Scapular Angular Positioning at End Range Internal Rotation in ... - jospt

Scapular Angular Positioning at End Range Internal Rotation in Cases of Glenohumeral Internal Rotation Deficit Michael R. Borich, DPT 1,2 Jolene M. Bright, DPT 1,3 David J. Lorello, DPT 1,4 Cort J. Cieminski, PT, MS, ATC, CSCS 5 Terry Buisman, PT 6 Paula M. Ludewig, PT, PhD 7

Study Design: Controlled laboratory study. Objectives: Investigate the relationship between glenohumeral internal rotation range-of-motion deficit and 3-dimensional scapular angular positioning during active arm movements in participants with recent participation in overhead sports activity. Background: Subacromial impingement is one of the most common shoulder pathologies and is multifactorial in etiology. Posterior glenohumeral joint capsule tightness has been theorized to contribute to one potential causal factor: abnormal scapular positioning. Methods and Measures: Twenty-three subjects, who had participated in competitive sports involving overhead activity within the last 5 years, were categorized into 2 groups based on the degree of glenohumeral internal rotation deficit (20% deficit threshold). Scapular angular positioning of subjects performing shoulder internal rotation from 90° flexion and abduction shoulder positions was evaluated using 3-dimensional electromagnetic surface tracking. Additional sensors monitored trunk and humeral motion. Scapular position data at end range glenohumeral internal rotation, along with glenohumeral internal rotation range of motion measurements, were used to analyze the relationship between glenohumeral internal rotation deficit and scapular position using 2-way ANOVA and regression analyses. Results: The internal rotation deficit group had significantly greater scapular anterior tilt (9.2° difference, P = .04) across positions, as compared to the control group. Regression analysis demonstrated a significant association between glenohumeral internal rotation deficit and scapular position (tilting) during flexed internal rotation (r 2 = 0.37, P = .03) and for scapular position (anterior tilting and upward rotation) during abducted internal rotation (r2 = 0.35, P = .036). Conclusions: These findings demonstrate a significant relationship between glenohumeral internal rotation deficit and abnormal scapular positioning, particularly increased anterior tilt. This 1 Student (at time of the study), Program in Physical Therapy, The University of Minnesota, Minneapolis, MN. 2 Doctoral Student, Program in Rehabilitation Science, The University of Minnesota, Minneapolis, MN. 3 Physical Therapist, Thomas Jefferson University Medical Center, Philadelphia, PA. 4 Physical Therapist, Maricopa Medical Center, Arizona Burn Center, Phoenix, AZ. 5 Doctoral Candidate, Program in Rehabilitation Science, The University of Minnesota, Minneapolis, MN; Assistant Professor and Program Director, Doctor of Physical Therapy Program, College of St Catherine, Minneapolis, MN. 6 Physical Therapist, Orthopedic Rehabilitation Specialists, Inc. Minneapolis, MN. 7 Associate Professor, Programs in Physical Therapy and Rehabilitation Science, The University of Minnesota, Minneapolis, MN. This study was approved by The Human Subjects Committee of The Institutional Review Board of The University of Minnesota. Address correspondence to Paula M. Ludewig, The University of Minnesota, 420 Delaware St SE, Minneapolis, MN 55455. E-mail: [email protected]

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relationship identifies a possible mechanism for development of excessive scapular anterior tilt. J Orthop Sports Phys Ther 2006;36(12):926934. doi:10.2519/jospt.2006.2241

Key Words: biomechanics, rotator cuff, scapula, shoulder

D

uring overhead activities, the shoulder must have adequate internal rotation (IR) and external rotation (ER) range of motion (ROM), and coordinated kinematic patterns between the humerus, scapula, clavicle, and thorax to function properly. Athletes involved in sports requiring repetitive overhead motion, such as, baseball, softball, swimming, volleyball and tennis, are identified as an at-risk population for the development of subacromial shoulder impingement secondary to repetitive placement of the shoulder into vulnerable positions as well as high forces and loads.10,14,15,25 Several factors have been implicated in the potential development of subacromial impingement syndrome, including posterior glenohumeral joint capsule tightness.25 This soft tissue tightness is theo-

