|Home | About | Journals | Submit | Contact Us | Français|
The importance of the scapular stabilizing muscles has led to an increased interest in quantitative measurements of their strength. Few studies have measured isometric or concentric isokinetic forces. Additionally, limited reports exist on the reliability of objective measures for testing scapular protraction and retraction muscle strength or scapular testing that does not involve the glenohumeral joint.
To determine the reliability of four new methods of measuring the maximal isometric strength of key scapular stabilizing muscles for the actions of protraction and retraction, both with and without the involvement of the glenohumeral (GH) joint.
The Isobex® stationary tension dynamometer was used to measure the maximal isometric force (kg) on thirty healthy females (ages 22–26 years). Three measures were taken for each method that was sequentially randomized for three separate testing sessions on three nonconsecutive days.
Intraclass correlations (ICC2,3) for intrasession reliability and (ICC3,3) for intersession reliability ranged from 0.95 to 0.98, and 0.94 to 0.96 respectively. The standard errors of measurement (95% confidence interval [CI]) were narrow. Scatter grams for both protraction and retraction testing methods demonstrated a significant relationship, 0.92 for protraction (95% CI 0.83 to 0.96) and 0.93 for retraction (95% CI 0.87 to 0.97). Bland-Altman plots indicated good agreement between the two methods for measuring protraction strength but a weaker agreement for the two methods measuring retraction strength.
The four new methods assessed in this study indicate reliable options for measuring scapular protraction or retraction isometric strength with or without involving the GH joint for young healthy females.
The importance of the scapular stabilizing muscles has led to an increased interest in quantitative measures of strength for protraction and retraction, especially for portions of the trapezius and serratus anterior muscles.1–16 Several studies have attempted to measure the strength of the scapular stabilizing muscles by measuring isometric or concentric isokinetic forces.16–26 Although strengthening of the scapular muscle stabilizers is a common intervention for scapular dyskinesia,27 objective measurement of scapular stabilizing muscle strength is likely uncommon in most clinical settings. Simple and reliable objective strength measurement capabilities for the scapular stabilizing muscles could assist the clinician in identifying weakness, muscle imbalance, or both during a shoulder evaluation and could be used as an outcome measure following selected intervention exercises. Objective strength testing of the scapular stabilizing muscles is difficult since the commonly used procedures require a resistive contact force on the upper extremity, or the glenohumeral (GH) joint is positioned at or above 90 degrees elevation, while assuming the GH joint and rotator cuff muscles are normal. The same difficulties exist for manual muscle testing (MMT) of the serratus anterior when using the upper extremity. For example, the original MMT position advocated by Kendall for testing the serratus anterior applied the resistance force against the fist with the arm extended. Using this method, Donatelli did not report protraction isometric force data in a study of baseball players since the intrarater reliability was only 0.266. Similarly, Michener modified the Kendall MMT procedure for the serratus anterior by flexing the elbow to 90° and applying the resistance force to the olecranon along the long axis of the humerus. While hand-held dynamometers used in objective strength testing have reported good reliability, the reliability is dependent on the strength of the tester.28
Isokinetic equipment that may be used for scapular stabilizing muscle testing22, 23 are hindered by equipment costs and manpower time. Further, movements and positions used are often uncommon in activities of daily living, work place, athletics, or with common positions used for training and therapy.29
Clinically there is a need for simple and objective strength measurements of the scapular stabilizing muscles.30 Strength can be defined as an individual's ability to exert maximum muscular force statically.29 The purpose of this study was to determine the reliability of four new methods that measure maximal isometric strength of key scapular stabilizing muscles for protraction (serratus anterior, upper trapezius, and pectoralis minor muscles) and retraction (middle trapezius and rhomboid muscles) movements, with and without the involvement of the GH joint and rotator cuff muscles.
