Disorders of the shoulder are the third most common

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1 CLIONA O SULLIVAN, MSc 1 SUSANNA BENTMAN, MSc 2 KATHLEEN BENNETT, PhD 3 MARIA STOKES, PhD 4 Rehabilitative Ultrasound Imaging of the Lower Trapezius Muscle: Technical Description and Reliability Disorders of the shoulder are the third most common musculoskeletal disorder presenting to general practitioners, after back and neck complaints, and affect between 7% and 36% of the population. 11 Abnormal scapular kinematics has been shown to be associated with shoulder pathology, such as impingement syndrome, frozen shoulder syndrome, and instability. 5,19,21,22 The scapular muscles, including the lower trapezius, have been established as essential components to optimum scapular kinematics. 8,13 During STUDY DESIGN: Exploratory and reliability study. BACKGROUND: Shoulder dysfunction is common and often difficult to diagnose and treat. The trapezius muscle is an important stabilizer and primary mover of the scapula. The potential use of rehabilitative ultrasound imaging (RUSI) to evaluate scapular muscle function warrants investigation. OBJECTIVES: To establish a procedure for imaging the thickness of the lower trapezius muscle and to examine reliability within and among investigators. METHODS: In 16 asymptomatic subjects (12 female, 4 male), aged 20 to 41 years, 3 investigators used RUSI to measure the thickness of the left lower trapezius muscle with the subject at rest in prone. Investigator 1 took 3 images on each of 2 days, while the other 2 investigators took 2 images each on the second day. All measurements of lower trapezius muscle thickness were made off-line, at a point 3 cm lateral to the lateral edge of the spinous processes. To also obtain within-scan reliability, 1 image taken by investigator 1, once displayed on the scanner s screen, was measured 3 times. Investigator 1 also measured lower trapezius muscle thickness 1 cm medial to this site. Reliability was examined using intraclass correlation coefficients (ICC) and the Bland and Altman plot. RESULTS: The intrarater within-scan reliability at the lateral site was ICC 3,3 = 0.99 (95% CI: 0.98 to 1.0). The intrarater between-scan reliability (within-day) at the lateral site, medial site, and combined sites (mean of medial and lateral) were ICC 3,3 = 0.96 (95% CI: 0.90 to 0.98), ICC 3,2 = 0.90 (95% CI: 0.78 to 0.96), and ICC 3,2 = 0.99 (95% CI: 0.99 to 1.0), respectively. Intrarater (betweenday) reliability was good for the lateral site and combined sites (ICC 3,3 = 0.91, 95% CI: 0.74 to 0.96, and ICC 3,3 = 0.90, 95% CI: 0.70 to 0.96, respectively) and moderate for the medial site (ICC 3,3 = 0.89, 95% CI: 0.68 to 0.96). Interrater reliability (among investigators) was also moderate (ICC 2,2 = 0.88, 95% CI: 0.73 to 0.96). Mean lower trapezius muscle thickness was approximately 3.1 mm (SD, 0.8 mm). CONCLUSION: Thickness of the lower trapezius muscle can be measured reliably with RUSI. J Orthop Sports Phys Ther 2007;37(10): doi:2519/jospt KEY WORDS: morphology, morphometry, muscle size, scapular muscles, ultrasonography arm elevation, the lower trapezius, together with the upper trapezius and serratus anterior muscles, produce upward rotation, external rotation, and posterior tipping of the scapula, all of which are integral to normal scapular kinematics. 8,21 Many studies have reported on altered scapular motion 13,21,22 and altered patterns of muscle activity of the upper and lower trapezius and the serratus anterior muscles in patients presenting with shoulder dysfunction. 19,21,22 It is not known whether such alterations precede or develop as a result of shoulder pathologies, such as impingement syndrome. 20 Various authors have suggested differences in scapular muscle activation patterns between patient groups. For example, patients with frozen shoulder syndrome display higher ratios of upper to lower trapezius muscle activity when compared with asymptomatic subjects, 19 while patients with impingement syndrome display increases in lower trapezius muscle activity relative to upper trapezius activity. 20 In a study of patients presenting with glenohumeral joint instability, the lower trapezius and serratus anterior muscles were active only during the last one third of arm elevation. 22 Authors have suggested that addressing abnormal scapular muscle activity and, hence, abnormal scapular kinematics, may be a fundamental component in the manage- 1 Lecturer, Department of Physiotherapy, School of Medicine, Trinity College Dublin, Ireland. 