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Shoulder Joint and Muscle Characteristics Among Weight-Training Participants With and Without Impingement Syndrome

Kolber, Morey J.1; Hanney, William J.2; Cheatham, Scott W.3; Salamh, Paul A.4; Masaracchio, Michael5; Liu, Xinliang6

Author Information
Journal of Strength and Conditioning Research: April 2017 - Volume 31 - Issue 4 - p 1024-1032
doi: 10.1519/JSC.0000000000001554
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Weight-training (WT) has been advocated as a means of developing muscular performance for injury prevention (18), rehabilitation (3), and fitness-related endeavors (2,17). Despite the well-documented benefits ascribed to WT, participation is not without risk, as researchers have reported injuries associated with participation at both competitive and recreational levels (19,24,32). Although improper use of equipment and loss of control using free weights are known sources of injury (27), reports are not limited to trauma, as WT injuries range along the spectrum of acute to adaptive disorders.

The shoulder complex, in particular, accounts for a considerable proportion of injuries attributed to WT (19,22,26,32,34). Evidence suggests that up to 74% of WT participants have experienced shoulder pain during training within the past year (32). Moreover, at any given time, up to 26% of those surveyed have reported shoulder pain during WT in the past 3 days (32,34). The susceptibility of the shoulder complex to injury is in part because of the demands placed on the shoulder during WT, which may predispose the shoulder to injury (31,32). From a biomechanical perspective, WT places considerable stress on the shoulder complex, requiring a traditionally non–weight-bearing joint to function as a weight-bearing joint during the course of repetitive lifting under heavy loads. Although load itself has not been identified as an independent risk factor (31), improper exercise form or training patterns carried out repetitively under heavy load present a viable injury source. Moreover, common training practices emphasize muscles that produce obvious gains in strength and hypertrophy, subsequently neglecting key stabilization muscles, potentially leading to impaired shoulder function (6,14,32,34).

Although the underlying origin of shoulder pain among WT participants is multifactorial, subacromial impingement syndrome (SIS) has been implicated as an etiological source of symptoms (31,32). The condition of SIS generally occurs when the rotator cuff tendons and subacromial bursa get impinged between the bony structures of the shoulder during movements requiring the arm to be raised overhead. This space where impingement occurs is referred to as the subacromial or suprahumeral region. Kolber et al. (32) investigated the prevalence of SIS among recreational WT participants and compared findings with an asymptomatic control group (non-WT participants). In the aforementioned investigation, 20% of WT participants tested positive for SIS (based on a validated clinical testing cluster) were compared with only 5% of controls (p < 0.001). Of the WT participants testing positive for SIS, a significantly greater proportion reported (at the time of data collection) having experienced shoulder pain during the past 3 days (p = 0.034) when compared with WT participants who had negative results of the SIS testing cluster. It should be noted that none of the participants identified an interruption in training despite reports of pain. Moreover, delayed onset muscle soreness was not considered pain for the purpose of the study.

Various factors are thought to contribute to and perpetuate SIS among WT participants. These factors include but are not limited to performance of exercises deemed to place the shoulder in biomechanically undesirable positions that promote SIS and training practices that lend to biased muscle recruitment. Biased muscle recruitment is of considerable interest as it may create imbalances of muscles that are required to synergistically function (coactivation) for mechanically correct impingement-free mobility (10,11,41,45,46). With respect to the shoulder abductors and external rotators, evidence exists to support the premise that muscle imbalances may be responsible for SIS. Specifically, during shoulder abduction, the vector of force from the abductors creates a superior translation of the humeral head (41,45,46). Thus, without coactivation (synergistic) of the external rotators, which creates an opposing inferior translation as a secondary function, SIS could occur (41,45).

