differences in scapular orientation, subacromial space and shoulder pain between the full can and...

Clinical Biomechanics xxx (2013) xxx–xxx

JCLB-03580; No of Pages 7

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Clinical Biomechanics

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Differences in scapular orientation, subacromial space and shoulder pain between thefull can and empty can tests

Mark K. Timmons a,b,⁎, Andrea Diniz Lopes-Albers b,c, Lindsey Borgsmiller b, Catherine Zirker d,Jeff Ericksen e, Lori A. Michener b

a Interprofessional Polytrauma and Traumatic Brain Injury Rehabilitation, Department of Veterans Affairs, Hunter Holmes McGuire VA Medical Center, Richmond, VA 23249, USAb Department of Physical Therapy, Virginia Commonwealth University, Medical College of Virginia Campus, Richmond, VA 23298, USAc Universidade Federal de Sao Paulo, Departamento de Medicina, Sao Paulo, SP, Brazild Inpatient Spinal Cord Injury, Craig Hospital, Englewood, CO 80113, USAe Department of Veterans Affairs, Hunter Holmes McGuire VA Medical Center, Physical Medicine and Rehabilitation Section, Richmond, VA 23249, USA

⁎ Corresponding author at: Department of Physical MeVA Medical Center, 1201 Broad Rock Blvd, Richmond, VA

E-mail address: [email protected] (M.K. Timmon

0268-0033/$ – see front matter. Published by Elsevier Lhttp://dx.doi.org/10.1016/j.clinbiomech.2013.01.015

Please cite this article as: Timmons, M.K., et aempty can tests, Clin. Biomech. (2013), http

a b s t r a c t
a r t i c l e i n f o
Article history:
Received 10 February 2012Accepted 29 January 2013
Keywords:Scapular orientationAcromio-humeral distanceJobe TestSubacromial impingement syndrome

Background: The empty and full can arm positions are used as diagnostic tests and in therapeutic exerciseprograms for patients with subacromial impingement syndrome. The adverse effects of these arm positionson the rotator cuff have not been fully described. The purpose of this investigation was to compare theacromio-humeral distance, three-dimensional scapular position, and shoulder pain during maximum isomet-ric contractions in the empty and full can arm positions.Methods: Subjects with subacromial impingement syndrome (n=28) and a matched control group withoutshoulder pain (n=28) participated. Acromio-humeral distance, scapular/clavicular positions and shoulderpain were measured during maximal isometric contractions in each position.
Findings: No difference was found in acromio-humeral distance (P=0.314) between the arm positions or be-tween the groups (P=0.598). The empty can position resulted in greater scapular upward rotation (Pb0.001,difference=4.9°), clavicular elevation (Pb0.001, difference=2.7°), clavicular protraction (P=0.001, difference=2.5°) and less posterior tilt (Pb0.001, difference=3.8°) than the full can position. No differences in the scapularpositions were found between the groups. Positive correlations were seen between the scapular positions in thecontrol and not in the subacromial impingement group.Interpretation: Our results did not show a difference in acromio-humeral distance between the arm positions orgroups, indicating that the kinematic differences between the positions are not associated with alteredacromio-humeral distance. The increased pain in the EC position might be due to the lack of an associationamongst the scapular positions rather than the deficiency of a single scapular motion.
Published by Elsevier Ltd.

1. Introduction

The empty can (EC) and full can (FC) test positions are used as diag-nostic tests and as therapeutic exercises in rehabilitation programs forpatients with rotator cuff disease. Specifically, the EC position (“Jobetest”) is used to assist in the diagnosis of injury to the supraspinatusmuscle and is theorized tomaximize the activation of the supraspinatusduring exercise (Jobe and Moynes, 1982; Kelly et al., 1996; Park et al.,2005). Prior research has indicated that these tests do not differ insupraspinatus muscle activity; therefore one is not recommendedover the other to activate the supraspinatus (Boettcher et al., 2009;Takeda et al., 2002). There may be other parameters that differ be-tween the FC and EC positions that will preferentially direct theuse of the two arm positions.

dicine, Hunter Holmes McGuire23249, USA.s).