2 Visiting Fellow, School of Health Professions and Rehabilitation Sciences, University of Southampton, UK. 3 Lecturer, Department of Pharmacology and Therapeutics, Trinity College Dublin, Ireland. 4 Director of Research, School of Health Professions and Rehabilitation Sciences, University of Southampton, UK. This study was supported in part by a grant from Trinity College Dublin. This study was approved by Trinity College Dublin Faculty of Health Sciences Research Ethics Committee. Address correspondence to Dr Maria Stokes, Professor of Neuromuscular Rehabilitation and Director of Research, University of Southampton, Highfield Campus, Southampton, Hants SO17 1BJ, United Kingdom. m.stokes@soton.ac.uk 620 october 2007 volume 37 number 10 journal of orthopaedic & sports physical therapy

2 ment of patients presenting with shoulder dysfunction. 19,20,22 Therefore, assessment of scapular muscle function should be an important part of the examination of patients with shoulder problems. In current clinical practice, assessment of scapular muscle function involves observation of the scapula both in static postures and during movement. 27 Usual methods of manual muscle testing are not sufficient to assess strength of the scapular muscles because it is difficult to isolate the individual muscles through testing. Scapular motion and muscle activity can be assessed using complex procedures, such as electromyography (EMG) and 3- dimensional motion analysis. 8,21 But there is a need for simple, objective, and reliable methods of assessing scapular muscles in the clinical setting to examine dysfunction and optimize treatments. Rehabilitative ultrasound imaging (RUSI) has been used successfully to measure different aspects of muscle morphology, including thickness, 7,23,26 crosssectional area (CSA), 2,16,31,36 and muscle volume 10,25 in a variety of muscles. When compared with magnetic resonance imaging (MRI), which is considered widely to be the gold standard of muscle imaging, RUSI has been shown to be valid for measuring the CSA and muscle thickness (resting and contracted) of the lumbar multifidus, 16 rectus femoris, 2 vastus lateralis, 31 and abdominal muscles. 15 RUSI is also increasingly used for visual feedback when teaching patients with low back pain to contract the deep abdominal muscles. 14,15 Ultrasound imaging also has advantages over other forms of medical imaging modalities because it does not involve exposure to radiation, is less costly, and is more easily accessible. 25,32 A muscle s ability to generate force is closely related to its CSA. Ultrasound studies of muscle CSA have demonstrated a close relationship with force in various muscles including the biceps 18 and quadriceps. 40,41 The ability to measure muscle CSA, therefore, enables estimation of the force generating capacity of a muscle (ie, providing an indirect measure of strength). Thickness has been shown to be highly correlated with CSA in various muscles 35 and has also been shown to be related to strength. 1,12 The potential clinical uses of RUSI in the evaluation of scapular muscle function and as a means of biofeedback during rehabilitation warrant investigation. The reliability and validity of the technique first need to be established. The aims of this study were to develop a technique for measuring the thickness of the lower trapezius muscle using RUSI and to establish intrarater and interrater reliability of measurements in a group of healthy asymptomatic subjects. METHODS Subjects Sixteen asymptomatic subjects participated: 12 females, aged years and 4 males, aged 21 to 36 years. Subjects were recruited from the staff and students of the Department of Physiotherapy and Occupational Therapy in Trinity College Dublin. Inclusion criteria were as follows: asymptomatic subjects with no history of neck, shoulder, or arm pain; full active pain-free range of motion at the cervical spine and shoulder; and over 18 years of age. Exclusion criteria were the following: neuromuscular or musculoskeletal disorders; history of neck, shoulder or arm trauma, pain or surgery; pregnancy; metal implants; and activity that included training of the scapular muscles. As this study examined the intrarater and interrater reliability of the RUSI technique rather than morphological characteristics of the lower trapezius muscle itself, it was decided to perform this initial study on asymptomatic subjects. It is recognized that reliability studies should be extended to patient groups. Subjects gave their written, informed consent to take part in the study. The study protocol was approved