Journal of Orthopaedic & Sports Physical Therapy

A

Downward rotation

Upward rotation

B RESEARCH

External rotation

REPORT

rized to develop as an adaptation to the high stresses on the posterior capsule, particularly during deceleration of the arm.32 The posterior glenohumeral joint capsule is a significant restraint to shoulder IR ROM, as demonstrated cadaverically.4,13,31 Clinically, measurement of posterior capsule tightness cannot be distinguished from tightness of other posterior shoulder soft tissues.25 However, the association between overhead athletic participation and loss of dominantside shoulder IR ROM is well documented.1,5,6,27,28,30,34 A clinical relationship between presumed posterior capsule tightness and limited IR ROM has also been established by previous authors using multiple methods of testing, including supine IR ROM, supine cross-body adduction, and sidelying cross-body adduction.1,3,5,11,19,32 Proper 3-dimensional (3-D) positioning of the scapula is crucial in allowing full and nonimpaired motion of the upper extremity.21,22,24 The resting 3-D orientation of the scapula on the thorax has been reported to include slight upward rotation, anterior tilting, and IR (protraction).21,24 During planar humeral elevation above shoulder level in asymptomatic subjects, the scapula moves into progressive upward rotation, slight ER (retraction) at higher elevation angles, and decreased anterior tilting (Figure 1).21,22,24 These scapular motions are believed necessary during glenohumeral elevation to maximize the distance between the greater tuberosity and acromion process, thus maintaining adequate size of the subacromial space.9 Deviations from these scapular kinematic patterns have been observed in subjects with subacromial impingement syndrome, as they tend to have increased anterior tilting and IR, as well as less upward rotation of the scapula.21,22 The combination of these motion deviations are theorized to result in a reduction of the subacromial space12 and increased risk for rotator cuff impingement. Alterations in scapular and humeral kinematics have been theorized as a result of posterior capsule tightness.13,19 However, research to date has only investigated glenohumeral translatory changes with induced posterior capsule tightness.13 Induced posterior capsule tightness in cadavers has resulted in alterations in glenohumeral kinematics, which may place the overhead functioning shoulder at greater risk for injury.13 For example, Harryman et al13 reported significantly increased anterior and superior translation of the humeral head during shoulder flexion after surgical tightening of the posterior capsule. It has also been postulated that in vivo posterior capsule tightness may result in abnormal scapular motion on the thorax, particularly during the follow-through phase of throwing, in which the humerus is in a flexed, horizontally adducted, and internally rotated orientation.18 This posterior capsule tightness may lead to passively moving the scapula over the thorax resulting in greater anterior

Internal rotation

C

Posterior tilting

Anterior tilting

FIGURE 1. Scapular motions from (A) posterior (upward/downward rotation); (B) superior (internal/external rotation); and (C) lateral (anterior/posterior tilting) views. Axes of rotation are indicated as black dots.

J Orthop Sports Phys Ther • Volume 36 • Number 12 • December 2006

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tilting and IR (protraction) of the scapula, subsequently placing the shoulder at risk for rotator cuff impingement. To date, however, no study has quantified the effect of glenohumeral IR deficit, presumed secondary to posterior capsule tightness, on the 3-D positioning of the scapula during shoulder motion. The purpose of this study, therefore, was to investigate the relationship between glenohumeral IR deficit and 3-D scapular positioning during end range active glenohumeral IR at 90° humeral elevation. We hypothesized that there would be a significant relationship between glenohumeral IR deficit and abnormal 3-D scapular positioning, specifically increased IR, anterior tilting, and decreased upward rotation in subjects exposed to repetitive overhead-throwing activities.

tional volunteer did not meet inclusion/exclusion criteria, and 1 did not complete testing.

Instrumentation

METHODS

The Flock of Birds electromagnetic motion capture system and Minibird 800 motion sensors (Ascension Technology Corporation, Burlington, VT) were used to quantify 3-D position and orientation of each subject’s scapula, humerus, and thorax during kinematic testing. A fourth sensor was attached to a hand-held stylus to digitize various manually palpated anatomical landmarks. The translational range of the unit is ±76.2 cm in any direction forward of the transmitter. Position and orientation root-meansquare accuracy has been reported to be 1.8 mm and 0.5°, respectively, for this system. The Motion Monitor software (Innovative Sports Training, Inc, Chicago, IL) collected the 3-D sensor data at a sampling rate of 100 Hz for each sensor.