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l., Differences in scapular orie://dx.doi.org/10.1016/j.clinbio

The EC and FC tests are performed by resisting isometric arm eleva-tion in the scapular plane at 90° elevation; the tests differ in the positionof the glenohumeral joint. The EC is performed in glenohumeral internalrotation (thumb pointing down) and the FC is performed in neutralglenohumeral rotation (thumb pointing up). The glenohumeral internalrotation in the ECmay place the greater tuberosity of the humerus closerto the acromion, leading to a decrease in the volume of the subacromialspace (SAS) and therefore increasing the risk for subacromial impinge-ment of the rotator cuff and producing shoulder pain (De Wilde et al.,2003; Roberts et al., 2002).

The SAS contains the tendons of the rotator cuff and is defined bythe borders of the coracoacromial arch and the humeral head. Theacromio-humeral distance (AHD) is the linear distance between infe-rior acromion and humerus. This distance is used to represent thewidth of the SAS outlet (Fig. 1) (Azzoni and Cabitza, 2004; Azzoniet al., 2004; Desmeules et al., 2004). The SAS outlet allows for theexcursion of the supraspinatus tendon into the SAS. Patients withsubacromial impingement syndrome (SAIS) have been shown to

ntation, subacromial space and shoulder pain between the full can andmech.2013.01.015

http://dx.doi.org/10.1016/j.clinbiomech.2013.01.015

mailto:[email protected]


http://www.sciencedirect.com/science/journal/02680033


Fig. 1. Acromio-humeral distance with the arm at 90° abduction in the plane of thescapula in the full can position.

2 M.K. Timmons et al. / Clinical Biomechanics xxx (2013) xxx–xxx

have a decreased AHD measurement when compared to patientswithout SAIS (Graichen et al., 1999b; Hebert et al., 2003). Changesin the AHD measurement may be related to the changes in scapularmotion or position (Kalra et al., 2010; Seitz et al., 2011; Silva et al.,2008; Solem-Bertoft et al., 1993).

Decreased scapular posterior tilt, upward rotation, and external rota-tion have been theorized to cause extrinsic impingement of the rotatorcuff tendons by decreasing the size of the subacromial space, (Ludewigand Reynolds, 2009; Michener et al., 2003; Timmons et al., 2012) con-versely, increased upward rotation and posterior tilt of the scapulahave been theorized to increase the subacromial space (McClure et al.,2006). Evidence indicates that limited scapular upward rotationmobility(Atalar et al., 2009), scapular dyskinesis (Silva et al., 2008), and scap-ular protraction (Solem-Bertoft et al., 1993) decrease the size of thesubacromial space, while a position of increased scapular upward ro-tation, posterior tilt, (Seitz et al., 2011) and scapular retraction (Kalraet al., 2010; Solem-Bertoft et al., 1993) is associated with an increasein subacromial space. It is unclear if the FC and EC test positions ad-versely affect scapular kinematics, the subacromial outlet and increas-ing risk of shoulder pain.

Improved understanding of the effects of positioning the arm in theFC and EC positions during resistedmaximal isometric force productionon subacromial space outlet and scapular kinematics will assist healthcare providers in use of the FC and EC positions. The purpose of this in-vestigation was to compare the three-dimensional scapular position,AHD, and shoulder pain during maximum isometric contractions inthe EC and FC arm positions. We hypothesized that during the ECthere would be increased shoulder pain, decreased acromio-humeraldistance, decreased scapular upward rotation and posterior tilt, and

Table 1Subject demographic information by group (means and standard deviations).

Control (n=28,female=10, male=18)

SAIS (n=28, fema

Mean SD Mean

Age (years) 37.9 14.3 38.7Height (cm) 172.8 11.4 174.8Mass (kg) 74.1 15.1 82.5PENN pain 29.3 1.0 19.9PENN function 58.8 3.2 42.9PENN total 97.1 4.2 67.1

Please cite this article as: Timmons, M.K., et al., Differences in scapular orieempty can tests, Clin. Biomech. (2013), http://dx.doi.org/10.1016/j.clinbio

increased scapular internal rotation as compared to the FC. Secondarilythis investigation had the purpose to determine if scapular position andAHD in the two test positions differ between subjects with and withoutSAIS.

