8), sophomores (n=6), and freshmen (n=8). No history of major previous elbow or shoulder injuries were reported, and no shoulder impingement was recorded during pre- or postseason testing. Relative strength measures were adjusted according to current body weight at the pre- and postseason time of data collection. Not all twenty-nine participants who completed the season encountered any time-loss injuries. In the FMSTM shoulder mobility screen, no differences were observed across the season, nor were there gender differences to report (Table 1).

Table 1. Frequency of FMSTM scores, Preseason and Postseason (Males n=13, Females n=16)

FMSTM Score Sex Preseason* Postseason*
1 Male 2 (15.4%) 2 (15.4%)
Female 2 (12.5%) 1 (6.3%)
2 Male 3 (23.1%) 4 (30.8%)
Female 1 (6.3%) 4 (25%)
3 Male 8 (61.5%) 7 (53.8%)
Female 13 (81.3%) 11 (68.8%)

 Note: FMSTM = Functional Movement ScreenTM shoulder mobility scores
*Values are counts (%)

The trend in glenohumeral ROM changes were similar across the season in both genders, as all postseason ROM measures (IR and ER for dominant and nondominant arms) significantly decreased in both (Table 2). Independent samples t tests revealed gender differences existed with IR in the preseason, as males exhibited less IR in both dominant (males = 51.23 ± 10.83°, females 64.03 ± 8.79°; t = 3.51, p= .002) and nondominant (males = 52.79 ± 8.39°, females 66.2 ± 10.31°; t = 3.78, p= .001) arms. In the postseason, the only difference was found in the nondominant arms (males = 31.51 ± 13.86°, females 47.45 ± 13.49°; t = 3.12, p= .004).

Paired-samples t tests revealed females’ strength significantly improved in three of the four measures collected (dominant and nondominant internal rotation, dominant external rotation), with no significant changes in the IR/ER ratio across the season (Table 3). Males demonstrated no significant strength improvements across the season, and exhibited a decline in three of the four measures collected, along with the dominant IR/ER ratio (although none were enough to be statistically significant).

Independent samples t tests revealed several gender differences in the MVIC data collected. In the preseason, males were significantly stronger than females relative to body mass in all four MVIC measures collected and demonstrated a significantly higher IR:ER ratio in their nondominant arm (males = 1.46 ± .209, females = 1.26 ± .175; t = -2.71, p = .041). Postseason data revealed no significant gender differences in IR or ER alone; the significantly higher IR/ER ratio remained in the nondominant arm among males (males = 1.47 ± .227, females = 1.26 ± .181;   t = -2.65, p = .014).

Table 2. Shoulder Range of Motion (degrees) for Swimmers’ Dominant and Nondominant Arms, Preseason and Postseason

Measure Gender Arm Preseason,
mean (SD)		 Postseason,
mean (SD)		 Effect Size P Value
Internal Rotation Male dominant 51.23 (10.83)♦ 39.04 (14.16) 0.34 .002*
nondominant 52.79 (8.39)♦ 31.51 (13.86)♦ 0.66 < .001*
Female dominant 64.03 (8.79) 45.52 (13.76) 0.51 < .001*
nondominant 66.20 (10.31) 47.45 (13.49) 0.55 < .001*
External Rotation Male dominant 96.54 (8.37) 78.09 (12.17) 0.62 < .001*
nondominant 90.67 (9.61) 76.32 (14.63) 0.41 .003*
Female dominant 96.75 (12.9) 84.16 (17.54) 0.29 .006*
nondominant 96.02 (10.46) 78.57 (14.61) 0.49 < .001*

*Significant change in postseason (Bonferroni Correction: P < .0125)
♦Significant sex difference  (P < .0125)
Note: Effect size = Cohen’s d

Table 3. MVIC Relative Shoulder Strength Measures for Swimmers’ Dominant and Nondominant Arms, Preseason and Postseason

Measure Sex Arm Preseason,
mean (SD)		 Postseason,
mean (SD)		 Effect Size P Value
Internal Rotators Male dominant .248 (.054)♦ .277 (.084) -0.14 .193
nondominant .262 (.063)♦ .260 (.075) 0.01 .884
Female dominant .176 (.043) .214 (.05) -0.28 .008*
nondominant .173 (.039) .209 (.04) -0.32 .005*
External Rotators Male dominant .189 (.04)♦ .167 (.07) 0.14 .179
nondominant .177 (.038)♦ .175 (.035) 0.01 .895
Female dominant .141 (.021) .166 (.04) -0.3 .017
nondominant .138 (.02) .168 (.034) -0.4 .001*
IR:ER Ratio Male dominant 1.33 (.232) 1.48 (.215) -0.24 .018*
nondominant 1.46 (.209)♦ 1.47 (.227)♦ 0.03 .726
Female dominant 1.26 (.227) 1.30 (.205) -0.06 0.384
nondominant 1.26 (.175) 1.26 (.181) 0.01 0.937