by the Trinity College Dublin Faculty of Health Sciences Research Ethics Committee. Subjects rights were protected. Procedure An Aquila Pie Data real-time ultrasound scanner (Pie Data Medical, Maastricht, The Netherlands), with an 8-MHz linear probe (40-mm footprint) was used. The scanner s accuracy was confirmed by calibration using a phantom 2 days prior to the study. Scanning was performed with the subject lying prone, with the head and neck in neutral alignment and the face through the facial cut-out section of the plinth. Subjects rested their arms by the sides of their body, with palms facing upwards. A pillow was placed under the abdomen to reduce the lumbar lordosis and another under the feet for comfort. To standardize the procedure for the purposes of the reliability study, the left lower trapezius muscle was scanned. The decision to scan the left lower trapezius as opposed to the right was arbitrary. The scanning site for imaging the muscle was determined with the subject seated by palpating and marking the inferior angle of the scapula on the skin with an eyeliner pencil, then marking the vertebral level on the thoracic spine at the same level as the inferior angle of the scapula. Then, with the subject prone, the spinous process of the sixth cervical vertebra (C6) was palpated to determine which vertebral level was to be used for imaging in each subject. The C6 spinous process was identified by asking the subjects to extend their neck, upon which, it is believed that the C6 spinous process becomes less palpable. 24 The spinous processes inferior to C6 were then palpated and counted until the spinous process closest to the skin mark on the thoracic spine was reached. This level was at T8 in 67% (10) of subjects (T7 in 2 subjects, T9 in 3 subjects, and not recorded on 1 subject). For more precise orientation and location of anatomy on the ultrasound screen, the probe was first placed centrally and horizontally over the relevant spinous process. This produced a bilateral image of the lower trapezius muscle, which resembled a butterfly (FIGURE 1). The probe was then moved laterally to the left, towards the inferior angle of the scapula, journal of orthopaedic & sports physical therapy volume 37 number 10 october

3 FIGURE 1. The Butterfly. Ultrasound image showing the spinous process (SP) centrally and the medial portions of the lower trapezius (LT) muscles bilaterally between the 2 echogenic white fascial layers. with the lateral edge of the spinous process kept in view (FIGURE 2). From preliminary observations and considering the local anatomy and the flat nature of the lower trapezius muscle, it was decided that this was the most practical method to locate the scanning site. It had been intended that images would be taken at the end of expiration, as performed for other muscles, such as the lateral abdominal muscles 38 ; but for the lower trapezius breathing did not appear to affect muscle thickness. Therefore, the breathing maneuver was not performed, as it tended to cause the probe to move. Once the probe was in the correct location and the left lower trapezius muscle was well visualized, the images were frozen on the screen, stored on a compact flash card, and then downloaded onto a computer for off-line analysis, using onscreen callipers, which took place 1 day later. All measurements were made offline using ImageJ software available from html. Measurements of muscle thickness were taken on all scans 3 cm lateral to the lateral edge of the spinous process. Investigator 1 also examined another site, which was 1 cm medial to the first site (ie, 2 cm lateral to the lateral edge of the spinous process), to see if the mean of the 2 measurements produced more reliable results (FIGURE 2). The cursors were placed on the inside edge of the muscle borders to measure their vertical distance on the transverse scans. Scans were coded so that the subject could not be identified. FIGURE 2. Ultrasound scan of the left lower trapezius in a female. The spinous process (SP) is kept in view at the right of the screen. The lower trapezius (LT) muscle is easily visible between the 2 echogenic fascial layers. The 2 white vertical lines indicate the sites (lateral and medial) where muscle thickness was measured. Reliability Assessments by Investigators Three investigators performed the scanning technique and measurements. One of the investigators (M.S.) was experienced in RUSI and widely published in the area. 35,36,40,41 The other 2 investigators had undergone introductory training in RUSI (a course approved by the