Normal overhead mobility in the coronal plane (abduction) as seen with exercises such as lateral deltoid raises requires concurrent external rotation as a means of preventing impingement of the rotator cuff and bursa. External rotation provides the biomechanical benefit of increasing space in the region between the bony structures where impingement would otherwise occur (43). From a range of motion perspective, the subacromial space is more narrow at the 90° elevation (reaching overhead) position (20,21), thus making this angle a risk point for exercises requiring elevation (such as abduction) that traditionally are not performed with external rotation. Previous research investigating the effect of aberrant shoulder positioning during WT has identified strong associations between performing lateral deltoid raises above 90° or upright rows with elbows above a height of 90° (Φ ≥ 0.30) and the presence of SIS (32). The phi (Φ) coefficient used in the aforementioned study is a measure of association for categorical data, and values of ≥0.30 indicate a strong association. Moreover, significant differences were present (p ≤ 0.004) when comparing SIS among WT participants who performed either lateral deltoid raises or upright rows to a height above 90° when compared with those who did not. These findings support biomechanical literature, which proposes that abduction with the arm internally rotated may lead to or perpetuate SIS (20,21). Exercise selection that favors larger muscle groups (e.g., deltoids or upper trapezius) while neglecting stabilizing muscle groups (e.g., external rotators and lower trapezius) has been identified as a risk factor for SIS and general shoulder pain among WT participants (14,30,32,34). A recent investigation of WT participants identified a decreased prevalence of SIS among participants who routinely performed strengthening of the external rotators when compared with those who did not (p < 0.001) (32). Although it has been postulated that biased training patterns promote muscle imbalances that predispose WT participants to SIS, a paucity of evidence exists to support this premise.

The purpose of this study was to determine if WT participants with SIS present with risk-related joint and muscle adaptations (muscle imbalances and mobility impairments). Additionally, we sought to determine if there is a significant difference in joint and muscle characteristics among WT participants with and without SIS. To our knowledge, there are no previous studies that have evaluated risk-related shoulder complex adaptations among WT participants with SIS. Moreover, an investigation of WT participants with and without SIS is necessary to determine if indeed joint and muscle adaptations are present among these individuals who present with SIS. Identifying aberrant shoulder joint and muscle characteristics in the WT population may provide the basis for detecting risk factors associated with shoulder disorders a priori and provide evidence for specific injury prevention efforts and exercise modifications. We hypothesized that a significant difference would be present for all tested dependent variables with reduced strength (external rotators and lower trapezius) and mobility among WT participants with SIS. Additionally, we hypothesized muscle ratio imbalances to be greater among WT participants with SIS.


Experimental Approach to the Problem

This investigation was a descriptive comparison of shoulder joint and muscle characteristics among men who participate in WT. The independent variables were group assignment consisting of WT participants with and without SIS based on a previously validated testing cluster that includes testing positive for both the Hawkins-Kennedy test and having a painful arc sign (44). The dependent variables measured to describe joint and muscle characteristics were (a) mean bodyweight-adjusted shoulder muscle strength values (abductors, external rotators, internal rotators, upper trapezius, lower trapezius), (b) shoulder agonist to antagonist strength ratios (internal rotator to external rotator, abductor to external rotator, upper trapezius to lower trapezius), and (c) active range of motion (AROM) (flexion, abduction, internal rotation, external rotation). The dependent variables investigated have previously been associated with shoulder disorders in the general and athletic populations, therefore were of interest for identifying intrinsic risk factors among the WT participants (5,7,9,12,41). To our knowledge, and as previously stated, no previous investigations have quantified shoulder strength ratios in the WT population with SIS.


The project was approved by the Institutional Review Board at Nova Southeastern University, and all participants provided informed consent before being enrolled in the study. All questions were answered to each participant's satisfaction by the principal investigator before commencing data collection.

