Establishing Normative Data on Scapulothoracic Musculature Using Handheld Dynamometry

Transcription

1 Journal of Sport Rehabilitation, 2009, 18, Human Kinetics, Inc. Establishing Normative Data on Scapulothoracic Musculature Using Handheld Dynamometry Nichole Turner, Kristen Ferguson, Britney W. Mobley, Bryan Riemann, and George Davies Context: Scapular strength deficits have been linked to shoulder dysfunction. Objective: To establish normative data on the scapulothoracic musculature in normal subjects using a handheld dynamometer. Design: Descriptive normative data study. Setting: Field research. Subjects: 172 subjects with varying levels of overhead activity. Methods: A handheld dynamometer was used to test the upper, middle, and lower trapezius; rhomboids; and serratus anterior. Main Outcome Measures: A 2-factor ANOVA was performed for each of the muscles by activity level and unilateral ratio by activity-level analyses. Post hoc analysis included multiple pairwise comparisons, using the Dunn-Bonferroni correction method. Results: Activity level did not significantly affect the unilateral ratios: Elevation:depression was 2.5:1, upward:downward rotation was 1.5:1, and protraction:retraction was 1.25:1. A rank order from strongest to weakest was established through significant comparisons. Conclusion: The unilateral ratios along with the rank order should be considered when discussing scapular rehabilitation protocols. Keywords: handheld dynamometer, scapulothoracic strength, shoulder impingement Weakness, abnormal positioning, and abnormal timing of the scapular muscles are all contributing factors to scapular dyskinesia. Impairments in scapular motion can lead to problems such as abnormal stresses on the anterior capsular structures of the shoulder, increased risk of rotator-cuff compression, and decreased muscle performance. 1,2 Also, dyskinesis influences the position and decreases the size of the subacromial space, leading to subacromial impingements. 3,4 Changes in scapular motion, such as decreased protraction or imbalances between the upper and lower trapezius, have been reported in patients with impingement. 3 8 Inadequate scapular stabilization has been shown to contribute to altered biomechanics of the shoulder complex and to increase the risk of musculoskeletal problems such as instability and impingement. 2 5,7 9 Turner, Ferguson, and Mobley are practicing physical therapists who graduated from Armstrong Atlantic State University. Riemann is with the Dept of Health Sciences, and Davies, the Dept of Physical Therapy, at the university. 502

2 Normative Data on Scapulothoracic Musculature 503 Research has demonstrated that alterations in scapular kinematics are connected to decreased serratus anterior activity, increased upper trapezius muscle activity, and imbalances between the upper and lower trapezius. 2,7,10,11 Research has shown that scapular upward rotation, posterior tilt, and external rotation were decreased in patients with impingement syndrome when compared with healthy subjects. 7 Ludewig and Cook 7 also demonstrated that subjects with symptoms of impingement had significantly more upper trapezius muscle activity and less serratus anterior activity than a control group. Cools et al 5 found that overhead athletes with impingement demonstrated decreased protraction. These studies underline the importance of scapular kinematics and strengthening exercises in shoulder rehabilitation protocols. One way to objectively measure strength in the clinical setting is through the use of a handheld dynamometer, which is more accurate and less subjective than manual muscle testing. The intrarater and interrater reliability of handheld dynamometry have been supported in previous studies The strength of the rater, experience, and tester stability all can alter the reliability of the measurement. 21,23 However, if these variables are controlled, the handheld dynamometer can be a valuable assessment tool. Adequate assessment of the scapulothoracic musculature is essential to designing better rehabilitation protocols. Research has demonstrated that some cases of impingement problems have been adequately resolved with rehabilitation protocols involving scapular muscle reeducation and strengthening exercises. 1,24,25 By restoring the normal balance of force couples, physical rehabilitation can improve the position and motion of the scapula to decrease impingements and also increase the strength of rotator-cuff muscles. 1 Therefore, outcome assessments in rehabilitation protocols should address these kinematic and muscle-activity alterations to restore normal scapulothoracic and shoulder-complex movements. Michener et al 22 examined the construct validity, reliability, and error of handheld dynamometry testing of scapular muscles in subjects with shoulder pain and functional loss. They stated, At present, the error value of a single-time handheld dynamometry scapular muscle measurement is unclear without the establishment of normative values of scapular muscle measurements. 22(p1135) To our knowledge, no studies have been performed to develop normative strength data on the scapular muscles using a handheld dynamometer. Therefore, the purpose of this study was to establish normative data for strength of the scapulothoracic muscles in healthy individuals, as well as examine the effects of overhead-activity level on these measurements. We hypothesized that those who participate in activities that require increased scapular stabilization, such as overhead athletes, would have higher strength values than those who do not. Subjects One hundred ninety subjects were recruited from local high school, college, and adult sports teams. A cross-section of sports and activities was sampled to examine the differences in subjects with bilateral, unilateral, or no overhead activity. A sample of convenience was used based on the availability, accessibility, and characteristics of the surrounding community. Because this was a unique study, we did not have reasonable estimates of effect sizes to perform an actual power analysis