Note: Data are expressed as relative strength (kg/body weight)
MVIC = Maximal voluntary isometric contraction
Effect size for paired t tests= Cohen’s d
*Significant change in postseason (Bonferroni Correction: P < .0125)
♦Significant sex difference  (Bonferroni Correction: P < .025)

DISCUSSION

In athletes and nonathletes alike, normal IR/ER strength ratios have demonstrated approximately a 3:2 ratio (14,22),which has also been found in competitive swimmers (4,19). The sample of collegiate swimmers in this study all exhibited lower strength ratios compared to previous studies, representing greater ER strength relative to IR strength (Table 3). There were significant gender differences in the pre and postseason nondominant ratios with females exhibiting lower ratios in both measures. It is uncertain why females demonstrated a greater relative ER strength to IR strength compared to males. All participants regularly engaged in weekly team-lifting activities in the weight room two days per week. One possible explanation could be due to males adapting to a greater extent in the internal-rotator muscles (pectoralis major, latissimus dorsi, and subscapularis) compared to females through upper body exercise training.

In the preseason, males demonstrated significantly greater relative strength in dominant and non-dominant IR and ER. Postseason data changed considerably, with no relative measures greater than females. It is unclear why such gender differences existed during preseason and why males and females responded differently to the same training program. The reduced relative shoulder strength scores could possibly been a result of not training as extensively in the summer. In addition, since females recorded significantly greater improvements in relative shoulder strength of both arms during postseason testing, they could have simply responded better to the high training demand of a collegiate season.

A novel explanation for these observances may be related to body mass. Female swimmers demonstrated a significant increase in three of the four postseason relative strength measurements collected (dominant and nondominant IR, and nondominant ER) with all effect sizes in the small range (0.28-0.4). Male swimmers actually decreased in relative strength in three of the four measures (not enough to be statistically significant). Over the course of the season, all swimmers performed the same dry-land exercise regimen and followed a swim-training program designed to focus on their specific distance (sprint, middle, or long-distance) as opposed to their gender or stroke specialty. Therefore, male and female sprinters all completed the same workouts each practice. The same sustainment of practice completion for both genders also held true for middle and long-distance swimmers. This specialized training according to distance may conceivably explain the gender differences in postseason relative strength because the males weighed an average of 15.08 kg greater than females. The increased strength males demonstrated in the preseason may have been enough to handle the increased effort necessary to swim ~8,000 meters in one practice, or even across many practices. However, over the course of an entire season, repetitively swimming 20,000-25,000 meters/week with a greater body mass may have cumulatively been too stressful for males who collectively developed more chronic upper-extremity fatigue relative to females.

One of the most common methods to improve swim performance utilized by coaches is the concept of tapering which is characterized by an incremental reduction in training volumes ranging anywhere between 7 to 21 days prior to a championship race (13). Muscular power tends to decrease with prolonged endurance training due to either residual neural fatigue or muscular inhibition and tapering has been shown to increase muscular power in swimmers (15). The males in the current study completed a 14-day taper beginning their tapering two practice-days prior to the female swimmers’ 12-day taper, due to the men’s squad requiring a greater amount of time to fully rest and prepare for championships. The differences noted in post-season MVIC strength tests for the male swimmers may suggest their taper was not enough to allow full recovery for strength and power adaptations to occur. Alternatively, a longitudinal study tracking elite female collegiate swimmers across several years noted improvement or maintenance in maximal power and torque when greater amounts of high intensity training accompanied their taper compared with lower intensity training (30). However, the type of training (high intensity vs. low intensity) during the taper was not recorded for this study.  

In nonathletic populations, shoulder ROM measures are typically similar when comparing males and females (22). It is often believed that swimmers exhibit greater shoulder mobility due to the cumulative stress swimming places on the shoulder girdle, however, this is not consistently demonstrated in previous studies as Hibberd et al. (10) found no differences in posture, subacromial space, or glenohumeral ROM when comparing adolescent swimmers to non-overhead athletes. Yet, decreased glenohumeral IR and increased glenohumeral ER has been previously noted among male and female elite swimmers compared with controls in several studies (4, 25). In collegiate water polo athletes, only the dominant arms in both genders showed similar alterations in ER, most likely due to the additional throwing stress on the dominant arms (2). The findings of the study herein differ from previous studies due to both genders exhibiting significant decreases in postseason IR and ER.