United Kingdom Association of Sonographers) and had completed practice sessions of imaging the lower trapezius muscle prior to commencing the study. Four types of reliability were assessed. Intrarater Intraimage (Within-Scan) Reliability The principal investigator (investigator 1) took 3 images on 2 separate days in the same week, at the same time of day in each subject. The reliability of investigator 1 to measure the same image on the scanner s screen for each subject was examined. This was termed the intrarater intraimage (within-scan) reliability and was assessed using ICC 3,1 based on 3 observations from a single image for each subject. Intraimage measurements were performed for the lateral site only. Intrarater Interimage (Between-Scan) Reliability The ability of the same investigator to take and measure different images from the same subjects within the same session reliably was examined for investigator 1 on day 1. Each subject had 3 images taken and measurements of muscle thickness were analyzed for single sites (lateral and medial), as well as for both locations combined (mean value). The lateral site was measured 3 times on each image and the mean values were used to calculate the ICC (model 3,3). The medial site was measured twice on each image and therefore ICC model 3,2 was used. For both sites combined (lateral and medial), model 3,2 was also used. Intrarater Interimage (Between-Day) Reliability The reliability of the same investigator to measure 3 different images of the same subjects taken on 2 separate days was examined and termed intrarater between-day reliability. This was assessed for single sites (lateral and medial), as well as both locations combined (mean value). Mean values for each day were calculated from the 3 images, with the value for each image derived from repeating the on-screen measure 3 times for the lateral site and twice for the medial site. Because the unit of interest is the number of scans measured on each day (not the number of times each scan was measured), an ICC 3,3 was used to assess intrarater between-day reliability. Interrater Reliability In addition to the 3 images taken by the first investigator, the other 2 investigators also took 2 images each on day 2. To establish interrater reliability among the 3 investigators, all images were taken using the same surface markings to locate the scanning site and with the subject remaining in the same position. The order of imaging was randomized among the 3 investigators. Each investigator made 3 on-screen measurements at the lateral site for the 2 scans they took and the mean of the 2 scans for each investigator was used in the analysis (for investigator 1, only the first 2 of the 3 scans taken were used). An ICC 2,2 was used to assess interrater reliability between the 3 investigators, based on a mean value of 2 images for each of the 3 investigators. The investigators were not blind to the subjects during scanning, but were blind to whose scans they were measuring. While making measurements of the stored images, the results on the computer screen were not visible to the investigator and were recorded by another person. Investigators were also blind to their own previous results and those of the other investigators. 622 october 2007 volume 37 number 10 journal of orthopaedic & sports physical therapy

4 Data Analysis The dependent measure for analysis was the thickness of the left lower trapezius muscle. Both intrarater (intraimage, between-scan, and between-day) and interrater reliability were examined using ICCs 6,29 and standard error of measurement (SEM). The SEM provides an estimate of error in mm when interpreting muscle thickness values and was calculated using the formula (SEM = S 1 ICC), where S is the pooled standard deviation and ICC is the reliability coefficient. For TABLE 1 each of the ICC values, 95% confidence intervals were also calculated. Reliability between the 2 days for investigator 1 was also assessed using the Bland and Altman plot, 3,4 to provide a visual representation of the degree of agreement and to allow easy identification of bias and outliers. ICCs and Bland and Altman plots have been advocated as the statistical methods of choice in reliability studies. 