3 504 Turner et al for the group by muscle comparisons. Thus, based on the central-limit theorem, a goal of 50 subjects per group was set to ensure a reasonable chance of obtaining normally distributed data for the statistical analyses. The subjects each fit into only 1 category, with little crossover training between unilateral and bilateral activities. For example, there were no baseball, tennis, or volleyball subjects who regularly participated in swimming. Eighteen subjects were excluded based on the following criteria: history of shoulder surgery or macrotrauma; current shoulder, neck, or thoracic pain; scoliosis; or observable winging of the scapulae. Institutionalreview-board approval was obtained, and the remaining 172 subjects (and parents when applicable) gave informed consent or assent. All testing was carried out in keeping with the spirit of the Helsinki declaration. After testing, an exercise program for the glenohumeral and the scapulothoracic musculature was provided to the subjects as an incentive to participate in the study. 25,26 Methods Subjects were asked to fill out a questionnaire regarding their activity level, height, weight, history of previous injury to the upper extremities, and basic demographic information. Body-mass index (BMI) was then calculated based on height and mass. Total arm length and upper-arm circumference were recorded for later comparisons with strength data. Honesty in reporting height and weight was assumed in some cases because of equipment limitations; most of the testing took place at team practices or meetings. Total arm length was defined as the distance from the lateral edge of the acromion to the end of the middle finger. We also measured shoulder-girdle length, defined as the distance from the mastoid process to the lateral edge of the acromion. The halfway point of this measurement was marked for dynamometer placement over the muscle bulk of the upper trapezius. In addition, upper-arm length was measured, defined as the length from the lateral acromion to the lateral epicondyle, and the halfway point was marked for dynamometer placement during rhomboid testing. Arm circumference was also measured at this halfway point of the upper arm. The manual-muscle-testing positions described by Hislop and Montgomery 28 were adapted for use with the Baseline hydraulic push pull handheld dynamometer with analog gauge (see Figures 1 and 2). Muscles rarely work in an isolated manner, so these muscle tests can be considered biased toward each muscle. For example, several studies have shown increased middle-trapezius electromyographic activity during the position described in Figure 3 for the lower trapezius. 11,22,27 However, isolating the middle trapezius action of scapular retraction is commonly done in clinical practice, as shown in Figure Michener et al 22 established construct validity for the lower- and upper-trapezius muscle tests using the positions described in Figures 3 and 5, respectively. The testing procedure for the serratus anterior targets both functions of the muscle: upward rotation and protraction (see Figure 6). 11,27,29 The testing procedure for the rhomboids is illustrated in Figure 7. Smith et al 30 demonstrated that the rhomboid manual muscle test described by Hislop et al was not significantly different than the rhomboid manual muscle test described by Kendall et al when considering the percent maximum voluntary contraction of the rhomboids.

4 Figure 1 Rectangular end piece used for testing the middle and lower trapezius. Figure 2 Curved end piece used for testing the upper trapezius, serratus anterior, and rhomboids. Figure 3 Lower-trapezius bias. The subject was prone with the arm in 150 of scaption with the thumb up. The dynamometer was placed on the posterolateral corner of the acromion, and the rectangular end piece was used. The clinician stabilized the contralateral hip while matching the force of the subject. 505