Thirty females (mean age = 24 years, SD 1.5, range 22 to 28 years) volunteered to participate in the study. The study was approved by the East Tennessee State University/Veteran's Administration Medical and the Rocky Mountain University of Health Professions Institutional Review Boards, and written consent was obtained from all subjects. Only females were recruited because of the likely inherent variances and strength differences between males and females, and to avoid the possibility of healthy males exceeding the force measuring capabilities of the testing apparatus. Exclusion criteria included pregnancy and history of neck or shoulder pain/dysfunction within the last three months that required a medical consultation or intervention. Potential subjects were then asked to complete the modified American Shoulder and Elbow Surgeons Questionnaire (ASESQ).31 A previous study reported a test-retest intra-class correlation coefficient (ICC) of 0.96 for the ASESQ.31 Individuals who scored lower than 95 on the ASEQ were informed that they were not eligible to participate in the study.
After meeting all study inclusion criteria, subjects were assigned a random identification number to aid in assuring confidentiality. All subjects completed a short questionnaire that inquired about their age (years) and upper extremity dominance relative to their preference for writing or throwing a ball. Height (cm) and weight (kg) were measured using a standard stadiometer.
The Isobex® static tension dynamometer (Medical Device Solutions, AG; Burgdorf, Switzerland) was used to measure each subject's isometric force. The Isobex® dynamometer has been used in post-operative shoulder follow-up studies for measuring the strength portion of the Constant-Murley shoulder score.32 The dynamometer was attached to a special track that was anchored to the wall that then allowed vertical level adjustment of the dynamometer. The instrument's measurement range is 10-450 Newtons (N), resolution of 1 N, precision ± 2 N, and the frequency of measurement is 10 measurements/second. Leggin et al33 reported the intrarater reliability of the Isobex® instrument was 0.95 to 0.98 and the interrater reliability was 0.90 to 0.97 when measuring maximal isometric force for humeral abduction and internal/external rotation.
A portable stabilizing bench with a padded vertical trunk pad (A87 Pressing Chair, Magnum Fitness Systems, Milwaukee, WI, USA) was used to position each subject (Figure 1). A laser beam (Acculine Laser Levels Pro, 40-6164) was used to align the center of the humerus with the tension attachment perpendicular to the dynamometer that was anchored on the wall. The height of the dynamometer was adjusted so that the tension line was parallel to the floor and in the sagittal plane. The subject was seated facing away from the wall with the chest against the vertical bench pad. A padded stabilizing strap was draped around the subject's trunk, vertical bench pad, and over the opposite shoulder for increased trunk and hip stabilization. The humerus of the subject's dominant shoulder was elevated to 90° with the scapula in a neutral resting position (midway between maximal protraction and retraction) and the elbow extended. The dynamometer handle was positioned with the subject's thumb up. Tension in the dynamometer tension line was adjusted until there was no slack in the dynamometer cable. During testing, emphasis was placed on the subject pushing against the handle while trying to isolate protraction of the shoulder without using the trunk or lower extremity muscles.
The position and stabilization were the same as Position 1. However, the arm position and resistive contact area were different (Figure 2). The humerus of the subject's dominant shoulder was placed at the subject's side with the elbow flexed at 90°. A padded shoulder strap circumscribed the shoulder in the sagittal plane while passing under the axilla and attached to the dynamometer tension line. If necessary, the tester lightly held the top of the strap to keep it from sliding down prior to achieving the maximal tension force. The primary contact area over the shoulder for the resisted movement was the anterior aspect of the shoulder and the lateral aspect of the pectoral region. Tension in the dynamometer tension line was adjusted until there was no slack in the dynamometer tension line. During testing, emphasis was placed on the subject pushing forward against the strap while trying to isolate protraction of the shoulder without the use of the trunk or lower extremity muscles.
The positioning and stabilization were similar to Positions 1 and 2. However, the subject's back was stabilized against the vertical trunk pad and the subject faced the wall and the dynamometer (Figure 3). The humerus of the dominant shoulder was elevated to 90°, the scapula in a neutral resting position, and the elbow extended. Subjects were then instructed to grasp the dynamometer handle with their thumb up. The dynamometer tension line was adjusted until there was no slack. During testing, emphasis was placed on the subject pulling against the handle while trying to isolate retraction of the shoulder without using the trunk or lower extremity muscles.