2. Methods

This was a prospective cross-sectional controlled laboratory study.This study was approved by the Institutional Review Board at theinvestigator's university. Participating subjects reviewed and signedthe informed consent, completed the intake questionnaires andunderwent an eligibility examination. Next, subjects underwent studytesting in both the FC and EC positions.

2.1. Subjects

Two groups of subjects were recruited to participate in this investiga-tion, a control group not reporting shoulder pain (n=28) and a groupwith a clinical diagnosis of SAIS (n=28). Descriptive data is available inTable 1. The control group and SAIS group were matched based on age(within 5 years), sex, and shoulder tested (dominant or non-dominantside). Control group inclusion criteria were 18–65 years of age withoutshoulder pain in the previous 6 months. Control group subjects were ex-cluded if they had positive finding on any of the SAIS tests (painful arc,pain or weakness with resisted external rotation, Neer, Hawkins, andJobe tests) (Michener et al., 2009), a history of upper arm fracture, shoul-der surgery, or shoulder pathology. The SAIS group inclusion criteriawere pain with resisted arm elevation or external rotation as well as 3of 5 positive SAIS tests (stated above). In order to assure that subjectsdid not have adhesive capsulitis; subjects were excluded from the SAISgroup if they could not elevate their shoulder greater than 150° norhad a 50% limitation of passive shoulder range of motion in more than2 planes of motion. Additional exclusion criteria included shoulder paingreater than 7/10, a history of fracture to the shoulder girdle, systemicmusculoskeletal disease, shoulder surgery, or a positive clinical examina-tion for a full thickness rotator cuff tear.

2.2. Procedures

Subjects sat with their feet flat on the floor, and shoulder-widthapart, and they were instructed to sit up straight with head facingforward. The subject's arm was positioned with the shoulder in 90°of elevation in the scapular plane. The scapular plane is defined asbeing rotated 40° anterior to the coronal plane. For the FC, the armwas placed in neutral rotation standardized by the thumb pointingtowards the ceiling (Fig. 2A). The EC was standardized by the thumbpointing down towards the floor. Arm elevation and scapular planeangles were verified with a digital inclinometer (Acumar, LafayetteInstruments, Lafayette, IN, USA). During 2 trials, each in the FC andEC positions, the subject performed a 6 second maximal voluntaryisometric contraction resisted against shoulder elevation. A minimumof a one minute rest was given between the 2 trials. During the isomet-ric contraction, dependent variables were measured including 1 —

shoulder elevation force measurements with hand-held dynamometer,

le=10, male=18)

SD Mean difference t P value

13.4 0.9 0.221 0.8269.1 1.9 0.691 0.492

16.1 7.7 1.89 0.0644.6 9.4 −10.577 b0.0016.9 15.9 −11.784 b0.001

10.5 30.0 −14.064 b0.001



A

B

Fig. 2. A) Setup and subject position for scapular kinematic testing, with the subject inthe full can arm position, B) ultrasound transducer placement.

3M.K. Timmons et al. / Clinical Biomechanics xxx (2013) xxx–xxx

2 — shoulder pain with the numeric pain rating scale, 3 — ultrasoundimaging of the subacromial space for measurement of the acromio-humeral distance, and 4 — scapular position with 3-dimensional kine-matic electromagnetic sensors. The EC and FC test positions wereconfirmed by calculating the glenohumeral internal/external rota-tion position in each test position during data processing. The meansof the two trials for all dependent variables were used for data analysis.Kinematic and ultrasound data were collected during separatemaximalvoluntary isometric contraction (MVIC).