5 Figure 4 Middle-trapezius bias. The subject was prone with the arm in 90 of abduction and the elbow in 90 of flexion. The dynamometer was placed on the posterolateral corner of the acromion, and the rectangular end piece was used. The subject was instructed to squeeze your shoulder blades together to retract his or her scapula. The clinician stabilized the contralateral scapula while matching the force exerted by the subject. Figure 5 Upper-trapezius bias. The subject was seated with his or her arms at the sides. The dynamometer was placed halfway between the mastoid and lateral acromion over the muscle bulk, and the curved end piece was used. The examiner was allowed to use both hands, so no stabilization was necessary. Figure 6 Serratus anterior bias. The subject was seated with the arm in 130 of flexion. The dynamometer was placed on the humerus distally to the deltoid attachment, and the curved end piece was used. The clinician stabilized the inferior angle of the scapula being tested while matching the force of the subject. 506

6 Normative Data on Scapulothoracic Musculature 507 Figure 7 Rhomboids bias. The subject was prone with the hand on the small of the back. The dynamometer was placed on the humerus halfway between the acromion and lateral epicondyle, and the curved end piece was used. The clinician stabilized the contralateral scapula. For each test, the subject was asked to perform the motion through his or her full range of motion, back off into midrange, and hold the position. A make muscle contraction was used rather than a break muscle contraction. A make test was used to avoid overpowering the subjects in an effort to measure their force-producing capabilities. Make tests are used almost exclusively with handheld dynamometry ,23 Subjects were asked to build their force gradually to a maximum voluntary effort over a 2-second period. They maintained a maximum voluntary effort for a 5-second period. The examiner kept the dynamometer in place by matching the force exerted by the subject, and the peak force was recorded. If the subject broke against resistance, the data were not recorded, and the muscle test was repeated. Strength measurements were collected for each subject s dominant upper extremity. One trial of each muscle test was used, which has been established in the literature as adequate for measuring muscle strength in healthy subjects. 20 To avoid any possible fatigue factor, all the testing occurred before practices, competitions, or heavy exercise. The order for testing the muscles was semirandomized to facilitate the speed of testing. Because the end piece of the dynamometer had to be changed, muscle tests requiring the curved end piece were tested together. These muscles included the upper trapezius, serratus anterior, and rhomboids. The middle- and lower-trapezius muscle tests required the rectangular end piece and were therefore tested last. For the next subject, the rectangular end-piece configuration was used first. Within these limitations, the order of testing was randomized. The handheld dynamometer with both end pieces used in testing is pictured in Figures 1 and 2.

7 508 Turner et al Reliability Study Before actual data collection, the 2 testers involved in the study performed extensive practice with subjects not included in the study analysis to develop consistent techniques, adopt stable positions for resisting subjects force, and improve reliability. After sufficient practice, a pilot study was performed to determine the intrarater and interrater reliability of the 2 examiners. Sixteen subjects were recruited, all of whom met inclusion criteria for the study. Both examiners were blinded to the measurements, and the procedure used was identical to that of the larger study. None of the subjects were included in the larger investigation. Time between testers (intertester) was 15 minutes, and intratester-reliability interval was 72 hours. Again, the subjects were similar in anthropometric and demographic characteristics to the subjects in the larger study. Rationale for sample size used was based on a reliability power analysis. 31 Data were analyzed using Statistical Package for the Social Sciences (version 15.0) to calculate interclass coefficients (ICCs) for intrarater and interrater reliability. The (3, 1) formula was used to calculate these ICCs. After ICC calculation, the standard errors of measurement were calculated. See Table 1. Data Analysis Subjects were classified into 1 of the following 3 groups: no overhead activity, unilateral overhead activity, or bilateral overhead activity. No overhead activity was defined as not participating in any athletic activity that required the arm to be elevated above 90, including runners, soccer players, and nonathletic participants. Unilateral overhead activity was defined as participating in any athletic activity that required predominately 1 arm to be elevated above 90, such as tennis and any throwing-sport athletes. Bilateral overhead activity was defined as participation in Table 1 Reliability Analysis for the Pilot Study ICC SEM (n) Muscle E1 E2 IR E1 E2 IR Upper trapezius (left) Upper trapezius (right) Middle trapezius (left) Middle trapezius (right) Lower trapezius (left) Lower trapezius (right) Rhomboids (left) Rhomboids (right) Serratus anterior (left) Serratus anterior (right) E1, examiner 1; E2, examiner 2; IR, interrater.