The position and stabilization were the same as Position 3. However, the subject's arm position and the resistive contact area were different (Figure 4). The humerus of the subject's dominant shoulder was placed at the subject's side with the elbow flexed at 90°. A padded shoulder strap circumscribed the shoulder in the sagittal plane while passing under the axilla and attached to the dynamometer tension line. If necessary, the tester lightly held the top of the strap to keep it from sliding down prior to achieving the maximal tension force. The primary contact area over the shoulder for the resisted movement was the posterior aspect of the shoulder and the lateral aspect of the scapula. The dynamometer tension line was adjusted until there was no slack. During testing, emphasis was placed on the subject pulling back against the strap while trying to isolate retraction of the shoulder without using the trunk or lower extremity muscles.
For each of the three data collection sessions, three measurements were conducted for each of the four methods (Figures 1–4) for a total of twelve strength measurements per session. Two testing methods were administered to determine protraction strength (one each with or without involvement of the GH joint or rotator cuff muscles), and two testing methods were for retraction strength (again, one each with or without involvement of the GH joint or rotator cuff muscles). The sequence of the four methods tested was randomized for each of the four sessions to minimize the influence of learning effect or fatigue.
The subject was positioned in the appropriate sitting position for each testing method. A brief warm-up consisted of three to four repetitions of active shoulder rolls, humeral circumduction, shoulder elevation, protraction, and retraction. The procedure was explained to the subject, followed by sub-maximal tests to practice the sequence of testing until the subject felt comfortable with the testing protocol, generally one to three times. The dynamometer automatically calibrated prior to each measurement. Then, an acoustic start signal from the dynamometer was triggered, and the subject pushed or pulled against the dynamometer maximally for a total duration of 5 seconds as recommended by the American Society of Exercise Physiologists.34 A consistent verbal command was used during the 5 second period of testing to encourage maximal effort. The subject would stop exerting force when a second acoustic sound was heard from the dynamometer. The average force over the 5 seconds that subjects exerted their maximal force for each test was measured and recorded in kilograms (1 kg = 9.81 N).
Subjects were given 1 minute rest periods between measurements for each method.34 A minimum of three-minute rest intervals were used between the four methods. Testing was measured and recorded over three separate testing sessions with intervals of one to three days. One investigator administered all testing procedures. All testing was performed on the dominant extremity.
Descriptive statistics (mean, SD, and range) were used to describe the subjects' age (yrs), height (cm), weight (kg), body mass index (BMI) [kg/m2], and score on the the modified American Shoulder and Elbow Surgeons questionnaire (ASESQ). Means and standard deviations were also calculated for the maximal isometric force measurements (kg) of the four methods of testing key scapular stabilizing muscles. Intersession reliability of the measurements was determined using intraclass correlation coefficients (ICC) 35, 36 with 95% confidence intervals along with the standard error of measure (SEM) to provide an estimate of error associated with the measurement. The intrasession reliability was determined using intraclass correlation coefficients (ICC) 35, 36 with 95% confidence intervals along with the standard error of measure (SEM) to provide an estimate of error associated with the average force measurement for each method between the three testing sessions. Additional comparisons of the two methods for protraction and retraction were conducted using Pearson Product-Moment Correlation Coefficient,36 scattergrams,36 and a Bland-Altman Plot.36–38 Pearson Product-Moment Correlation Coefficients determined the amount of relationship between the two methods. The scatter-gram provided a graphic representation between the force measurements of each subject for one method against the force measurements of the other method. Bland-Altman plots37 were created to visually inspect agreement of the two testing methods for both protraction and retraction (with and without the GH joint and rotator cuff muscles). The Bland-Altman plot provided a visual means to look at the difference between the two methods plotted against the mean score for each subject. Within the Bland-Altman plots were lines that indicated the mean difference between methods and lines for two standard deviations above and below the mean difference. To assist in visualizing any bias between methods, data points were fitted with a linear regression line. All statistical calculations were performed using the Statistical Package for the Social Sciences 15.0 statistical package (SPSS, Inc., Chicago, IL) and Microsoft Excel software package (Microsoft, Redmond, Washington).