Subjects

Procedures

Subjects were recruited from local competitive baseball and softball teams as well as postings and announcements. Athletes involved with overheadthrowing motions were sought. Twenty-three asymptomatic volunteers were included in this study (18 males and 5 females). The sample contained 9 baseball pitchers, 12 fielders, and 2 athletes from other overhead sports. Seven baseball players participated at the collegiate level, while the remaining subjects were current recreational-league participants. The study was approved by The Institutional Review Board of The University of Minnesota, and informed consent was obtained prior to testing. Upon consent, subjects were given a questionnaire to provide background information (age, height, body mass, type and amount of sports participation, etc) as well as to provide a history of any shoulder injury or pathology. Involvement in competitive overhead-throwing sports within the last 5 years was required for inclusion. To prevent confounding of conditions other than glenohumeral IR deficit that might impact shoulder kinematics, exclusion criteria included any of the following: current shoulder pain; shoulder ROM loss (less than 150° of abduction); self-report of past fracture, dislocation, separation, surgery, labral tear, or other diagnosed pathology of the shoulder; current treatment for shoulder pain or dysfunction; or positive results on a clinical screening examination, including positive load-and-shift test,23 greater than 2 positive tests for impingement (Neer, Hawkins Kennedy, Jobe’s, Speed’s),20,23,26 or a positive cervical quadrant test.23 These criteria allowed for testing the potential mechanistic effect of glenohumeral IR deficit. All clinical screening was completed by a single investigator. Beyond the 23 subjects analyzed, 1 addi928

With the subject in a supine position, the shoulder was placed in 90° abduction of the humerus relative to the trunk, verified by goniometric measurement, and glenohumeral ER/IR passive ROM was measured using an inclinometer aligned on the forearm (Figure 2). Scapular motion was manually restricted by direct pressure over the anterior acromion process during glenohumeral IR assessment. High intratester goniometric reliability of passive shoulder IR ROM measures has been previously reported (ICC, 0.93).29 Percent deficit of IR was calculated by dividing the difference between the dominant (D) arm and nondominant (ND) arm IR values by the nondominant IR and multiplying by 100 [((ND – D)/ND) × 100]. Clinically, this has been referred to as glenohumeral IR deficit or ‘‘GIRD.’’1,5,6 To classify subjects into deficit or control groups, a cutoff for percentage deficit had to be selected. Groups were

FIGURE 2. Measurement of glenohumeral internal rotation range of motion.


A

Scapular sensor

Thorax sensor Humeral sensor

B

RESEARCH REPORT

subsequently divided based on their percent IR deficit. The experimental or IR deficit group was operationally defined as subjects with a dominant-arm IR loss of 20% or more, while the control group had nondominant arm IR loss or a loss of IR less than 20%. The choice of a 20% cutoff for assignment into deficit or control groups was based on unpublished data of deficits discriminating Major League baseball pitchers with shoulder pain from asymptomatic major league pitchers.7 Ten subjects were subsequently assigned to the IR deficit group and 13 subjects to the control group. Sensors were attached with double-sided tape to the skin overlying each subject’s sternum inferior to the sternal notch, and the posterior flat aspect of the scapular acromion process just lateral to the junction with the spine of the scapula.21 The third sensor was attached to a thermoplastic cuff held around the distal humerus with Velcro straps (Figure 3A). Bony landmarks of the trunk, scapula, and humerus (Table 1) were manually palpated and digitized with the hand-held stylus to create 3-D anatomical coordinate systems for trunk, scapular, and humeral segments.21 The trunk axes created represent the cardinal planes. The digitizing process allows sensor position and orientation to be transformed into anatomically based position and orientation data for each segment.21,35 All palpation and subsequent digitizing was done by 1 investigator who underwent training and practice until able to reproduce angular values within 3° with subjects in a standing resting position (arms relaxed at the side). Subjects were required to perform 2 active shoulder ER/IR motions, one at 90° shoulder flexion (Figure 3B), the other at 90° shoulder abduction (Figure 3C). The 90° elevation angle for these 2 motions was maintained by a sling around the distal humerus that was fixed to an overhead support beam. The sling provided tactile input to maintain position but not rigid support, so subjects still needed to contribute to active elevation of the arm in this position. Data were collected from each subject’s dominant arm, and subjects were asked to complete each motion twice through their full available motion, starting at maximum ER and proceeding to maximal IR. To standardize velocity of the testing motions, subjects were asked to take approximately 6 seconds to complete each cycle of ER/IR. No attempt was made to control the ROM between subjects, as we wanted subjects to obtain their individual end ROM. We did not attempt to control force or torque at end range beyond the instruction to complete their full active ROM. Subjects completed 1 to 2 practice repetitions to ensure they understood the motions to be performed and could complete the motions at the proper velocity. Subjects did not perform any stretching prior to testing. Subjects were wearing their normal footwear. Trunk postural alignment was not

C

FIGURE 3. Subject setup (A); and demonstrating test motions including (B) humeral internal rotation in 90° flexion; and (C) humeral internal rotation in 90° abduction.