Fifty-five male adults, aged 21–56 (mean age 27) years, who participated in WT (based on survey reports entering this study) at a frequency of 2–5 d·wk−1 (mean 3 days) were recruited from a University setting and local fitness centers over a 3-year duration. Participants included 24 individuals presenting with SIS based on a validated clinical testing cluster and 31 individuals who did not have SIS based on the testing cluster. Experience with WT, before data collection, ranged from 12 weeks to 30 years (mean 9 years). Participants were surveyed on the type of upper-body exercises that they routinely performed as part of their WT program. The survey included pictures of exercises with an open-ended description section. The survey was reviewed by the primary researcher to provide any needed participant clarification. Greater than 75% of participants reported performing at least 3 or more of the following exercises: (a) flat bench press (free weight or Smith machine), (b) incline bench press (free weight or Smith machine), (c) chest flies (supine or incline), (d) military press (dumbbells or barbells), (e) latissimus pull-downs to the front, and (f) lateral deltoid raises as part of their routine. The nondominant arm was used for data collection among participants to control for factors common to the dominant extremity, which may confound the results, such as compulsory use or participation in throwing sports. In cases where participants reported ambidextrous use, their writing or throwing arm was considered dominant. Participants were excluded from participation if they had partaken in professional bodybuilding or competitive power lifting or participated in overhead sports with their nondominant extremity. Individuals who had received orthopedic or neurological medical care for the nondominant upper extremity or cervical spine in the past 6 months were excluded from participation as well.

Because the nondominant arm was of interest, participants in the SIS group were required to test positive for both the Hawkins-Kennedy Maneuver and Painful Arc Sign on their nondominant extremity. According to the literature, this testing cluster has a positive likelihood ratio of 5.05 for identifying SIS (44). Individuals in the non-SIS were required to have no pain during training or activities of daily living in the past 72 hours and did not satisfy the SIS testing cluster.

Statistical analysis revealed no significant differences (p ≥ 0.28) between groups for the variables of age, height, body mass, body mass index, WT days per week, and participation years. Descriptive subject characteristics are listed in Table 1.

Table 1.:
Demographic characteristics of participants.


Testing was conducted in an air-conditioned environment, and all participants had not trained on the day of testing. Testing was routinely carried out between 12:00 pm and 7:00 pm. There were no instances where participants were recruited during a time of required fasting. Standardized warm-up exercises were completed by all participants before testing and included the pendulum exercise and standing scapular adduction without resistance. The warm-up lasted approximately 3 minutes and was not intended to offer a mobilizing effect or influence symptoms in any manner. The principal investigator (M.J.K.), a licensed physical therapist who holds a board specialist certification in orthopedics, conducted all testing. The detailed procedures (with illustrations) used for measuring the dependent variables have previously been described and reported among a similar cohort of WT participants (30,32). A trained assistant recorded measurements on a data collection sheet, which the investigator was blinded to during data collection. All tests were performed in consecutive order, and participants were provided with illustrations of the testing positions before performance. During all seated tests, participants were placed in an armless chair with their trunk supported, feet flat on the floor, and a stabilization belt to restrict trunk movement.

Strength Testing

Strength tests were performed with a microFET2™ digital handheld dynamometer (HHD) (Hoggan Health Industries, Draper, UT). All strength tests were performed for 3 repetitions with 6-second hold times. Participants were provided with a 10-second rest between repetitions and a 3-minute rest between testing positions. A 3-minute rest was selected as it is sufficient time to allow adenosine triphosphate repletion as required for optimum muscle contraction. For each series of strength measurements, the peak score of the attempts was used to derive the group mean. Strength ratios were derived by dividing the mean strength value of one measurement of interest by another.

Internal and external rotation strength was assessed in a seated position using a previously established protocol found to have excellent reliability, intraclass correlation coefficient (ICC) (3,1) = 0.97 for intrasession test-retest trials (29). Abduction strength was tested in a seated position based on previously described procedures using WT participants (30,33). The intrarater reliability of the abduction test among WT participants has been reported at ICC (3,1) = 0.96 (30). Upper and lower trapezius strength was tested using previously established muscle testing protocols (16,26,39). The upper trapezius test procedure was performed seated with the tested elbow actively flexed to 90°. Participants were asked to perform one active shoulder shrug on the tested shoulder to determine the mid-range position. The mid-range position optimized the length-tension relationship of the tested musculature (25,39) and provided a horizontal contact point for the HHD. The intrarater reliability of the upper trapezius test among WT participants has been reported at ICC (3,1) = 0.87 (30). The lower trapezius test required participants to lie prone with the proximal 50% of their arm supported on the testing table and diagonally elevated in the range of 130–145° in line with the fibers of the lower trapezius (25). The intrarater reliability of the lower trapezius test among WT participants has been reported at ICC (3,1) = 0.79 (30).