8 Normative Data on Scapulothoracic Musculature 509 Table 2 Subjects Classified by Activity Level Overhead Activity None Unilateral Bilateral Number Age range, y Mean age 24.9 ± ± ± 13.2 Men Women athletic activity that required both arms to be elevated above 90, such as swimmers and triathletes. Based on the operational definitions used, to be considered for either the unilateral or bilateral athlete categories, one had to have actively participated in an organized sport (ie, high school, college, master s level) for a minimum of 1 year. Descriptive statistics for the demographics for each group are provided in Table 2. First, the data were correlated with the anthropometric measurements to attempt to normalize scapulothoracic strength to each individual. The dominant arm was used for all data analysis. A 2-factor repeated-measures ANOVA was used to analyze the effects of activity level on scapulothoracic strength measurements. The 5 muscles were entered as a within-subject factor using 5 levels. Activity level was used as a between-subjects factor. For each ANOVA, the average force production of each muscle across each activity level was analyzed to determine a rank order for the strength of the 5 muscle groups tested. Then the unilateral strength ratios were determined. The 3 ratios studied were elevation versus depression (upper vs lower trapezius), protraction versus retraction (serratus anterior vs middle trapezius), and upward versus downward rotation (serratus anterior vs rhomboids). For simplicity, the serratus anterior was used to represent upward rotation, instead of the forcecouple concept using upper trapezius, lower trapezius, and serratus anterior. The middle trapezius was selected because it has the unilateral function of retraction, as opposed to the rhomboids, which have the dual function of retraction and downward rotation. As stated before, muscle weaknesses and imbalances can lead to impingement, so using these unilateral ratios may help establish normal balance of the scapulothoracic muscles. 3 8 A separate 2-factor ANOVA of ratio by activity level was used to examine the effects of activity level on the strength ratios. Each ratio served as a within-subject factor with 3 levels, and activity level constituted a between-subjects factor with 3 levels. When significant interactions on main effect were revealed, main interactions, only effects post hoc comparisons were conducted using the Dunn Bonferroni procedure. Specifically, for significant interactions, only within-group betweenmuscles and within-muscle between-groups comparisons were considered. Statistical significance was considered P <.05.

9 510 Turner et al Results Correlations With Anthropometric Data Correlations are shown in Table 3. Analysis demonstrated very weak relationships between muscle force and these anthropometric relationships. Muscle by Overhead Activity-Level Analysis The 2-factor ANOVA demonstrated a significant interaction between muscle strength and activity level (F 5.2,438 = 4.364, P =.001). A significant main effect was observed for muscle (F 2.6,438 = , P <.001), as well as activity level (F 2,169 = , P <.001). The means, SDs, and 95% confidence intervals are presented in Table 4. Generally, the overhead athletes (both unilateral and bilateral) had significantly higher muscle strength than the group with no overhead activity (Figure 8). This pattern was true for every muscle except the lower trapezius, which was the weakest across all 3 groups. There were no significant differences between the unilateral and bilateral overhead-activity groups. There were no statistically significant differences found among any of the groups with respect to the lower trapezius (Table 4). The upper trapezius was significantly stronger than any other muscle (Figure 9). Both the middle trapezius and serratus anterior were significantly stronger than the rhomboids and the lower trapezius. There were no significant differences between the middle trapezius and the serratus anterior. In addition, there were no significant differences between the lower trapezius and the rhomboids. The upper trapezius was significantly stronger than any other muscle (Figure 10). The serratus anterior was significantly stronger than the middle and lower trapezius, as well as the rhomboids. The middle trapezius was significantly stronger than the lower trapezius and rhomboids. There was no significant difference between the lower trapezius and the rhomboids. The upper trapezius was significantly stronger than any other muscle (Figure 11). The middle trapezius was significantly stronger than the lower trapezius and the Table 3 Correlations (r Values) Between Anthropometric Measurements and Scapulothoracic Strength Bodymass Muscle index Height Mass Total arm length Upper-arm circumference Upper trapezius.213*.288**.349**.372**.443** Middle trapezius.250*.260*.367**.375**.451** Lower trapezius.271**.154*.334**.280**.436** Rhomboids *.230*.454**.295** Serratus anterior.184*.209*.293**.398**.425** *P <.05. **P <.001.

