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Original Research

Shoulder Joint and Muscle Characteristics in the Recreational Weight Training Population

Kolber, Morey J; Beekhuizen, Kristina S; Cheng, Ming-Shun S; Hellman, Madeleine A

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Journal of Strength and Conditioning Research: January 2009 - Volume 23 - Issue 1 - p 148-157
doi: 10.1519/JSC.0b013e31818eafb4
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Abstract

Introduction

The popularity of weight training is evident by the more than 45 million Americans who engage in strength training regularly (12). The Centers for Disease Control analyzed data from the National Health Interview Survey to determine the prevalence of strength training in the adult population from 1998 to 2004 and estimated that nearly 20% of adults aged 18-65 years participate in some form of resistance training 2 or more times a week (12). Weight training has been advocated as a means of developing musculoskeletal strength for sports (9,23), rehabilitation of injuries (4,21), and for various health and fitness benefits (3,22,48). The health and fitness benefits ascribed to weight training, however, are not obtained without risk. Researchers have reported, in both descriptive and prevalence-based investigations, numerous injuries associated with weight training participation (25,29,45,56). The incidence of injuries has increased during the past decade, with 25-30% of individuals who participate in weight training reporting injuries severe enough to seek medical attention (29,45). Research indicates that up to 36% of weight training-related injuries and disorders occur at the shoulder complex (25,31,35). Although shoulder injuries attributed to weight training have been reported in the literature (25,26,28,35,43,53), the specific adaptations and stresses imposed on the shoulder complex as a result of training practices have not been sufficiently investigated.

Weight training places considerable stress on the shoulder complex because it requires a traditionally non-weight-bearing joint to become a weight-bearing joint during the course of repetitive lifting. Common weight training exercises often place the shoulder in unfavorable positions such as end-range external rotation while under heavy loads, creating joint and muscle imbalances and predisposing the shoulder to injury (26,37). Furthermore, it has been postulated that weight training routines emphasize large muscles that produce obvious gains in strength and hypertrophy, subsequently neglecting the muscles responsible for stabilization (9,26). Normal shoulder function requires a delicate balance between mobility and strength of muscle groups that function synchronously during activities. Training routines that are biased toward specific muscle groups or exercises often neglect the required strength and mobility balance necessary for unimpaired shoulder function.

A paucity of research has described shoulder joint and muscle characteristics among individuals participating in weight training, thus limiting the ability of clinicians and strength and conditioning specialists to recognize “at-risk” training patterns and provide the evidence-based education and instruction necessary to minimize shoulder injuries/disorders in this population. Identifying aberrant shoulder joint and muscle characteristics in the weight training population may provide the basis for detecting risk factors associated with shoulder disorders a priori and also may provide evidence for specific injury prevention efforts and exercise modifications.

The purpose of this investigation was to examine shoulder joint and muscle characteristics in the recreational weight training (RWT) population and to determine whether a significant difference in joint and muscle characteristics (dependent variables) was present between individuals participating in RWT and a control group. The results will serve as the basis for describing shoulder joint and muscle characteristics within the RWT population and may identify those at risk for shoulder disorders.

Methods

Experimental Approach to the Problem

This investigation was a descriptive analysis of shoulder joint and muscle characteristics in the men's RWT population. The independent variables were group assignment consisting of the RWT and control group. The dependent variables measured to describe joint and muscle characteristics were (a) mean body weight-adjusted shoulder strength values (abductors, external rotators, internal rotators, upper trapezius, lower trapezius), (b) shoulder agonist/antagonist strength ratios (internal rotator/external rotator, abductor/external rotator, upper trapezius/lower trapezius), (c) active range of motion (flexion, abduction, internal rotation, external rotation), and (d) posterior shoulder tightness (PST). The dependent variables investigated have been associated previously with shoulder disorders in the general and athletic population; therefore, they were of interest for identifying intrinsic risk factors for injury (7,8,10,14,16,26,46). To our knowledge, there are no previous investigations that have quantified shoulder strength ratios or PST in the weight training population.

Subjects

Approval from the Nova Southeastern University (NSU) institutional review board was obtained before commencing the study. All volunteers completed an NSU institutional review board-approved informed consent before participation.