Active Range of Motion

Active flexion, abduction, and external and internal rotation ranges of motion were tested using a clear plastic 12-inch goniometer. Verbal cues and manual cues for trunk posture were provided as needed to ensure measurements were performed in the intended movement planes. Flexion and abduction measurements were performed sitting upright in a supportive chair, whereas internal and external rotations were performed in prone and supine positions, respectively. Each measurement was performed once as a warm-up to ensure correct movement followed by the actual movement that was recorded. Previous investigations have shown these procedures to have excellent intrarater reliability with ICC (3,k) ≥ 0.95, k = average of trials (35).

Statistical Analyses

Collected data were transferred to SPSS (SPSS, Inc., Chicago, IL) statistical program, Version 15.0, for Windows for analysis. The mean, SD, and 95% confidence intervals of the descriptive data from both groups were generated for comparison. Inferential statistical analysis for each test result was performed with the appropriate parametric and nonparametric tests. Continuous variables of AROM and mean adjusted strength values (strength divided by bodyweight) were compared between groups using an independent t-test. Strength ratios were analyzed as ordinal data using the Mann-Whitney U-test. The p value was considered significant at the 0.05 level using a 2-tailed test (α2 = 0.05) for all hypotheses. An a priori power analysis using the G-Power statistical software, Version, determined that a total sample size of n = 52 would be required for 80% power if a large effect size was posited.


Mean bodyweight-adjusted strength values (strength [kg]/bodyweight [kg]) of the external rotator and lower trapezius musculature were significantly less in the WT group with SIS (p ≤ 0.02). Significant differences were not identified for the abductor and internal rotator muscle groups (p ≥ 0.38) (Table 2). Upper trapezius muscle strength was significantly greater in the group with SIS (p = 0.03).

Table 2.:
Data analysis of adjusted strength values (mean strength/bodyweight).

Significantly greater median strength ratios in the WT group with SIS were identified (p ≤ 0.004) based on the Mann-Whitney U-test (Table 3). In the WT group with SIS, external rotator strength was 49% of internal rotator strength compared with 64% in the group without SIS. External rotator strength was 31% of the abductors in the WT group with SIS compared with 39% among WT participants without SIS. Lower trapezius strength was 9.6% of the upper trapezius in the WT group with SIS compared with 12% among WT participants without SIS.

Table 3.:
Strength ratio comparison for muscle imbalances.

The AROM was significantly less in the WT group with SIS for shoulder internal and external rotations (p ≤ 0.016). No significant differences were identified for flexion and abduction (p ≥ 0.37) despite pain with overhead reaching among participants in the SIS group. Results from the data analysis for AROM are presented in Table 4.

Table 4.:
Data analysis for AROM.


The goal of this investigation was to build on the current knowledge base of shoulder joint and muscle characteristics among WT participants. Previous research suggests that individuals (men and women) who participate in WT are likely to have joint and muscle characteristics that differ from those who do not participate in WT (30,33). However, before this investigation, it was unclear if such characteristics would be different among WT participants with shoulder pain or pathology. Thus, we sought to compare strength and mobility characteristics among WT participants with SIS with those without SIS. Our results support the hypothesis that WT participants with SIS have a greater loss of mobility and more pronounced strength imbalances than WT participants without SIS.

Previous data have reported an increased presence of shoulder pain among individuals who participate in WT when compared with controls; however, the actual prevalence of shoulder disorders among WT participants is likely underrepresented (32,34). Despite having at-risk training patterns (performing exercises that require undesirable positioning or biased exercise selection that neglects stabilizing musculature) and joint and muscle impairments, many WT individuals are asymptomatic with nondemanding activities of daily living, thus not seeking care. Unlike many functional activities, WT places considerable demand on the shoulder joints, potentially exposing otherwise occult pathology. Thus, pain during participation and identification of impairments associated with pathology may lend to preventative efforts.