10 Normative Data on Scapulothoracic Musculature 511 Table 4 Mean Force Production for Each Muscle Across Overhead- Activity-Level Groups Overhead activity Mean (n) SD 90% CI n Upper trapezius None Unilateral Bilateral Total Middle trapezius None Unilateral Bilateral Total Lower trapezius None Unilateral Bilateral Total Rhomboids None Unilateral Bilateral Total Serratus anterior None Unilateral Bilateral Total rhomboids. The serratus anterior was significantly stronger than the middle and lower trapezius, as well as the rhomboids. The rhomboids were significantly stronger than the lower trapezius, which is a unique finding to the bilateral overheadactivity group. Therefore, given the data, a rank order cannot be established for every activitylevel group. However, Table 5 demonstrates a general template for rank ordering the strength of the scapulothoracic muscles based on overhead-activity level. After a 2-way ANOVA to compare muscle-strength ratios across activitylevel groups, no significant interaction was observed (F 2.6,217 = 1.929, P =.135). A significant main effect for muscle ratio was noted (F 1.3,217 = , P <.001), but the main effect for activity level was not significant (F 2,169 = 0.919, P =.401). Therefore, it is not necessary to discuss each muscle ratio separately by overheadactivity level. Post hoc analysis revealed that the elevation:depression ratio was statistically significantly higher than both of the other ratios. The upward:downward rotation ratio was significantly higher than the protraction:retraction ratio (Table 6 and Figure 12).

11 Figure 8 Strength of the scapulothoracic muscles between activity-level groups. *Significantly greater than the group with no overhead activity. Error bars represent the SD. Figure 9 Strength of the scapulothoracic muscles in the no-overhead-activity group. *Significantly greater than all other muscles for this activity group. Significantly greater than the lower trapezius and rhomboid. Error bars represent the SD. 512

12 Figure 10 Strength of the scapulothoracic muscles in the unilateral overhead-activity group. *Significantly greater than all other muscles for this activity group. Significantly greater than the lower trapezius and rhomboid. Significantly greater than the middle trapezius, lower trapezius, and rhomboid. Error bars represent the SD. Figure 11 Strength of the scapulothoracic muscles in the bilateral overhead-activity group. *Significantly greater than all other muscles for this activity group. Significantly greater than the lower trapezius and rhomboid. Significantly greater than the middle trapezius, lower trapezius, and rhomboid. Significantly greater than the lower trapezius. Error bars represent the SD. 513

13 Table 5 Rank Order for Scapulothoracic Muscle Strength Based on Post Hoc Analysis of Overhead-Activity Level Overhead Activity None Unilateral Bilateral Upper trapezius a Upper trapezius a Upper trapezius a Serratus anterior or middle Serratus anterior a Serratus anterior a trapezius a Rhomboids or lower trapezius Middle trapezius a Rhomboids or lower trapezius Middle trapezius a Rhomboids a Lower trapezius a Indicates the strength of this muscle was significantly greater than those below. Table 6 Ratio Comparison as Grouped by Overhead-Activity Level Elevation:depression (upper trapezius to lower trapezius) Upward:downward rotation (serratus anterior to middle trapezius) Protraction:retraction (serratus anterior to rhomboids) Overhead activity Mean (n) SD 95% CI n None Unilateral Bilateral Total None Unilateral Bilateral Total None Unilateral Bilateral Total

14 Normative Data on Scapulothoracic Musculature 515 Figure 12 Unilateral muscle-strength ratios by overhead-activity level. *Significantly greater than the protraction:retraction ratio and the upward:downward rotation ratio. Significantly greater than the protraction:retraction ratio. Error bars represent the SD. Discussion This study was the first to examine the relationships between scapulothoracic strength using a handheld dynamometer and anthropometric measurements such as BMI or arm length. These measurements are clinically relevant in a myriad of ways. For instance, these data can be used to establish normative strength values for use during injury-prevention screenings, initial evaluations, and serial reassessments and when setting discharge criteria, as well as being used as guidelines for designing strength and conditioning programs to ensure that the glenohumeral joint has a stable scapulothoracic base. Future research in this area, along with our current study, could lead to an extensive database of strength values for clinicians. When the handheld-dynamometer measurements were compared with measurements such as height, weight, BMI, total arm length, and arm circumference, weak relationships were found. Examining BMI, in addition to height and mass, was one of the original intents of the current study because currently there is no other research comparing handheld-dynamometer strength data and anthropometric data for any body part. However, because there were very weak relationships (Table 3) we did not think it was appropriate in the final analysis of the data. For example, the highest correlation was between BMI and the lower trapezius (r =.271), which only leads to 7.3% shared variance. Although normalizing to body weight or BMI would have been useful for general application, the data did not support this relationship. A few possible explanations for the weak relationships