Ninety participants, men between the ages of 19-47 (mean age 28.9), were recruited from a university setting and local fitness centers during a 3-month period. Participants included 60 individuals who participated in upper-extremity RWT an average (mean) of 3 days per week (range 2-5). Weight training experience before data collection ranged from 12 weeks to 25 years (mean 8.7 years). Participants were surveyed on the type of upper body exercises that they routinely performed as part of their weight-training program. Greater than 75 percent of participants reported performing at least 3 or more of the following exercises: (a) flat bench press, (b) incline bench press, (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. Twenty-two of the 60 RWT participants reported either having their program designed by a professional who is certified or licensed to prescribe exercise programs (personal trainer, certified strength and conditioning specialist, athletic trainer or physical therapist) or being certified or licensed themselves. Thirty participants who did not participate in any type of upper-extremity weight training exercise served as controls. The nondominant arm was used was used for data collection in both groups to control for factors common to the dominant extremity, which may confound the results, such as compulsory use of the dominant extremity. In cases where participants reported ambidextrous use, the writing and/or throwing arm was considered the dominant arm. Volunteers were excluded from participation if they participated in professional body building or competitive power lifting or participated in overhead sports with their nondominant extremity.

Statistical analysis revealed no significant differences (p > 0.16) between the RWT and control groups for the variables of age, weight, height, and body mass index. Descriptive subject characteristics are listed in Table 1.

Table 1
Table 1:
Demographic characteristics of participants.

Procedures

Standardized warm-up exercises were completed by all participants before testing and included the pendulum exercise and standing scapular adduction without resistance. A trained assistant recorded measurements on a data collection sheet to which the investigator was blinded 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, each participant was placed in an armless chair with his 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) (Hogan Health Industries, Draper, Utah). 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 because 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 (Figure 1) using a previously established protocol found to have excellent reliability (intraclass correlation coefficient [ICC] (3,1) = 0.97 for intrasession test-retest trials) (34). A preconstructed arm support was placed in each participant's axillary region to maintain the tested arm in 30° of scapular plane elevation to ensure consistency of position. The arm was placed in 0° of rotation and the elbow at a 90° angle with the wrist in neutral rotation for testing. A PVC stabilization device that housed the HHD was then positioned against the wall at a level that accommodated the desired testing angles for each participant, with the circular padded contact surface of the HHD positioned at the distal aspect of the radial/ulnar region as illustrated in Figure 1. Participants applied pressure directly into the circular pad in the transverse plane for both internal and external rotation.

Figure 1
Figure 1:
Testing position for shoulder internal rotation and external rotation.

Abduction strength was tested in the seated position with the participant's arm elevated by the investigator to approximately 20° in the scapular plane, with the elbow flexed to 90°. The PVC stabilization device was positioned with the HHD at the lateral epicondyle. Each participant was instructed to grasp the seat of the chair with his nontested arm for stabilization. Each participant applied pressure to the HHD in the coronal plane with the lateral aspect of his elbow (Figure 2).

Figure 2
Figure 2:
Testing position for shoulder abduction.

Upper and lower trapezius strength was tested using previously established muscle testing protocols (20,30,41). The upper trapezius test procedure (Figure 3) 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 midrange position. The midrange position optimized the length-tension relationship of the tested musculature (30,41) and provided a horizontal contact point for the HHD. The tester stood behind the participant using a stable stool for leverage (as necessary) and placed the HHD on the superior lateral aspect of the scapula. Once positioned, participants were asked to unilaterally shrug (elevate) the tested shoulder while the investigator, with both hands and torso, met the resistance equal to the participant. Verbal cues, in addition to the stabilization belt, were provided to ensure that participants maintained a vertical trunk position during the test.

Figure 3
Figure 3:
Testing position for upper trapezius.

The lower trapezius test required each participant to lie prone with the proximal 50% of his arm supported on the testing table and diagonally elevated in the range of 130-145° in line with the fibers of the lower trapezius (30). The HHD was applied to the lateral aspect of the distal radius, and each participant was asked to raise his arm off the table toward the ceiling while pulling the scapula downward in the direction of scapular depression (Figure 4).

Figure 4
Figure 4:
Testing position for lower trapezius.