2.3. Shoulder scaption moment

During each MVIC trial, the maximum force exerted was recordedusing a hand held dynamometer (microFET3, HogganHealth Industries,Draper, UT, USA) located immediately proximal to the hand. Force (N)was recorded, and the shoulder scaption net moment (Nm) was calcu-lated bymultiplying the arm scaption force by the arm length inmeters.Joint moment was then normalized to the subject's body mass (kg).Handheld dynamometry for assessment of shoulder strength in symp-tomatic (Hayes et al., 2002) and healthy subjects (Agre et al., 1987)has shown excellent inter-rater and intra-rater reliability.

2.4. Shoulder pain

Subjects' shoulder pain was assessed in 2 ways. First, the subjectscompleted the PENN Shoulder Pain and Function questionnaire as partof the pretest screening. A second assessment of shoulder pain occurredduring the shoulder testing session. Immediately following each con-traction subjects were asked to rate the shoulder pain they experiencedduring the contraction on a numerical patient-reported 0–10 painscales (NPRS). Subjects were shown the numeric pain scale with 0equating to no pain at all and 10 equating to the most extreme painimaginable.


2.5. Scapular position

The 3-dimensional kinematics of the scapula, clavicle and humeruswere measured with the 6 degree of freedom Polhemus 3Space Fastrakelectromagnetic-based motion capture system (Polhemus, Colchester,VT, USA) integrated with Motion Monitor software (Innovative SportsTechnologies Inc., Chicago IL, USA). Kinematic data were sampled at30 Hz. The International Society of Biomechanics (ISB) protocol wasfollowed for sensor placement, creation of anatomical coordinatesystems, and Euler angle sequence of rotations (Wu et al., 2005).Electromagnetic sensors were placed and secured over the distal hu-merus with an elastic neoprene cuff, on the posterior lateral acromionwith double sided tape, and over the third thoracic vertebrawith a dou-ble sided tape. A fourth sensor was used to digitize bony landmarks forcreation of local anatomical axis and coordinate systems.

Twelve subjects from the SAIS groupwere asked to returnwithin oneweek for a repeat kinematic testing in order to test the reliability of thesemeasurements. The intra-class correlation coefficient (ICC) ranged be-tween 0.93 and 0.97 for all scapular and clavicular motions. Measure-ment error for the scapular and clavicular positions was determinedprior to this investigation. The standard error of the measure (SEM)and minimal detectable change (MDC) for upward rotation (SEM=2.22°, MDC=3.14°), posterior tilt (SEM=1.52°, MDC=2.15°), internalrotation (SEM=1.86°, MDC=2.62°), clavicular elevation (SEM=1.02°,MDC=1.44°) and clavicular protraction (SEM=1.43°, MDC=2.03°) at90° of scapular plane elevation were all calculated.

2.6. Ultrasound imaging

A diagnostic ultrasound unit (LogiQ e; GE Healthcare, Wisconsin,USA) with an adjustable 5.0–12.5 MHz frequency linear array transduc-er was used to capture ultrasound images of the subacromial space out-let. All ultrasound imaging was performed by the same examiner. Thelinear transducer frequency and focal image depth were adjusted foreach subject in order to produce the best image of the subacromialspace outlet. The position of the probe was standardized as previouslydescribed by Desmeules et al. (2004). Three coronal plane view imageswere collected with the transducer placed over the lateral acromion atits most anterior aspect (Fig. 2B). An image of the subacromial outletfor AHDmeasurement was saved when both the hyperechoic acromionand humeral head were clearly visualized on the screen of the ultra-sound unit. The AHD was measured as the shortest distance betweenthe humeral head and the lateral inferior tip of the acromion (Azzoniand Cabitza, 2004; Desmeules et al., 2004; Seitz and Michener, 2010).The AHDmeasurementsweremade using software embedded in the ul-trasound unit. The AHD measurements from the 3 separate ultrasoundimages taken during the FC and EC positions were used for statisticalanalysis. From preliminary data the intra-rater test–retest reliabilitywas calculated on n=9 subjects without shoulder pain. The ICCwas cal-culated as 0.90, measurement error was SEM=0.07 mm, and MDC=0.18 mm. Both are well below the resolution of the ultrasound unit.