29 Statistical analyses were performed using SPSS Version 12.1 for Windows (SPSS Inc, Chicago, IL). In 1 subject, the thickness of the lower Intrarater Intraimage (Within-Scan) Reliability for Lower Trapezius Muscle Thickness Measurements* Thickness (mm) Measurement 1 Measurement 2 Measurement 3 ICC 3,3 (95% CI) SEM (mm) ( ) 0.07 the measurement. * Measurements are mean SD. Three measurements made on the same scan for each subject (n = 16). TABLE 2 Intrarater Between-Scan Reliability for Lower Trapezius Muscle Thickness Measurements* Thickness (mm) Scan 1 Scan 2 Scan 3 ICC (95% CI) SEM (mm) Lateral site (ICC 3,3 ) ( ) 0.14 Medial site (ICC 3,2 ) ( ) 0.28 Combined sites (ICC 3,2 ) ( ) 0.07 the measurement. * Measurements are mean SD. Based on 3 images taken on the same day. TABLE 3 Intrarater Between-Day Reliability for Lower Trapezius Muscle Thickness Measurements* Muscle Thickness (mm) Day 1 Day 2 ICC 3,3 (95% CI) SEM (mm) Lateral site ( ) 0.24 Medial site ( ) 0.27 Mean of lateral and medial sites ( ) 0.25 the measurement. * Measurements are mean SD. Based on mean of 3 images taken on each day. trapezius muscle was imaged and measured at 3 different vertebral levels to see if muscle thickness varied between levels. RESULTS Intrarater Reliability Intrarater intraimage (withinscan) reliability of 3 repeated measurements of the first scan on day 1 taken at the lateral site by investigator 1 was good, with an ICC 3,1 of 0.99 (95% CI: 0.98 to 1.0) (TABLE 1). Investigator 1 also showed good intrarater interimage (between-scan), within day reliability for the single and combined sites (lateral site, ICC 3,3 = 0.96 [95% CI: 0.90 to 0.98]; medial site, ICC 3,2 = 0.90 [95% CI: 0.78 to 0.96]; medial and lateral sites combined, ICC 3,2 = 0.99 [95% CI: 0.99 to 1.0]) (TABLE 2). Intrarater interimage (between-day) reliability results for the same investigator (investigator 1) for the lateral site was ICC 3,3 = 0.91 (95% CI: 0.74 to 0.96), for the medial site 0.89 (95% CI: 0.68 to 0.96), and for the combined sites 0.90 (95% CI: 0.70 to 0.96). The SEM values shown in TABLES 1 through 3 were similar for all intrarater reliability conditions (ie, scanning sites, repeated measurements). A Bland and Altman plot for reliability at the lateral site between the 2 days is shown in FIGURE 3. The values for the mean difference (d) were close to zero ( 0.3 mm), with 1 outlier. The SD diff was 0.42 mm and the 95% limits of agreement were 1.2 to 0.5 mm. There was no association (using Pearson correlation coefficient) between differences in muscle thickness and mean muscle thickness for subjects (r = 0.36; P =.17). This indicates that the amount of error in measurements made between days is not associated with the thickness of the muscle. Interrater Reliability When results were compared among the 3 investigators within the same session (using the lateral site only), the ICC 2,2 was 0.88 (95% CI: 0.73 to 0.96) (TABLE 4). The SEM value for interrater reliability was journal of orthopaedic & sports physical therapy volume 37 number 10 october

5 Difference (mm) FIGURE 3. Bland and Altman plot showing between-day reliability for scans of lower trapezius taken at the lateral site by investigator 1. The difference in muscle thickness between day 1 and day 2 is plotted against mean muscle thickness for each subject. The middle line shows the mean difference ( 0.3 mm). The 95% upper and lower limits of agreement represent 2 standard deviations above and below the mean difference ( mm). Values for difference plotted on the x-axis are in millimeters. TABLE mm, which is similar to values for the intrarater conditions. Descriptive Data The mean thickness of the lower trapezius muscle was approximately 3.1 mm at the lateral site (SD, 0.8; range, ). Three levels were imaged in 1 subject, at T7, T8, and T9, and lateral thickness was similar at all levels (3.4, 3.4, and 3.6 mm, respectively). DISCUSSION 1.0 Aprocedure has been described for imaging the lower trapezius, adding to the list of skeletal muscles explored with RUSI. The small thickness value for the lower trapezius Interrater Reliability for Lower Trapezius Muscle Thickness Measurements* Thickness (mm) 2.0 Mean Thickness (mm) Investigator 1 Investigator 2 Investigator 3 ICC 2,2 (95% CI) SEM (mm) ( ) ( ) ( ) 0.88 ( ) 0.30 measurement. * Measurements are mean SD (95% CI of the mean). Based on mean of 2 images taken by each investigator at the lateral site muscle makes the possibility of error in measurement higher than that of larger muscles. But the SEM for each of the measurements taken was quite small ( mm) in relation to the mean muscle thickness of 3 mm, indicating that RUSI produces very precise and accurate estimates of the lower trapezius muscle thickness in asymptomatic individuals. The reliability of actual cursor measurements was good, indicating that taking only 2 measures on each scan would be justified in future