Active Range of Motion

Active range of motion was tested for flexion, abduction, and external and internal rotation using a clear plastic 12-inch goniometer. The procedures followed guidelines established by Clarkson and Gilewich (13). Flexion and abduction were both measured with participants seated in direct contact with a high-back chair. For flexion, the arm was elevated in the sagittal plane with the goniometer axis placed at the lateral aspect of the humeral head, 2.5 cm inferior to the acromion process, with the stationary arm parallel to the trunk and the movable arm aligned with the humerus pointing toward the lateral epicondyle. For abduction, the arm was elevated in the coronal plane with the thumb pointed up toward the ceiling to permit the required external rotation necessary to avoid greater tuberosity impingement on the acromion process (44). The goniometer axis was placed 1.3 cm inferior and lateral to the coracoid process, the stationary arm parallel to the sternum, and the movable arm parallel to the longitudinal axis of the humerus. External rotation was tested supine with the arm abducted to 90° and the elbow flexed to 90° throughout the test. A towel roll was placed under the humerus to ensure neutral horizontal positioning. Each participant was asked to rotate his arm back into external rotation to the end available range without discomfort, with the goniometer axis placed on the olecranon process of the ulna, the stationary arm perpendicular to the ceiling, and the movable arm aligned with the ulna. Each participant was instructed to keep his lower back flat on the table during this measurement. Internal rotation was measured in the prone position with the tested arm supported on the table in 90° of abduction and the forearm flexed to 90° throughout the test. A towel roll was placed directly under the arm to ensure neutral horizontal positioning. Each participant was instructed to internally rotate his arm while maintaining the 90° abducted position. The goniometer axis was placed on the olecranon process of the ulna while the stationary arm was aligned perpendicular to the floor and the movable arm was aligned with the ulna.

Posterior Shoulder Tightness

Posterior shoulder tightness (PST) was quantified using a side-lying horizontal adduction technique (Figure 5). The measurement required each participant to lie on his nontested (dominant arm) side with half the length of his humerus from the edge of the plinth to allow clearance of his forearm past the plinth during horizontal adduction. The nontested extremity was placed under each participant's head, and a pillow was placed between the head and arm to support a neutral position for comfort. A Baseline bubble inclinometer (Fabrication Enterprises Inc, White Plains, NY) attached to a Velcro strap was placed on each participant's midhumerus. The investigator stood facing the participant at the level of the participant's shoulders and grasped the participant's elbow with one hand, passively abducting the humerus to 90° while maintaining 0° of humerus rotation and approximately 90° of elbow flexion. The investigator maintained humeral positioning with the initial contact hand while the other hand manually contacted the participant's lateral scapular border and placed it in a fully adducted position. The investigator then passively lowered the humerus toward the plinth in the transverse plane across the participant's torso, maintaining neutral humerus rotation and scapular adduction. Once the investigator determined that the scapula was unable to be further stabilized and/or movement stopped, a trained assistant recorded the angular measurement from the inclinometer. Participants with PST were expected to have decreased angular measurements (mobility) compared with those without PST.

Figure 5
Figure 5:
Testing position for posterior shoulder tightness.

Statistical Analyses

Collected data were transferred to SPSS (version 15.0 for Windows; SPSS Inc, Chicago, Illinois) for analysis. Mean, SEM, and 95% confidence intervals (95% CIs) of the descriptive data from the RWT and control 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, mean adjusted strength (strength/body weight), and PST were compared between the RWT and control groups using an independent t-test. Strength ratios were analyzed as ordinal data using the Mann-Whitney U-test. A pilot intrarater reliability analysis using the ICC model (3,1) was performed for the variables of strength and PST.

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 G-Power statistical software (version 3.03) determined that a sample size of n = 84 would be required for 95% power if a large effect size were posited.

Results

Reliability Analysis

The intrasession reliability of the strength measurements ranged from ICC = 0.79 to 0.96 (95% CI: 0.72-0.97). Reliability of the PST measurement was excellent; ICC = 0.84 (95% CI: 0.35-0.96). A pilot reliability analysis was not conducted for AROM because the procedures were carried out according to established guidelines (13), and the reliability of goniometry has previously been established in the literature, with both merits and limitations being considered (11,24).

Strength Testing

Mean body weight-adjusted strength values (strength [kg]/body weight [kg]) were significantly greater in the RWT group for the abductor, internal rotator, and upper trapezius muscle groups (p < 0.001) (Table 2). No significant differences were identified in the external rotators (p = 0.18) when comparing the RWT group with the control group. The lower trapezius was found to be significantly less (p = 0.03) in the RWT group when compared with the control group. Significantly greater median strength ratios in the RWT group were identified when compared with the control group (p ≤ 0.001) using the Mann-Whitney U-test (Table 3). In the RWT group, external rotator strength was 61% of internal rotator strength compared with 76% in the control group. When compared with abduction, external rotator strength was 46% of the abductors in the RWT group compared with 57% in the control group. Lower trapezius strength was 12% of the upper trapezius in the RWT group compared with 18% in the control group.