2.7. Statistical analysis

Means and standard deviations were calculated for all demo-graphic data and dependent variables. A 2×2 (group by arm posi-tion) repeated measure ANOVA was used to test for differences independent variables. Paired t-tests were used as post-hoc to determinestatistical differences between levels of the independent variableswhensignificant group main effects were found. The relationships betweenvariables were determined using Pearson correlation. Statistical signifi-cance was determined a priori at P≤0.05; all statistical analyses werecompleted using SPSS 19 statistical software (SPSS Inc., Chicago, IL,USA).



Table 2Means and standard deviations for scaption moment, acromio-humeral distance and scapular and clavicular positions by arm position collapsed across groups.

Full can Empty can

Mean SD Mean SD Mean difference F value P value

Scaption moment (Nm/kg) 1.74 0.66 1.51 0.57 0.23 13.166 0.001AHD (mm) 8.6 1.7 8.4 1.6 0.2 1.034 0.314Scapular upward rotation (degrees) 25.3 15.6 30.2 13.9 −4.9 31.430 b0.001Scapular internal rotation (degrees) −32.5 7.6 −33.1 7.3 0.6 1.120 0.295Scapular posterior tilt (degrees) −7.0 7.5 −10.8 7.4 3.8 66.053 b0.001Clavicular elevation (degrees) 19.6 5.1 22.3 6.7 −2.7 15.948 b0.001Clavicular protraction (degrees) −20.5 5.1 −23.0 6.6 2.5 12.811 0.001


3. Results

The PENN shoulder pain and function scores were higher in the con-trol group than the SAIS group, indicating greater impairment amongthe subjects with SAIS (Table 1). Glenohumeral internal/external rota-tion position was significantly different (t=21.120, Pb0.001) betweenthe EC (mean=−4.2°, SD=12.2°) and FC (mean=48.1°, SD=19.3)test positions. During the maximal voluntary isometric contractions,subjects reported greater pain in the EC than in the FC position(F(54,1)=20.130, Pb0.001; mean difference=0.76) and produced alower shoulder scaption moment in the EC (F(54,1)=13.166, P=0.001; mean difference=0.23 Nm/kg). The group by arm positioninteraction for shoulder pain was significant (F(54,1)=13.234, Pb0.001).Control subjects reported no difference (P=0.355) in shoulder pain be-tween the two test positions, the pain reported during the maximumcontraction did not differ from zero (t=1.441, P=0.161). Subjects inthe SAIS group reported greater pain in the EC position than the FCposition (t=−4.387, Pb0.001; mean difference=1.4). Scaption mo-ment was greater in the control group as compared to the SAIS group(F(54,1)=7.691, P=0.008; mean difference=0.40 Nm/kg).

3.1. Acromio-humeral distance

Means and standard deviations for AHD by arm position andgroup are available in Tables 2 and 3. The difference in the AHD be-tween the FC and EC positions was not significant (F(54,1)=1.034,P=0.314; mean difference=0.19 mm). The difference in AHD be-tween control and SAIS group did not reach statistical significance(F(54,1)=0.281, P=0.598; mean difference=0.22 mm).

3.2. Scapular position

Means and standard deviations for scapular and clavicular posi-tions by arm position and group are available in Tables 2 and 3.There were significant differences in scapular and clavicular posi-tions between the FC and EC positions. In the FC position, subjectshad less upward rotation (UR) (F(54,1)=31.43, Pb0.001; mean differ-ence=4.9°), greater posterior tilt (F(54,1)=66.053, Pb0.001; mean dif-ference=3.8°), less clavicular elevation (F(54,1)=15.948, Pb0.001;mean difference=2.7°), and less clavicular protraction (F(54,1)=12.811, P=0.001; mean difference=2.5°) as compared to the EC

Table 3Means and standard deviations for scaption moment, acromio-humeral distance and scapu