studies. Reliability was also good between scans, between raters, and between days. Portney and Watkins 28 suggested that ICC values above 0.75 indicate good reliability, but that ICC values for clinical measurements should exceed Using these general guidelines, intrarater reliability for measurements between scans and between days at both lateral and combined locations (all ICCs greater than 0.90) supports the clinical measurement of the lower trapezius using RUSI. Intrarater reliability (between-day) for the medial measurement location was slightly lower (ICC 3,3 = 0.89). Although reliability between raters for the lateral location was good (ICC 2,2 = 0.88), investigator 3 appeared to systematically overestimate muscle thickness compared with the other 2 investigators. Examination of the 95% CI (TABLE 4) suggests a bias between investigators 1 and 3, as the confidence intervals do not overlap. The reason for this is unclear. The protocol required investigators to place the cursors on the inside edge of the muscle boundaries, but, as measurements of lower trapezius muscle thickness are small (mean thickness, 3.1 mm), there is little room for error. Our findings suggest that, when using RUSI, lower trapezius muscle thickness can be measured reliably by the same person. But caution is necessary when interpreting measurements taken by different raters, despite high ICCs. It is recommended that for any study involving different investigators, their interrater reliability needs to be established. For the present study, the level of the inferior angle of the scapula was used to standardize the scanning site. The inferior angle of the scapula is generally said to be level with the seventh thoracic vertebra. 33 In the present subjects, T8 was the most common vertebral level, so it is recommended that this level be used for RUSI measurements of the lower trapezius muscle in the future. The precise distance from the spinous process was not considered crucial at the time of scanning, given the flat nature of the muscle in most subjects. But for repeated measurements within a subject, stricter criteria would aid reliability. In some subjects, the shape of the muscle was irregular, in that it curved in the medial region (FIGURES 4 and 5), so measurements were taken in the lateral part of the 624 october 2007 volume 37 number 10 journal of orthopaedic & sports physical therapy

6 FIGURE 4. Irregular shape of left lower trapezius (LT) muscle seen on an ultrasound scan from a female. In some subjects, the muscle curved in the medial region. For this reason, the lateral part of the image where the muscle was more regular and horizontal was considered to be the preferable site for taking measurements of muscle thickness. Abbreviation: SP, spinous process. FIGURE 5. Ultrasound scan from a male subject showing a regular shaped left lower trapezius (LT), with clear hyperechoic fascial borders. image where the muscle was more regular and horizontal. The probe was only 4 cm in width, so a larger probe would have allowed the option of a more lateral site, further from the irregular area, ensuring uniformity of the procedure among subjects. Alternatively, the probe could have been moved laterally, at a proportionate distance between the vertebra and the scapula. However, as scapular position can vary among subjects, particularly where there are abnormalities of muscle function (weakness, causing lengthening of fibers, or overactivity, causing shortening), 39 this was not considered to be a reliable option. Another option, when measuring the thickness of irregularly shaped muscles, may be to measure the thickness at the location offering the maximum thickness. Measuring multiple sites is being explored and recommendations will be made for research purposes and clinical use, the latter requiring a more simple and rapid procedure. The relatively small values for thickness of this muscle (approximately 3 mm) might render the sensitivity of the measurement too poor to detect changes. However, the transversus abdominis muscle, which is only slightly thicker (mean of approximately 4 mm in females and 5.5 mm in males), 30 has been studied extensively using RUSI, 11,30,34,38 due to its believed important role in stabilization of the lumbar spine. 17 RUSI has been used to measure the thickness of transversus abdominis muscle at rest and during contraction. 11,34,38 One study that examined recruitment of the abdominal muscles (measured as a change in thickness with ultrasound imaging during contraction) demonstrated smaller increases in thickness of the transversus abdominis muscle among the individuals with low back pain, as compared to controls, during low-load contractions. 