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TABLE 1. Bony landmarks for digitization. Trunk Xyphoid process Sternal notch C7 spinous process

Scapula Root of spine Inferior angle Acromioclavicular joint

Humerus Medial epicondyle Lateral epicondyle

T8 spinous process

rigidly controlled, but visual inspection was used to monitor for any gross trunk substitution with motion.

Data Reduction The raw kinematic data were low-pass filtered with a 20-Hz cutoff frequency. Anatomic coordinate systems were created for the trunk, scapula, and humerus from the digitized anatomical landmarks.21 Scapular orientation relative to the thorax was described as IR/ER about the z-axis, upward/downward rotation about the y⬘-axis, and anterior/posterior tilting about the x⬙-axis (z,y,⬘x⬙; Cardan sequence).16,21 Humeral orientation relative to the thorax was described as plane of elevation about z, elevation angle about y⬘, and IR/ER about z⬙ (z,y⬘,z⬙ Euler angles).21 For statistical analysis, the values of scapular anterior/posterior tilting, IR/ER, and upward/downward rotation at the point of peak glenohumeral IR ROM were identified in both flexed and abducted positions and averaged across the 2 trials for each subject. To ease interpretation, scapular upward rotation and anterior tilting values were multiplied by –1 to result in positive values. Previous research has demonstrated high trial-to-trial reliability of similar scapular position measures (ICC, ⱖ.93; SEM, ⬍2°) and between-day reliability to be within 3.3° or less.21 Validity of surface measures as compared to bone-fixed tracking at 90° abduction has been reported to be within approximately 2° for scapular posterior tilting, approximately 6° for scapular IR, and approximately 7° for scapular upward rotation.17

Data Analysis The 3 dependent variables were analyzed using standard tests for normality, and were found to satisfy assumptions of normality.8 Demographic and ROM data were analyzed using 2-sample, independent t tests to detect between-group differences. These data included age, height, body mass, dominant-arm glenohumeral IR, nondominant-arm glenohumeral IR, dominant-arm glenohumeral ER, and non– dominant-arm glenohumeral ER. A critical level of P⬍.05 was considered statistically significant for all analyses. A mixed-model 2-way analysis of variance (ANOVA) was used to test for a main effect of group (between930

group factor) on the 3 dependent scapular variables of anterior/posterior tilt, IR/ER, and upward/ downward rotation, as well as test for an interaction of group and motion (flexion IR versus abduction IR; within-subjects factor). Post hoc 2-sample, independent t tests for each motion were planned if significant interactions between group and motion were found. It is possible the selected 20% cutoff for group classification could result in subject misclassification with regard to what constitutes GIRD, as there are little data from which to determine a valid cutoff selection. To ensure this choice did not bias the overall interpretation of how GIRD relates to scapular position, the association between glenohumeral IR deficit and scapular positioning for each motion was further analyzed using multiple regression, with no assignment of subjects to groups. A continuous variable regression analysis does not dichotomize subjects; rather, it measures the association between percent deficit and scapular position continuously across the range of subject values. Therefore, interpretation of the regression analysis results is not limited by the choice of group classification used in the ANOVA. The regression equation additionally allowed for a multifactorial assessment of all 3 scapular variables: scapular IR, upward rotation, and tilt. The model r2, partial correlation, and P values were used to determine significance of the model and contributing variables for each motion.

RESULTS Demographic and ROM data for each group are summarized in Table 2. Statistical analysis found no significant differences in demographic characteristics between groups (P⬎.05). As planned, glenohumeral IR passive ROM of the dominant arm was significantly decreased in the IR deficit group (P⬍.001). The glenohumeral IR deficit group included 10 males and no females, and 8 of the subjects were right hand dominant. The control group included 8 males and 5 females, and 12 of the subjects were right hand dominant. Subjects in the IR deficit group currently averaged 3.9 days per week and 4.5 hours per week of overhead throwing, while subjects in the control group averaged 5 days per week and 7.6 hours per week. The mean values (degrees) for the 3 scapular variables of IR/ER, upward/downward rotation, and anterior/posterior tilt identified at the peak of glenohumeral IR, separated by group and motion, are included in Table 3. An ANOVA found no interactions (P⬎.05) between motion and group for any of the 3 scapular variables, so group main effects were assessed averaged across motions. No significant difference between groups was found for either scapular IR or upward rotation in univariate analyses (IR: F1,21 = 0.37, P = .55; UR: F1,21


TABLE 2. Subject demographics and passive range of motion.