Control SAIS

Mean SD Mean

Scaption moment (Nm/kg) 1.83 0.61 1.AHD (mm) 8.4 1.8 8.Scapular upward rotation (degrees) 28.0 14.9 28.Scapular internal rotation (degrees) −33.1 8.2 −31.Scapular posterior tilt (degrees) −8.0 7.5 −9.Clavicular elevation (degrees) 20.6 5.3 21.Clavicular protraction (degrees) −21.8 5.6 −21.


position. No differences were seen between EC and FC positions(F(54,1)=0.013, P=0.91) in scapular internal rotation. Differences be-tween the control and SAIS groups and the two-way interaction effectsdid not reach statistical significance for all scapular and clavicular posi-tions. The significant differences found for upward rotation, posteriortilt, clavicular elevation and clavicular protraction were all greaterthan the SEM and MDC.

Correlations between the kinematic measurements are presented inTable 4. There was not a significant correlation between reported painduring the MVIC and any of the scapular and clavicular positions. InFC position significant positive correlations of moderate strength werefound between UR and internal rotation (IR) (r=0.343, P=0.010),and UR and posterior tilt (PT) (r=0.248, P=0.046), and a negative cor-relation was found between UR and protraction (PRO) (r=−0.462,Pb0.001). The data was separated by group and the correlation analysiswas performed on the separated data in order to determine differencesbetween groups. In the control group the correlations between UR andIR (r=0.471) and UR and PT (r=0.467) in the FC position werestrengthened. In the control group a positive correlation was found be-tween PT and IR in both the FC (r=0.572, P=0.001) and EC (r=0.426,P=0.0.24). In the SAIS group the correlation between scapular posi-tions was not statistically significant.

4. Discussion

It is theorized that shoulder pain during arm elevation in patientswith rotator cuff disease results from the compression of the rotatorcuff due to the narrowing of the SAS (Ludewig and Reynolds, 2009;Michener et al., 2003). In the current study, subjects in the SAIS grouphad greater shoulder pain during the MVICwith their arm in the EC po-sition than in the FC.We hypothesized that the AHDwould be narrowerin the EC position when compared to the FC position; however, we didnot find differences in the AHD between the EC and FC arm positions.We also did not find differences in AHD between those with and with-out SAIS. Other investigations have had mixed results, some showingsignificantly smaller AHD in subjects with shoulder pain, (De Wildeet al., 2003; Graichen et al., 1999a) while others have shown no differ-ence in AHD between subjects with and without shoulder pain (Hardyet al., 1986). These earlier studies did not measure AHD during anMVIC, as was done in the current study. Thompson et al. (2011) founda decrease in AHD during a loaded dynamic arm elevation and White

lar and clavicular positions by group collapsed across arm position.

SD Mean difference F value P value

43 0.54 0.4 7.691 0.0086 1.6 −0.2 0.281 0.5981 15.8 −0.1 0.001 0.9734 8.1 −1.7 0.751 0.3908 7.4 1.8 0.838 0.3644 6.3 −0.8 0.290 0.5937 6.1 −0.1 0.003 0.954



Table 4Scapular and clavicular positions correlation coefficients.