Table 2
Table 2:
Data analysis of adjusted strength values (mean strength/body weight).
Table 3
Table 3:
Strength ratio comparison for control and recreational weight training (RWT) groups.

Active Range of Motion and Posterior Shoulder Tightness

Active range of motion was significantly less in the RWT group than in the control group for shoulder flexion, abduction, and internal rotation (p < 0.001). External rotation was significantly greater in the RWT group when compared with the control group (p < 0.001). The RWT group had significantly less range in the PST measurement (indicating greater PST) than the control group (p < 0.001). Table 4 shows results from the data analysis for AROM and PST.

Table 4
Table 4:
Data analysis for active range of motion (AROM) and posterior shoulder tightness (PST).

Program Design Characteristics

Significant difference in the dependent variables were not identified when participants who received their program design from a certified or licensed professional or participants who possessed their own license or certification to provide exercise prescription were compared with participants who performed a routine without professional advice (p ≥ .076). An exception was that of flexion AROM, which was significantly greater (p ≤ .042) among participants who were certified or licensed or who had received their program design by a certified or licensed professional.

Discussion

Findings from this investigation support our initial hypotheses of aberrant strength characteristics among RWT participants. Body weight-adjusted strength values of the abductor, internal rotator, and upper trapezius musculature in the RWT group significantly exceeded those of the control group. This result was anticipated to occur from a training effect because the deltoids, upper trapezius, and internal rotators (i.e., pectorals and latissimus dorsi) are often targeted among weight training participants. Although strength of the commonly trained muscle groups such as the deltoids, upper trapezius, and pectorals were greater in RWT participants, the shoulder external rotator and lower trapezius musculature were not significantly greater, thus creating an imbalance of muscle groups that normally function synchronously as a force couple. We presumed that more common weight training routines neglect the strengthening of these muscle groups, thus accounting for the findings. In comparison with previous research, the results of this investigation have both consistencies and differences. Body weight-adjusted strength values in our investigation are comparable with those of Barlow et al. (9), who found greater strength in the abductor and internal rotator musculature among weight training participants when compared with controls. Barlow et al. (9) have reported significantly greater external rotator strength among weight training participants when compared with controls; however, our investigation found no significant difference. Lower trapezius strength values were lower in the RWT group when compared with controls in our investigation, in contrast to Barlow et al. (9), who report no significant difference.

When examining muscle performance in the athletic population, strength values may exceed those of the general population and normative values; therefore, alternate methods of quantifying muscle performance must be considered. Clinically, strength ratios are of greater clinical relevance because 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/external rotator, abductor/external rotator, and upper/lower trapezius force couples. The RWT group in this investigation had significantly higher median strength ratios of the tested agonist/antagonist muscle groups, suggestive of training-induced muscle imbalances. Although there exists no consensus as to precise strength ratios required for biomechanically correct shoulder function, the ratios quantified in the RWT group were significantly greater than those of control participants, which may indicate an imbalance of their force couple relationship. The strength ratios identified may result from the upper trapezius, internal rotator, and abductor muscle groups being routinely emphasized during RWT in comparison with the external rotator and lower trapezius muscles, which are often neglected.

The implications of muscle imbalance have previously been discussed in the literature regarding the general, athletic, and symptomatic populations (14,15,16,36,46). Normal shoulder function requires balanced strength of muscle groups that function synchronously during movements and activities. Researchers have reported that individuals with shoulder disorders possess greater deficits in external rotation strength than internal rotation or abduction strength (1,40,46,49,54,55). Moreover, descriptive studies have identified muscle imbalances among athletes with shoulder disorders along with imbalances that result from upper-extremity sports participation (7,14,18,36). During shoulder elevation, the external rotator musculature functions in a synchronous pattern, with the deltoids creating a force couple necessary for normal, unrestricted motion (19,44,46,47). 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. Additionally, impaired muscle performance of the lower trapezius has been associated with shoulder disorders such as impingement syndrome and frozen shoulder (15-18,38). 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 (5,17,19,30,39).