Variables Age (y) Height (cm) Body mass (kg) Dominant internal rotation (deg) Nondominant internal rotation (deg) Dominant external rotation (deg) Nondominant external rotation (deg)

Internal Rotation Deficit Group (n = 10)

Control Group (n = 13)

P Value

27.7 ± 12.9 179.6 ± 7.4 82.7 ± 10.4 39.5 ± 5.1 56.0 ± 6.6 97.9 ± 11.8 86.7 ± 8.3

23.5 ± 5.0 177.6 ± 8.1 79.0 ± 12.1 52.6 ± 9.0 54.1 ± 12.2 95.8 ± 5.8 92.5 ± 5.2

.29 .55 .44 .001* .66 .57 .05

*Significant group difference, P⬍.05.

TABLE 3. Scapular angular positioning by group and glenohumeral position. Internal Rotation at 90° Flexion Scapular Position Internal rotation* Upward rotation† Anterior tilting‡

Internal Rotation at 90° Abduction

Control

Deficit

Control

Deficit

53.9 ± 13.5 21.6 ± 8.8 5.0 ± 9.5

56.6 ± 15.7 23.7 ± 11.8 13.5 ± 10.7

38.2 ± 13.1 14.8 ± 13.4 14.8 ± 11.5

42.6 ± 14.9 13.1 ± 11.6 24.6 ± 11.7

DISCUSSION We were interested in examining scapular positioning in subjects with a self-reported history of participation in overhead-throwing sports. This population has been shown to develop subacromial shoulder impingement at a greater rate than the general population.10,20,33 This is thought to be due, in part, to repetitive overhead motion leading to capsular tightness and muscle imbalance.10,20,32 We investigated the effect of presumed posterior capsule tightness, as measured by glenohumeral IR ROM deficit, on 3-D scapular positioning. Significant differences of 8.5° to 9.8° were observed between the deficit and control groups for scapular tilt at peak glenohumeral IR across glenohumeral flexed and abducted positions. These positions are believed to most stress the posterior capsule.31

Glenohumeral IR at 90° humeral elevation significantly tensions the posterior capsule31 as well as brings the supraspinatus in closer contact with the acromion.9,12 The increased anterior tilt found is consistent with previous findings in subjects with subacromial impingement.21,22 Lukasiewicz et al22 reported a 10° increase in scapular anterior tilt in subjects with symptomatic impingement compared to nonimpaired individuals using static 3-D analysis in elevated arm positions. Similarly, Ludewig and Cook21 found approximately 6° more anterior tilt at 120° of elevation for subjects with impingement as compared to a matched control group. These findings support a possible relationship between presumed posterior capsule tightness, estimated by glenohumeral IR deficit, increased scapular anterior tilt, and subacromial impingement pathology.32 In univariate analyses, no significant group differences were found across glenohumeral positions for scapular IR or upward rotation. The greater variabil-

TABLE 4. Multiple correlation by glenohumeral position. Internal Rotation at 90° Flexion

Internal Rotation at 90° Abduction

Scapular Variable

Partial Correlation

P Value

Partial Correlation

P Value

Full model Internal rotation Upward rotation Anterior tilting

0.61 0.32 0.18 0.59

.030 .16 .43 .005

0.60 0.17 0.44 0.59

.036 .48 .048 .005


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REPORT

= 0.00, P = .96; Table 3). Analysis of scapular tilt revealed a significant difference between groups (F1,21 = 5.02, P⬍.04; Table 3), with the IR deficit group exhibiting a greater amount of anterior tilting of the scapula (9.2°). Multiple regression analysis identified a significant association between percent glenohumeral IR deficit and scapular position variables for glenohumeral peak IR at 90° flexion and 90° abduction, with model r 2 values of 0.37 and 0.35, respectively (Table 4). The strongest partial correlation (0.59) was present for scapular anterior tilt in both humeral positions, with scapular upward rotation also significantly associated with GIRD at 90° humeral abduction (Table 4).