15 516 Turner et al are that the upper extremity is not weight bearing and therefore may not be affected by body weight. In addition, the scapular muscles work as synergists and stabilizers, as opposed to prime movers, so this may affect the relationships with measurements such as body mass or BMI. Subjects with varying levels of overhead experience and a wide range of ages were studied, so perhaps a more homogeneous sample would have yielded stronger correlations. The strongest anthropometric correlations were seen with upper-arm circumference measurements. Arm circumference may represent muscle mass of the upper arm, so a higher measure would indicate increased cross-sectional area and therefore increased force production. The scapular muscles provide the proximal stability for the arm, so this increase in strength of the upper arm may translate to increased strength in the scapular muscles. Increased arm circumference could also indicate obesity, but in this study the mean BMI was ± The subjects were also generally physically active, so we feel that arm circumference better represents muscle mass. Further research is warranted in this area to explore all relationships between anthropometric measurements and scapulothoracic muscle strength. When comparing muscle strength and activity level, a common pattern was found. The overhead-activity-level groups (unilateral and bilateral) had significantly greater scapulothoracic muscle strength than the group with no overhead activity. This observation can most likely be explained by a training effect. Although direct training of the scapular stabilizers is not commonly seen in training programs, these muscles are a part of the kinetic chain and were therefore active during overhead activity for stabilization. There were significant differences between each muscle s strength in the bilateral overhead-activity level, which led to establishing a rank order for this group. The rank order is as follows: upper trapezius, serratus anterior, middle trapezius, rhomboids, and lower trapezius. For the remainder of the activity-level groups, an exact rank order could not be determined given the lack of significant differences between muscles; however, several trends were observed. These trends were similar to the order established by the bilateral overhead-activity group. Generally, the upper trapezius was the strongest, followed by the serratus anterior and middle trapezius, followed by the rhomboids and lower trapezius. As previously stated, scapulothoracic muscle weaknesses lead to imbalances that can result in abnormal stabilization and control of the scapula. 1 Identifying weakness that could potentially lead to shoulder dysfunction could potentially lead to a decreased rate of subacromial impingement. In one study, the rate of shoulder impingement was 55.1% of 878 patients with shoulder dysfunction. 32 A rank order can be used to identify weak muscles to modify training and conditioning programs with the goal of restoring normal muscle balance. This comparison would be analogous to considering quadriceps-to-hamstring ratios. However, with respect to the unilateral ratios, no differences were seen across activity level. The 172 normal subjects included in this study demonstrated uppertrapezius strength approximately 2.5 times that of the lower trapezius. The ratio between the upward rotators (represented by the serratus anterior) and the downward rotators (rhomboids) was approximately 1.5:1. The ratio between scapular protraction (serratus anterior) and scapular retraction was approximately 1.25:1. We recognize that the scapulothoracic muscles work as synergists to produce scap-