When considering muscle performance, our findings were not unexpected. Bodyweight-adjusted strength values of the abductor and internal rotator musculature were comparable among groups; however, the upper trapezius was significantly greater in the SIS group. These findings, when coupled with significantly decreased external rotator and lower trapezius strength as identified in our results among WT participants, create imbalances of muscles that function synchronously to promote normal unimpaired mobility. This result was anticipated to occur from a training effect as the deltoids, upper trapezius, and internal rotators (i.e., pectorals and latissimus dorsi) are often targeted by common WT routines (32). Additionally, WT routines often neglect strengthening of the external rotator and lower trapezius muscle groups, thus accounting for our findings (32). In comparison with previous research, the outcomes of this investigation share similar findings and in some cases present contrasting results. Barlow et al. (6) reported greater strength of the abductor and external and internal rotator musculature among WT participants when compared with controls; however, a comparison within WT participants was not made limiting our ability to compare our data. In comparison with previously reported research on WT participants without SIS (30), the bodyweight-adjusted strength values of the external and internal rotators, upper trapezius, and abductors were greater among the WT participants without SIS in this study. These differences likely represent a well-known heterogeneity among WT participants. With regard to bodyweight-adjusted strength values of the lower trapezius and external rotators, WT participants in this investigation with SIS had reduced strength values when compared with previous investigation of asymptomatic WT participants (30).

When examining muscle performance in an active or athletic population, strength values may exceed that of the general population; therefore, alternate methods of quantifying muscle performance must be considered. Clinically, strength ratios are of greater clinical relevance as they provide an interpretation of strength between muscle groups that normally function in a synchronous manner. Strength ratios as measured in this investigation provided an interpretation of the coordinated muscle function of the internal to external rotator and abductor to external rotator muscle groups and also the upper to lower trapezius force couples. The WT group with SIS in this investigation had significantly higher median strength ratios of the tested muscle groups when compared with participants without SIS, suggestive of muscle imbalances. Moreover, the strength ratios of the SIS group in this investigation were observed to be greater than previously reported data among asymptomatic WT participants (30), further supporting the postulated relationship of muscle imbalances to SIS.

With regard to shoulder pathology, impaired muscle performance of the lower trapezius has been associated with shoulder disorders, such as impingement syndrome (10–13). From a muscle performance perspective, upper to lower trapezius imbalances (strength or firing latency) have been associated with SIS among both the general and athletic populations (40,51). The ratio of upper to lower trapezius among individuals with SIS in this investigation was 10.67 compared with 8.45 among WT participants without SIS. Previous research using similar testing procedures identified an upper to lower trapezius strength ratio of 8.04 among asymptomatic WT participants. Strength deficits of the lower trapezius in the presence of a normally functioning upper trapezius may lead to impaired upward rotation and reduced posterior tilting of the scapula during elevation of the arm, subsequently leading to impingement of the shoulder structures and clinical signs of SIS (4,12,15,37).

Additionally, abductor to external rotator and internal to external rotator muscle performance imbalances have been identified among athletic populations and also the symptomatic general population (9,41,45,52). Researchers have reported that individuals with shoulder disorders possess greater deficits in external rotation strength than internal rotation or abduction strength (38,45,47,52,53). Moreover, descriptive studies have identified muscle imbalances among athletes with shoulder disorders that may result from upper-extremity sports participation or training (5,9,13,36). During shoulder elevation, the external rotator musculature functions in a synchronous pattern with the deltoids as necessary for normal unrestricted motion (15,43,45,46). Exercise routines that emphasize the deltoid musculature and neglect the rotator cuff may create an imbalance of the deltoid-rotator cuff force couple leading to altered muscle coordination, restricted AROM, and impingement of the shoulder complex during arm elevation.

Although no consensus exists as to what constitutes normal or desired strength ratios required for biomechanically correct shoulder function, the ratios quantified in the WT group with SIS were significantly greater than those without SIS. Moreover, the values were greater than previously reported values among both asymptomatic WT participants and controls, which suggests an imbalance of muscles that function in a synergistic relationship and may therefore be associated with and perpetuate SIS.