11 The feasibility of imaging the lower trapezius muscle during contraction warrants further investigation. The effects of gender, age, and activity on muscle thickness need to be studied in larger populations to produce normal databases for different groups, including different sports, to enable evaluation of the effects of pathology and interventions. While the findings of this study demonstrate that measuring the thickness of the lower trapezius muscle among asymptomatic subjects using RUSI is reliable, the generalizability of the technique among symptomatic subjects needs to be established before it can be used in clinical populations. A validation study is in progress, comparing RUSI with MRI of the lower trapezius muscle. Further exploration of different scanning sites, involving different vertebral levels (which did not vary in the subject studied) and distance from the spine, is warranted to confirm the robustness of reliability and validity. RUSI may have a role to play in the clinical evaluation of lower trapezius muscle dysfunction. The muscle is too large to allow its CSA to be measured but its thickness can be measured readily. The validation study mentioned above will also investigate the relationship between thickness and CSA measured from MRI scans. If thickness correlates with CSA and thickness values are similar for the 2 imaging techniques, then RUSI measures of thickness may potentially be used to assess lower trapezius muscle size, providing an indirect measure of strength. 40,41 This may prove beneficial for the clinical evaluation of patients with scapular muscle dysfunction. It is agreed that optimum functioning of the lower trapezius, upper trapezius, and serratus anterior muscles in an integrated fashion is necessary for full and pain-free shoulder movement. 13,22 Abnormal patterns of activity of these muscles have been identified in subjects with impingement and frozen shoulder syndromes, who display increased EMG activity of both upper and lower trapezius during arm elevation when compared to matched controls, 19,20,22 with altered ratios of activity resulting in excess scapular elevation in those with frozen shoulder. 19 RUSI may have a role to play in determining normal values for the ratio of upper to lower trapezius muscle thickness, and we are currently investigating this in healthy subjects. Such information may assist in identifying patients whose muscle thickness ratios are altered due to an imbalance between the 2 muscles. Dynamic RUSI may also be useful for assessing each muscle more specifically by examining changes in thickness during arm tasks. Currently, clinical evaluation of the scapular muscles involves observation of static and dynamic postures. There is a need for an objective tool to measure scapular muscle function to evaluate rehabilitation strategies. EMG has been used by many investigators to analyze scapular muscle activity in symptomatic groups and controls. However, EMG has several limitations for clinical use, including difficulty of use and interpretation, cross-talk of signals between muscles, dissociation from force when muscle is fatigued, 9 and access to equipment. The advantages of RUSI over EMG are that the imaging technique is more widely accessible, quick to use, and can easily differentiate between adjacent muscles. journal of orthopaedic & sports physical therapy volume 37 number 10 october

7 CONCLUSIONS Thickness of the lower trapezius muscle can be measured reliably with RUSI, both within and between investigators. The vertebral level suggested for measurement is T8. The potential use of RUSI in the clinical evaluation of lower trapezius muscle function remains unknown. The present findings form the basis for validation studies and to examine reliability in clinical populations. Future research in the area aims to explore whether thickness measured with RUSI correlates well with muscle thickness and cross-sectional area on MRI, or whether changes in thickness are a valid reflection of changes in muscle function. ACKNOWLEDGEMENTS We thank the staff and students at Trinity College Dublin for taking part in the study, Mr Jeremy Walsh and Ms Sinead Langford for their help with data entry, Jamie O Dwyer for help with illustration presentation, and Dr John Gormley for helpful comments on the manuscript. REFERENCES 1. Bakke M, Tuxen A, Vilmann P, Jensen BR, Vilmann A, Toft M. 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