UR IR PT ELE PRO UR IR PT ELE PRO UR IR PT ELE PRO

EC 0.124 0.196 -0.042 -0.381* 0.051 0.154 -0.004 -0.427* 0.180 0.253 -0.011 -0.324

FC 0.343* 0.267* 0.248 -0.462* 0.209 0.134 0.515* -0.466* 0.471* 0.467* -0.069 -0.480*

EC 0.124 0.199 -0.084 -0.310* 0.051 -0.075 0.100 -0.038 0.180 0.426* -0.289 -0.619*

FC 0.343* 0.159 -0.018 -0.511* 0.209 -0.296 -0.018 -0.373* 0.471* 0.572* -0.089 -0.615*

EC 0.196 0.199 -0.180 -0.143 0.154 -0.075 -0.117 -0.238 0.253 0.426* -0.277 -0.021

FC 0.267* 0.157 -0.077 -0.137 0.134 -0.296 0.013 0.038 0.467* 0.572* -0.113 -0.281

EC -0.042 -0.084 -0.180 0.361* 0.004 0.100 -0.117 0.548* -0.112 -0.289 -0.277 0.009

FC 0.248 -0.018 -0.077 -0.218 0.514* 0.018 0.013 -0.436* -0.069 -0.089 -0.113 -0.059

EC -0.381* -0.310* -0.143 0.361* -0.427* -0.038 -0.238 0.548* -0.324 -0.619* -0.021 0.009

FC -0.462* -0.511* -0.137 -0.218 -0.466* -0.373* 0.038 -0.436 -0.480* -0.615* -0.281 -0.059

Control SubjectsSAIS Subjects

UR

IR

PT

ELE

PRO

ALL Subjects

⁎Pb0.05.


et al. (2011) reported a decrease in AHDduring anMVIC shoulder exter-nal rotation. It is likely that duringmaximal muscle activity, the scapulais positioned such that the SAS would be narrowed and compression ofthe rotator cuff is increased.

We hypothesized that in the EC position the scapula would be po-sitioned in a manner that is believed to be associated with rotator cuffimpingement, i.e., less PT, less scapular UR and greater scapular IR. Inthe EC position, we found that the scapula was in less PT, a positionthat should produce a decrease in AHD. We also found an increasein scapular UR in the EC position, which should increase the AHD. Itis likely that the decrease in the AHD that resulted from the decreasedscapular PT was offset by the increase in AHD that resulted from theincrease in scapular UR. This might explain why we found no differ-ence in AHD seen between the FC and EC positions.

The secondary purpose of this investigation was to determine ifdifferences in the AHD and scapular position measurements seen be-tween the EC and FC would be greater in subjects with SAIS. The SAISgroup had greater pain during the MVIC in the EC position than theFC. We also found that the control and SAIS groups produced lowershoulder scaption moment in the EC position than in the FC. The de-crease in scaption moment in the EC position was greater in the con-trol group than the SAIS group, so it appears that the reduction inshoulder scaption moment was due more to the change in arm posi-tion than the increase in shoulder pain. The results of this investiga-tion did not show any differences in scapular position between thecontrol and SAIS groups.

Correlation analysis revealed relationships between the scapularpositions that could explain why we did not find a difference in theAHD between arm positions and groups. The positive correlationsfound between UR and IR, and UR and PT suggest that subjectswith greater scapular UR also showed greater PT and IR. The positivecorrelation between IR and PT suggests that subjects with greater IRalso had greater PT. Increasing IR is reported to decrease the AHDwhile increased PT increases the AHD. These correlations were notseen in the SAIS group. These relationships suggest that the com-bined scapular motions as opposed to a deficiency of a single motioncontribute to the reduction in rotator cuff compression and that dif-fering strategies might exist in order to reduce the compression. It ispossible that patients with SAIS might not be able to produce thecomplex combination of motions necessary to maintain the dimen-sions of the SAS. The SAIS group did have smaller AHD than thecontrol group even though the difference did not reach statisticalsignificance, and the association seen between PT and IR in the con-trol group might help the subject without SAIS to reduce compres-sion of the rotator cuff, while the dissociation of PT and IR seen inthe SAIS group might contribute to the pain reported during themaximal contractions.


It would seem logical that while experiencing pain, subjects wouldattempt to configure the shoulder girdle in a manner that would min-imize their pain. The options available to the subject would be tochange the orientation of the scapula in order to open up the SAS out-let and reduce compression of the rotator cuff tendons or to decreasethe length of the rotator cuff tendons in order to reduce the internalstrain of the tendon. The differences between the groups in the corre-lation between the scapular positions suggest that patients with SAISare not able to make the adjustment in order to maintain the width ofthe SAS outlet.

The SAIS group produced a lower scaption moment than thecontrol group in both EC and FC positions. Patients with rotatorcuff tendinopathy have been shown to produce significantly lowershoulder isometric, isokinetic, concentric and eccentric torques (Lerouxet al., 1994; MacDermid et al., 2004; Tyler et al., 2005). The increase inpain experienced by the SAIS group during the MVIC in the EC positionmay have deterred our subjects from producing greater muscle activity;preventing increased compression of the rotator cuff during the MVIC.