In addition to strength impairments, mobility dysfunction has been implicated as a predisposing factor for shoulder dysfunction (23,49,50,53). We hypothesized that WT participants with SIS would have reduced AROM when compared with controls. The results partially supported our hypothesis as decreased internal and external rotation AROM values were identified among WT participants with SIS. Interestingly, flexion and abduction were not significantly different among groups in this study. Given the inclusion criteria of a Painful Arc Sign in the SIS group, we anticipated reduced AROM for flexion and abduction. Although we are uncertain of the underlying reason for the AROM findings for flexion and abduction, a comparison of previous literature sheds some light on our finding. Specifically, the flexion and abduction AROM values in this investigation are comparable with previously published ranges among asymptomatic WT participants (30). Although conjectural, one explanation may reside in habitual movement during exercises such as military press and latissimus pull-downs preserving overhead motion among participants.

Among the motions measured in this investigation, internal rotation has received considerable attention in the literature because of an association with shoulder disorders. Normative values for internal rotation in the adult population are cited as 70° (1,8), whereas the WT participants in this study with SIS averaged 58°. The findings of Barlow et al. (6) of significantly less internal rotation in the shoulders of WT participants when compared with a control group lend support to WT being a potential risk factor for reduced internal rotation. The mobility characteristics among the WT participants in this investigation with SIS have clinical implications, as reduced internal rotation has been associated with shoulder disorders in both the general and athletic populations (28,42,48,49,53). With regard to external rotation range of motion, the SIS group had significantly less mobility than those without SIS. However, values were slightly greater (94–100°) than normative reports (90°); thus, an impairment was not present (8). One might postulate that the relative increase in external rotation when compared with asymptomatic controls may be the result of positioning during more common exercises, such as latissimus pull-downs or behind the neck military press.

The findings presented in this study provide insight into shoulder joint and muscle characteristics in the WT population; however, the results may not necessarily be generalized to all WT participants as professional bodybuilders and power lifters were excluded from participation. Furthermore, the inclusion criterion was limited to men and it cannot be assumed that women would have similar training patterns or joint and muscle characteristics. Moreover, the sample size itself is a limiting factor when making inferences to the broad population at large who participates in WT. Future investigations on women who participate in WT and professional WT participants may be of benefit to further delineate risk among other cohorts as results are just generalizable to men participating at the recreational level. Additionally, this investigation should be reproduced in different geographical regions to truly make the inference that the results are representative of the WT population at large. Lastly, the predictive validity of joint and muscle characteristics as an index for injury risk in the WT population could be established through future research focused on longitudinal prospective designs.

Practical Applications

Performance of WT is often recommended by physical therapists, athletic trainers, strength and conditioning professionals, and physicians for the benefits of increased strength, hypertrophy, athletic performance, and health variables. The ability to provide safe yet effective recommendations requires an understanding of the specific biomechanical stresses and adaptations associated with the more common exercises and training practices. The results of this investigation indicate that shoulder joint and muscle imbalances exist among WT participants with SIS to a greater extent than those without SIS. The WT routines often focus on the selection of large muscle groups, such as the pectoralis major, upper trapezius, and deltoids, subsequently neglecting muscles responsible for shoulder stabilization (i.e., rotator cuff and scapular musculature). Incorporating strengthening of the rotator cuff and scapular musculature and selecting flexibility exercises for the posterior shoulder tissues into WT routines should theoretically (a) balance strength ratios as necessary for coordinated shoulder function, (b) provide soft tissue mobility balance as required for normal internal rotation mobility, (c) improve strength of the humeral head depressors to avoid impingement with overhead exercises common to WT, and (d) reduce more common risk factors associated with shoulder disorders.

Although experience may be a necessary factor for understanding the diverse application of WT program design, evidence-based judgment and scientific research will dictate the success in managing or preventing shoulder disorders associated with WT. The information gleaned from this investigation provides WT participants and those professionals who contend with exercise prescription evidence-based guidelines. Efforts to address the lower trapezius and external rotator musculature and improve internal rotation AROM may serve to reduce impairments associated with shoulder pain and injury.


The authors would like to express their gratitude to the Nova Southeastern University, Health Professions Division, Faculty Research Grant for funding this investigation.


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mobility; muscle imbalance; shoulder complex; shoulder disorders

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