The timing of when data are collected may have an impact on shoulder ROM measurements. A clinically important finding of this research noted decreases in glenohumeral IR and ER measures in both arms of both genders from the postseason measurements collected, with many effect sizes in the medium range (0.5-0.8). Such changes in ROM have not consistently been related to shoulder pain or injuries (2,4,12), and no injuries were reported among the participants. Although not measured, the possibility exists that the decrease in shoulder ROM actually helped stabilize the glenohumeral joint during high training volumes as a protective mechanism from injury, as opposed to an increase in joint laxity. Subjective questionnaire data was not collected regarding how each swimmer ‘felt’ going into championships, yet many swimmers anecdotally noted on the final day of data collection they prefer feeling ‘tight’ going into races, and they feel stronger when pulling through the water during their strokes. The swimmers did not regularly perform traditional static stretching during in-season team training; instead. They performed mostly proprioceptive neuromuscular facilitation (PNF) with a partner after a general warm-up in each practice session. An increase in upper extremity ROM would likely occur with PNF as a result of post-activation potentiation (PAP), wherein activation of the muscles involved in a movement improves their contractile performance and function (1). PNF could also perhaps improve motor control in the area, secondary to increased proprioception in the upper extremity, since ROM has been shown to improve with the use of stabilization exercises, even in the absence of stretching (31).

Given this team strategy for upper extremity mobility during the season, it may explain why testing ROM in a ‘cold’ state shows significant decreases due to cumulative tightness from season-long training demands,, However, just prior to a race and when swimmers are competing, shoulder ROM is may likely be greater than the postseason recorded values.

FMSTM shoulder mobility scores did not reflect the decreases recorded in postseason glenohumeral ROM nor did they reflect any significant differences in postseason FMSTM scores for either gender. The reciprocal pattern of the FMSTM shoulder mobility screen requires the top arm to move into shoulder flexion, abduction, and external rotation, while the bottom arm simultaneously moves into shoulder extension, adduction, and internal rotation, along with necessary thoracic extension. The absence of changes noted in the current study could imply a low sensitivity of the FMSTM to catch such changes in ROM, or perhaps due to compensations in the other joint motions involved in the screen other than glenohumeral IR and ER.

This study does have limitations when interpreting the data. A convenience sample was used from a single university and statistically significant results in MVIC strength showed small effect sizes. Many of the swimmers competed in several different strokes as opposed to ‘specializing’ in one particular stroke, which could have influenced the findings. In addition to the FMSTM shoulder mobility screen, only glenohumeral IR and ER ROM was recorded which may not fully appreciate changes that might occur in collegiate swimmers across a season. There may have been significant changes in other shoulder joint motions or thoracic mobility that could have also influenced FMSTM scores.

In summary, both genders demonstrated a decrease in glenohumeral IR and ER ROM of both arms postseason, with no differences noted in FMSTM shoulder mobility scores. In females, relative shoulder strength significantly improved in three of four measures recorded postseason, whereas males showed no significant changes in relative strength taken postseason. The influence a collegiate swim season has on shoulder strength, ROM, and functional shoulder movement in male and female swimmers is essential for coaches to understand how to reinforce the multitude of factors that can affect performance and shoulder injury risk. This data may aid coaches’ understanding of how shoulder ROM changes across a season and how genders differ in shoulder strength following a collegiate season.

APPLICATIONS IN SPORT

Based on these results, postseason glenohumeral ROM measures all resulted in marked decreases in both arms for both genders. Male and female swimmers may respond differently to the demands of a collegiate swim season when comparing shoulder strength values. In particular, females demonstrated improved strength in most measurements recorded while males did not improve in any strength measurements. Such differences may occur due to differences in body mass. Coaches may need to account for such differences in body mass when considering the tapering duration allotted for male and female swimmers.

ACKNOWLEDGEMENTS

The authors would like to thank the university swimming team for their participation in this study. The authors have no conflicts of interest that are directly relevant to the content of this article. No financial support was provided for this study. Results of this study do not constitute endorsement of the product by the authors or the National Strength and Conditioning Association.

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