Narrowing of the SAS is not only related to the movement of thescapula, but also may be due to the intrinsic deficits of the rotator cuffmuscles producing a superior translation of the humeral head into theSAS (Chen et al., 1999; Deutsch et al., 1996; MacDermid et al., 2004;Royer et al., 2009). The results of this investigation, however, do notsuggest that the increase in shoulder pain in the EC position was dueto an increase in tendon compression in the SAS. It remains to beexplained why the subjects of the shoulder pain group reported thegreater pain in the EC position.

During testing subjects' arms were positioned at 90 degrees relativeto their trunk for both EC and FC tests, a position obtained by a combina-tion of scapular upward rotation and glenohumeral abduction. If subjectshad greater scapular UR, then theywould have less glenohumeral abduc-tion. We saw greater scapular UR in the EC position than in the FC posi-tion. With the greater scapular UR in the EC position, glenohumeralabduction contributed less to the elevation of the arm. The decrease inglenohumeral abduction would expose more of the rotator cuff tendonsto compression in the subacromial outlet (Bey et al., 2008; Flatowet al., 1994). Excursion of the rotator cuff is increased with increasedglenohumeral internal rotation, as in the EC arm position (Hughesand An, 1996; Nakajima et al., 1999). We did not see difference inthe AHD between the EC and FC positions, but in the EC position,there could be more rotator cuff tendon in the subacromial outletdue to the lower scapular upward rotation. There is an evidencethat patients with RCD have increased supraspinatus tendon thicknessof 1.5 mm. Other authors (Cholewinski et al., 2008) have found adecrease in tendon thickness of 1.1 mm in tendons in the shouldersof patients with rotator cuff disease (RCD). The thickness of thesupraspinatus tendon was not measured as part of this investigation,




if the thickness of the rotator cuff tendon was greater in the SAIS groupthan in the control group, it is possible that the rotator cuff tendonsmight have been impinged even if AHD did not change. The ultrasoundtechnique used to measure the width of the subacromial outlet wouldnot allow us to measure changes to the other areas of the SAS, it is pos-sible that the glenohumeral (GH) internal rotation in the EC position pro-duced changes in the SAS underneath the acromion and thus notdetected by the ultrasound imaging technique.

The measurement of AHD was conducted under static conditions. Itis likely that these measurements would be different under dynamicconditions as the SAS has been shown to decrease during active arm el-evation (Desmeules et al., 2004; Thompson et al., 2011). Thigpen et al.(2006) reported that subjects without shoulder pain showed less scap-ular PT and greater IR during dynamic arm elevation in the EC position.In our investigation during static conditions, subjects with SAIS had lessposterior tilt as in the Thigpen et al.'s (2006) study, but differed in thescapular IR and UR findings. The disparity in the findings between staticand dynamic investigationsmay be due to the differences in muscle ac-tivity between the test conditions producing differences in the position-ing of the scapula at lower arm elevation angles.

5. Conclusion

In our investigation we did not see differences in the AHD be-tween the EC and FC arm positions or between the groups. Subjectsin the SAIS group had greater pain in the EC position that was likelynot due to the compression of the rotator cuff tendon in the SAS out-let because both groups had similar AHD. We did see significant dif-ferences in scapular and clavicular kinematic data between the twoarm positions. The differences in scapular orientation between thetwo arm positions did not seem to affect the AHD, this might be dueto the association between scapular UR, IR and PT. The results ofthis investigation suggest that increased scapular UR is associatedwith increased IR and PT in order to reduce impingement of the rota-tor cuff tendons at the SAS outlet; we did not see this association inthe SAIS group. The adaptation of greater scapular UR seen in theSAIS group might very well expose more of the rotator cuff to injuryat the SAS outlet and increase the subjects reported pain. Further in-vestigation is needed to determine if these adaptations are also seenduring dynamic arm elevation in the EC and FC positions.

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differences in scapular orientation, subacromial space and shoulder pain between the full can and...

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