RESEARCH

* P = .55 between groups. † P = .96 between groups. ‡ Significant differences between the groups across both glenohumeral positions, P⬍.04.

ity of scapular IR is consistent with other studies22,24 and impacts the detection of significant differences, although group sample means for scapular IR were also separated by only approximately 3° to 4°, as compared to approximately 9° for tilting. There was also no significant difference in upward rotation, which may be in part a result of the glenohumeral positions assessed. The magnitude of scapular upward rotation is most strongly influenced by the amount of glenohumeral elevation,22,24 which was standardized at 90° for the motions performed. A retrospective power analysis at 80% power was conducted to find the number of subjects necessary to detect a significant difference between groups of 10° for scapular upward rotation and IR based on the observed variance, resulting in estimates of 22 to 35 subjects per group, respectively. Given the sample size of only 10 for the smallest group, our post hoc power for upward rotation and IR effects was only 45% and 31%, respectively. Subsequently, although this study did not support an effect of glenohumeral IR deficit on scapular upward rotation and IR positioning, further study in a larger sample is necessary to definitively rule out such an effect. To further investigate the relationship between 3-D scapular positioning and clinical measurement of glenohumeral IR ROM using a multifactorial approach, a multiple regression analysis demonstrated that scapular tilting was the only dependent variable consistently associated with percent deficit for both motions. Glenohumeral IR at 90° abduction identified that a higher percent IR deficit was significantly associated with reduced scapular upward rotation. Given the numerous factors that can influence 3-D scapular positioning, the degree of variance in scapular positioning explained by glenohumeral IR deficit (35%-37%) is substantive. This analysis removed the dichotomous group separation based on percent deficit, and considered the association between percent deficit and scapular angular position variables continuously across the range of subject variation. The regression analysis also allows for consideration of interactions between 3-D scapular variables. These findings support the univariate results, whose interpretation was limited by possible misclassification using the 20% deficit cutoff. The regression findings also add consideration of a lesser influence of posterior shoulder tightness on potentially reducing scapular upward rotation during glenohumeral IR in abduction. The subtle differences in the results of upward rotation in the 2 glenohumeral measurement positions may relate to differential tensioning of the posterior capsule or other soft tissues in flexion versus abduction.31 The sample used in this study consisted of athletes at varying levels of participation and competition. Overhead throwers, specifically baseball pitchers, have been most implicated in demonstrating IR deficits 932

and also shoulder impingement,18,32 but this study found significant IR deficits in recreational athletes as well as elite pitchers. Both groups contained each level of throwing athlete and no significant differences in demographics or participation amount were found between groups. Of greatest concern for possible bias would have been reduced participation in the control group. However, the control group did not subjectively report any reduced participation. It is possible the control group was more involved in or compliant with preventive measures, such as posterior capsule stretching. It is also possible there were differences in the numbers of pitches or throws typically completed during training and game situations between groups; however, this information was not gathered from the subjects. All 5 female subjects were in the control group, and if there were a larger sample size, it would be of interest to stratify the groups based on gender to examine any gender influence. Analysis of the data, excluding those of the female subjects, demonstrated minimal impact on the group means and no substantive change in the general interpretation of the results. To our knowledge, this study is the first to demonstrate a relationship between glenohumeral IR deficit believed related to posterior shoulder tightness and altered scapular positioning. The results support the theory of the scapula being pulled into increased anterior tilting during glenohumeral IR in 90° humeral elevation. Clinically, it is difficult to distinguish posterior capsule versus posterior soft tissue tightness with a glenohumeral IR deficit.25 Based on previous investigation, glenohumeral IR deficit has been significantly associated with a sidelying measure of posterior shoulder tightness.32 Further investigation is warranted to develop and/or validate clinical measures that attempt to distinguish these various soft tissue contributions. It has been reported that pitchers who demonstrate glenohumeral IR deficit have an equivalent compensatory gain in glenohumeral ER so that the total arc of rotation remains similar bilaterally.3,5,27,28,34 This could be related to excessive repetitive rotational forces on the humerus altering the humeral retroversion angle during ossification due to repetitive throwing in children and adolescents. This line of inquiry deserves further investigation; but in our sample, the significant reduction in glenohumeral IR was not associated with retention of the total rotational arc of motion between groups, thereby reinforcing the theory that their glenohumeral IR deficit was likely associated with soft tissue tightness. Other research has suggested that translations of the humerus in subjects with posterior capsule tightness may increase anteriorly and superiorly, subsequently decreasing the subacromial space.13 It is also possible such abnormal translations induced by the capsule could subsequently contribute to altered