Characteristics of the sample are presented in Table 1. Mean force (kg) and standard deviations for each of the four scapular muscle strength testing methods across three separate testing sessions are shown in Table 2. The intrasession reliability ICC (2,3) for each of the four methods with their corresponding 95% CI's and standard error of measurements (SEM) are presented in Table 3. The reliability correlation coefficients for the four methods of strength testing ranged from 0.95 to 0.98. The standard error of measurement (SEM) ranged from 0.9 to 1.10 kg for protraction with the GH joint, and 0.7 to 1.0 kg for protraction without the GH joint. The SEM for retraction ranged between 1.3 and 1.5 kg for retraction with the GH joint, and 0.9 to 1.5 kg for retraction without the GH joint. The ICC's (3,3) for each of the four methods with their corresponding 95% CI's and standard error of measurement (SEM) for intersession reliability are presented in Table 4. Intersession reliability correlation coefficient estimates ranged from 0.94 to 0.96 while the SEM were between 1.2 and 1.5 kg.
A scattergram of the force output for protraction with the GH joint as a function of the force output without the GH joint indicated a significant relationship of 0.92 between the methods (95% confidence interval: 0.83 to 0.96) (Figure 5). The Bland-Altman plot for protraction (Figure 6) was representative of the differences between the two methods as a function of the average of the two methods. The line for the mean difference between the methods was -1.31. For protraction, there were a minimal number of outliers outside the two standard deviations of the mean differences, indicating general agreement between the two methods. However, as the force output increased, the difference between the methods also increased.
The scattergram of the force output for retraction with the GH joint as a function of the force output without the GH joint is shown in Figure 7. A significant relationship of 0.93 was found between the methods (95% confidence interval: 0.87 to 0.97). The Bland-Altman plot for retraction (Figure 8) again was representative of the differences between the two methods as a function of the average of the two methods. However, the middle line indicating the mean difference between the two methods was 5.83. The two lines indicating two standard deviations from the mean difference showed there was one outlier and a varied spread of the mean difference. Furthermore, the regression line indicated that as more force was generated, a greater difference between the methods was observed.
The two methods for testing protraction maximal isometric strength and the two methods for testing retraction maximal isometric strength indicated high intrasession and intersession reliability with a small standard of error of measurement. According to Portney and Watkins,36 values of ICC above 0.90 are indicative of good reliability for clinical studies.
The importance of these findings is twofold. First, these new procedures for measuring scapular muscle strength for protraction and retraction offer a possible clinical alternative to manual muscle testing. Second, two of the procedures offer an alternative to measuring protraction and retraction strength while avoiding the use of the humerus and rotator cuff.
The intraclass correlations (ICC's) found in our study were high and were similar to reports that used the Isobex® static dynamometer33 or measured protraction and retraction strength with other static dynamometers17, 19, 20, 25, 26 with a custom stationary apparatus16 or a modified isokinetic machine.22, 23 The ICC's observed in this study were higher than the 0.27 intrarater reliability for protraction (serratus anterior muscle) performed on baseball players using the traditional manual muscle testing method of applying a downward force on the subject's hand.21 This may have been due to the disparity of strength between the baseball players and testers, as well as the difficulty with the subject's coordination and stability when a force is applied at the distal end of the upper extremity kinetic-chain. One aim of this study was to determine if there was similar agreement between two alternative methods of measuring force output for protraction and retraction, with the difference between the two methods being the inclusion or exclusion of the GH joint during testing. Many studies have used Kendall et al's39 testing protocols which involve the GH joint when testing protraction and retraction. However, because some individuals may also present with GH joint dysfunction, it raises the issue if excluding GH joint during testing may affect force outcome measures. Although our ICC's agreement findings were considered strong, the Bland-Altman plots indicated that not involving the GH joint affected the readings.
For the two protraction methods there were only two outliers for the thirty force outputs at two standard deviations from the mean difference of −1.31. The small mean difference visually indicated that there was minimal disparity between the two methods of force measurement for protraction. However, as the force output increased there was a small bias towards the method not involving the GH joint. This bias may be due to the unaccustomed movement pattern and also to the perceived increased coordination necessary to involve the unsupported upper extremity.
For the two retraction methods there was only one outlier for the thirty force outputs at two standard deviations from the mean difference of 5.83. While both methods of measuring retraction were reliable, a mean difference of 5.83 indicated that the methods were not likely interchangeable. The bias towards retraction involving the GH joint may be due to the fact that pulling is a common movement pattern. In addition, since the humerus was elevated to ninety degrees when involving the GH joint, there may have been some affect from the deltoid and latissimus dorsi muscles. Furthermore, for retraction, there was a greater spread of force measurements within two standard deviations at all levels of force output.