In addition to strength impairments, mobility dysfunction has been implicated as a predisposing factor for shoulder dysfunction (27,51,52,55). We hypothesized that RWT participants would have decreased mobility in the PST measurement and reduced AROM of internal rotation, flexion, and abduction when compared with controls. The results support our hypothesis in that decreased AROM (flexion, abduction, and internal rotation) and greater restrictions of the posterior soft tissue were identified in the RWT participants. We found that PST was significantly greater (less range) among RWT participants, as expected, given the internal rotation loss and lack of exercises or positioning that would increase the extensibility of these structures. Among the motions measured in this investigation, internal rotation and PST have received considerable attention in the literature because of their association with shoulder disorders. Normative values for internal rotation in the adult population are cited as 70° (2,13), whereas the RWT participants in this study averaged 60.1°. The results of this investigation were similar to Barlow and colleagues' (9) findings of significantly less internal rotation in the shoulders of weight training participants when compared with a control group. We did, however, hypothesize that external rotation would be greater because many of the common weight training exercises such as latissimus pull-downs, military presses, and incline presses require end-range external rotation or the frequently assumed “high 5” position. As expected, external rotation was significantly greater in the RWT participants; it was 104° in the RWT participants compared with 96° in the control group and with normative values of 90° (2,13).

The mobility characteristics among the RWT participants in this investigation have clinical implications because reduced internal rotation AROM and increased PST have been associated with shoulder disorders in both the general and athletic populations (32,33,42,50,51,55). Posterior shoulder tightness has been identified as a factor responsible for both limited mobility and shoulder disorders such as glenoid labrum detachment and impingement syndrome (6,50,51). PST is, therefore, an important biomechanical characteristic to measure when assessing injury risk and joint mobility. PST in combination with muscle imbalance and hypertrophy may be responsible for the overall loss of mobility when compared with the control group and normative values. Lastly, these findings, combined with greater shoulder external rotation, support the hypothesis that mobility imbalances exist as a result of RWT.

Our findings support the notion that weight-training programs must be properly supervised or designed by individuals who possess the knowledge of anatomy and biomechanics. Programs designed by properly credentialed professionals may mitigate some of the more common imbalances reported in this investigation.

The findings presented in this study provide insight into shoulder joint and muscle characteristics in the RWT population; however, the results may not necessarily be generalized to all weight-training participants as professional body builders and power lifters were excluded from participation. Furthermore, the inclusion criterion was limited to men, and it can not 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 RWT. Lastly, efforts to delineate between the various credentials among those individuals prescribing exercise programs would be useful when making inferences as to the merit of using properly trained professionals.

Future investigations on the women's weight-training and professional weight training population may be of benefit to delineate weight-training risk. Additionally, this investigation should be reproduced in another cohort and in different geographical regions to truly make the inference that the results are representative of the RWT population at large. Lastly, the predictive validity of joint and muscle characteristics as an index for injury risk in the RWT population could be established through future research focused on longitudinal designs.

Practical Applications

Weight training 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 for the weight training population requires an understanding of the specific biomechanical stresses and adaptations associated with the more common exercises. The results of this investigation indicate that common weight training routines lack the necessary exercise emphasis required for maintaining shoulder joint and muscle balance. Weight training 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 select flexibility exercises for the posterior shoulder tissues into RWT routines will theoretically 1) balance strength ratios as necessary for coordinated shoulder function of the agonist/antagonist force couples, 2) provide soft-tissue mobility balance as required for normal mobility, 3) improve the strength of the humeral head depressors to avoid impingement with overhead exercises common to RWT, and 4) reduce more common risk factors associated with shoulder disorders.

Although experience may be a necessary factor in understanding the diverse application of weight-training program design, evidenced-based judgment and scientific research will dictate the success in managing or preventing shoulder disorders associated with RWT. Recognizing the association of improper exercise selection and shoulder joint and muscle characteristics may reverse or prevent injury in the RWT population.

Individuals pursuing weight-training without prior knowledge of proper exercise form and program design should seek consultation from a competent provider with the appropriate qualifications. Moreover, individuals responsible for the operation of fitness centers and gyms should recognize the need to hire trained professionals with recognized credentials.

Acknowledgments

The authors would like to express their gratitude to the Nova Southeastern University, Health Professions Division, Faculty Research Grant for funding this investigation. Additionally, the authors would like to acknowledge Nova Southeastern University, Department of Physical Therapy for providing the facilities for data collection.

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Keywords:

shoulder complex; weight lifting; shoulder disorders; muscle imbalance

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