16 Normative Data on Scapulothoracic Musculature 517 ular motion. Therefore using 1 muscle to represent each motion may be a limitation and an oversimplification. Nonetheless, we felt it was more appropriate than having muscles with multiple functions. The ratios, which are rounded for each clinical interpretation, were very consistent among this population of healthy subjects. These findings warrant further research into muscle ratios in patients with shoulder pathologies, which may identify specific deficits relative to these pathologies. This information can help focus evidence-based rehabilitation programs. Given that the number of subjects in each activity-level group ranged from 52 to 66, we realize that this is a limited number for a normative database covering the 3 subgroups. We purposefully chose to include all 3 activity groups in our sample, as opposed to just concentrating data collection on 1 of the subgroups (ie, only considering unilateral activity), to provide clinicians with some initial direction concerning normal scapulothoracic muscle function across various athletes evaluated clinically. Therefore, we recommend using the 90% confidence intervals given as a practical clinical guideline. Again, given the lack of other published data addressing this issue, this work becomes a foundation and we strongly recommend future research to expand on it. There were several limitations of this study. The scapulothoracic muscles are difficult to test with a handheld dynamometer without crossing multiple joints. We decided that it was important to use previously studied and widely accepted manual muscle tests with as few modifications as possible when using the handheld dynamometer. Several studies have demonstrated increased middle-trapezius EMG activity with the position used to test the lower trapezius. 11,22 The middle trapezius may help eccentrically control upward rotation of the scapula by limiting scapular abduction, which may contribute to this increased activity during the lower-trapezius test. 27 However, isolating the middle trapezius function as a scapular retractor in 90 of abduction and neutral rotation (elbow flexed to 90 ) helps bias the test to measure only the middle trapezius strength. 28 We used the position presented in Figure 4 for several reasons. First, it is the most commonly described position in most manual-muscle-testing texts. Even though the lower trapezius produces high EMG activity in this position, it also is heavily involved with the upward-rotation force couple. Thus, the high EMG activity previously described likely represents the lower trapezius producing a synergistic activation to prevent scapular rotation. By using the position in Figure 4, we isolated the movement pattern for scapular retraction without the confounding rotations or force-couple coactivations. Furthermore, to establish unilateral ratios, we elected to establish the protraction:retraction ratios and felt that this muscle was a better indicator because it has a uniplanar function as opposed to multiplanar function. Michener et al 22 previously established the validity of the positions used for upper and lower trapezius using a handheld dynamometer. Given that Smith et al 8 found no differences between the Kendall, Kendall-alternative, and Hislop-Montgomery rhomboid-testing positions using surface EMG, it seems that the test position used in this study does adequately recruit the rhomboid muscles. Michener et al did not establish construct validity for the serratus anterior using a modified Kendall position. However, testing the patient in sitting and with the arm elevated above 120 may have helped reduce compensations and increase EMG activity. 22 Challenging the serratus anterior as an upward rotator, as well as scapular protractor, has been shown to increase EMG activity in the serratus anterior. 11

17 518 Turner et al In addition, with the particular model of handheld dynamometer used, there were issues when using a rigid end plate instead of a hand. Pain inhibition may have been a factor in our study, because of the shape of handheld-dynamometer force plates (end pieces). Attempts were made to reduce pain inhibition by avoiding placement directly over bony prominences, but maintaining consistency of dynamometer placement limited our ability to avoid this completely. With respect to dynamometer positioning and pain inhibition, any test that did not appear to be the subject s maximal effort was repeated. We feel that any dynamometer with similar pads would yield similar results. However, further research in this area may benefit from using a more comfortable dynamometer. Even though pilot testing demonstrated acceptable intrarater and interrater reliability, having 2 testers increases the variance in the measurements. Several studies have shown that tester strength affects the measurements; however, we feel that this was not a factor in our study, given the results of our reliability study. However, a few subjects did present a problem to the testers when performing the upper-trapezius strength testing. This issue would be anticipated to be a bigger problem when testing athletes in strength-dominant sports (ie, football, power lifting, bodybuilding, etc). In the current study, the upper trapezius was the only muscle with the potential to produce a ceiling effect with strength testing. However, given the results that demonstrate the significant differences between the upper trapezius and all other muscles tested, we do not feel that this is a clinically relevant issue. This sample included subjects across a large age range and with varying levels of overhead activity. Perhaps future studies attempting to establish normative data should examine the effects of age and sex on scapulothoracic strength. Future studies in this area would also benefit from having a larger sample size in an effort to establish normative data that are more representative of the general population. In addition, the findings of this study can only be generalized to the dominant arm of a given subject or patient. Future studies need to address the effects of arm dominance on scapulothoracic strength. Further research could also consider volume of training and sport specificity with scapular muscle function to better refine the normative values begun with this manuscript. The strength difference between sexes is an important area for further consideration, as well. Conclusion The strength of the 5 scapulothoracic muscles was measured in 172 subjects with no history of shoulder dysfunction using a handheld dynamometer. When the subjects were classified by overhead-activity level, a rank order for strength was established in the bilateral overhead-activity group (upper trapezius, serratus anterior, middle trapezius, rhomboids, and lower trapezius). When the strength of the scapulothoracic muscles as a unilateral ratio was examined, no significant differences occurred across activity levels. The elevation:depression ratio was approximately 2.5:1, upward to downward rotation approximately 1.5:1, and protraction to retraction approximately 1.25:1. The consistency of the ratios across overheadactivity levels could provide a practical clinical guideline for establishing exercise progression and meeting discharge criteria. We hope this information can lead to better objective documentation and customized rehabilitation programs based on the objective evidence and normative data.

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