There were limitations to this study. First, restricting the subject sample to young healthy females limits the generalizability of the findings. However, the intent was to establish reliability of the new measuring procedures in a confined homogenous population to minimize confounding. The reliability of these new testing methods and positions need to be further investigated with various population groups, validated, and taught to clinicians so they can assess scapular musculature without incorporating the GH joint. Second, the testing procedures appear to require stabilization from both sides of the trunk in the upright sitting position, both for countermovement stabilization and for prevention of synergistic trunk muscle activity in the direction being tested. Stabilization benches used for weight training purposes are readily available; however, one has to consider the width so it allows stabilization of the trunk while offering mobility of the shoulder girdle being tested. Alternative positioning methods using the supine and prone positions also deserve further study. The width of the trunk stabilization pad and the determination of the neutral position of the scapula should be considered. Third, some subjects had difficulty learning to quickly respond to the audio stimulus, which may have initially made it difficult to provide a maximal contraction effort for the full 5-second duration. Subjectively, it appeared that subjects that were athletic and familiar with resistance exercises learned the procedure more easily. Fourth, there was a trend of increasing mean force scores noted over the three testing sessions for all four methods. This suggested that there may have been some learning effect in spite of randomization and a strict testing procedure. However, further post hoc analysis using repeated-measures ANOVA revealed there was no real learning effect observed. Using three trials and using the mean measure helped to insure the truest score possible. Finally, while the Isobex® static dynamometer maximum measuring capability of 450 Newtons (approximately 46 kg) is appropriate for testing a normal patient population, it may not be sufficient for measuring individuals with strong, athletic physiques. With the appropriate stabilization it is unlikely that most athletes would exceed 450 Newtons. If extremely strong athletes are being tested there are various tension dynamometers that are available that could be used for the same protocol.
There were two key strengths of the study. First, several advantages of our new procedures using the Isobex® static tension dynamometer is that it is 1) a relatively inexpensive dynamometer, 2) is a more objective than manual testing grades,40 3) is relatively easy to learn to use, 4) is not dependent on the tester's strength, and 5) can be set-up in a typical patient management setting. Although the Isobex® dynamometer was chosen for this study, there are many other dynamometers that are commercially available. Second, it has been common to incorporate the GH joint when evaluating scapular dysfunction.39, 41–43 This study did not resolve the issue of whether to include or not include the GH joint when evaluating the scapular musculature in the healthy individual, and further study is warranted. However, in a clinical situation where the patient being evaluated for scapular muscle strength has problems such as limited GH range of motion, impingement with the GH joint at 90 degrees flexion, painful or dysfunctional rotator cuff musculature, elbow or wrist problems, or problems with their grip, then methods not requiring the upper extremity would seem prudent. The maximal strength force measurement is limited by the weakest link in the kinetic-chain.
The clinical application of this study is that it offers the clinician additional reliable, objective, and efficient quantitative methods for measuring force in the scapular stabilizing muscles, as well as being able to test subjects even if the subject has GH joint dysfunction or upper extremity problems. The clinician will be able to collect quantitative baseline information during the initial evaluation to determine muscle weakness or imbalances, as well as using the testing procedures for outcome measurements to determine the effect of interventions used for strengthening the scapular stabilizing muscles.
The four new methods in our study provided reliable options for quantitatively measuring scapular protraction and retraction maximal isometric strength with a stationary static tension dynamometer that was not dependent on the tester's strength as when testing with a hand-held compression dynamometer. Additionally, two of the new methods allowed measurement of scapular protraction and retraction isometric strength without involving the GH joint or rotator cuff muscles. Therefore, the findings of our study suggest that these four methods of measurement might be trustworthy when measuring protraction and retraction strength in a healthy population of females. Further research with patient and athletic populations will be needed to validate the usefulness of these new testing procedures.
This work contains data and text originally written as a dissertation for Rocky Mountain University of Health Professions. Small Research Grants from The Research Development Committee of East Tennessee State University paid for the stabilization bench and reimbursement of subjects.