would be to reduce the precision of the measures, increasing the difficulty to find differences between groups or to find an association between GIRD and scapular positioning. Subsequently, as previously noted, it is possible there were differences in scapular IR or upward rotation that existed between groups that we did not detect. This study found a significant relationship between dominant-arm glenohumeral IR deficits in overhead throwing athletes and altered scapular positioning, specifically increased anterior tilt. This relationship has been discussed theoretically but not previously quantified.18,25 With this information, further research can be conducted to evaluate the effect of glenohumeral IR limitation on scapular kinematics in patients with symptomatic subacromial and internal impingement versus healthy subjects. If technology allows, in future research it would also be advantageous to examine the impact of glenohumeral IR deficits on scapular kinematics during high-velocity throwing. Altered scapular positioning, including increased anterior tilting, has been previously implicated in potentially decreasing the subacromial space and leading to increased risk of subacromial impingement.21,25 The results of this current study identify a possible mechanism for development of excessive scapular anterior tilt.

CONCLUSIONS

ACKNOWLEDGEMENTS The authors would like to acknowledge the support of the Minnesota Medical Foundation in funding equipment necessary for this study. We would also like to thank Katherine L. Derr, ATC and Robert Broxterman, ATC for their assistance with various phases of this project.

REFERENCES 1. Bigliani LU, Codd TP, Connor PM, Levine WN, Littlefield MA, Hershon SJ. Shoulder motion and laxity in the professional baseball player. Am J Sports Med. 1997;25:609-613. 2. Borstad JD, Ludewig PM. The effect of long versus short pectoralis minor resting length on scapular kinematics in healthy individuals. J Orthop Sports Phys Ther. 2005;35:227-238.


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REPORT

These findings demonstrate a significant increase in scapular anterior tilt at end range glenohumeral IR with the arm in 90° of flexion or abduction in subjects with glenohumeral IR deficit as compared to controls. A positive relationship between glenohumeral IR deficit and abnormal scapular positioning (increased anterior tilt and decreased upward rotation with humeral IR from 90° abduction) was also identified.

RESEARCH

scapular positioning. These translations are small motions that are challenging to measure and were not assessed in our study, although they may further influence the risk of subacromial impingement. Muscular contributions to abnormal scapular kinematics have been previously demonstrated during 3-D analyses.2,21 Reductions in serratus anterior activation21 and reduced resting length of the pectoralis minor2 have been associated with scapular kinematic abnormalities. Scapular muscle imbalances have the potential to alter normal scapular position and movement.21,25 This study focused on the relationship between passive glenohumeral IR ROM measurement and active scapular positioning. Muscle activity was not assessed in this study and future research would benefit from EMG data to help explain potential muscular contributions to altered scapular positioning. In addition, EMG data would have indicated if subjects had reduced muscle activation in the elevated arm positions due to the support of the sling. Muscular influences, humeral translations, and anatomical factors are all possible contributors to the 63% to 65% of unexplained variance in this analysis. During tested motions, velocity and torque production were not tightly controlled, which might further influence variability in scapular position between subjects. Significant differences in peak humeral velocity (available as the first derivative of position data over time) were not found between groups during performance of either motion. Rigid experimental control of end range torque might have accounted for some of the unexplained variance, and this limitation should be considered when interpreting the results. However, we believe subjects self-selected end ROM results in individual control of end range force and torque more similar to what might occur while performing motions in a natural environment. It is also important to note that, during motion testing, a subject’s group assignment was not yet known, preventing possible bias between groups in instructions given to a subject. There is always a degree of error due to skin motion versus underlying bone motion whenever surface sensors are used. This error can be minimized by placing the surface sensor on areas of minimal skin slip, such as the scapular acromion process. The ability to reliably21 and accurately17 track scapular positioning with surface sensors has been previously demonstrated. The magnitude of scapular tilting differences found between groups was about 3 times the 2° to 3° measurement errors reported in past reliability and validity studies.17,21 However, greater errors (6°-7°) have been reported for surface measures of scapular IR and upward rotation as compared to bone-fixed tracking.17 We also did not test the reliability of our specific examiner for the supine glenohumeral IR ROM measures. The effect of these possible measurement errors

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