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

JCLB-03580; No of Pages 7 Clinical Biomechanics xxx (2013) xxx–xxx

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Differences in scapular orientation, subacromial space and shoulder pain between the full 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, USA Department of Physical Therapy, Virginia Commonwealth University, Medical College of Virginia Campus, Richmond, VA 23298, USA Universidade Federal de Sao Paulo, Departamento de Medicina, Sao Paulo, SP, Brazil d Inpatient Spinal Cord Injury, Craig Hospital, Englewood, CO 80113, USA e Department of Veterans Affairs, Hunter Holmes McGuire VA Medical Center, Physical Medicine and Rehabilitation Section, Richmond, VA 23249, USA b c

a r t i c l e

i n f o

Article history: Received 10 February 2012 Accepted 29 January 2013 Keywords: Scapular orientation Acromio-humeral distance Jobe Test Subacromial impingement syndrome

a b s t r a c t Background: The empty and full can arm positions are used as diagnostic tests and in therapeutic exercise programs for patients with subacromial impingement syndrome. The adverse effects of these arm positions on the rotator cuff have not been fully described. The purpose of this investigation was to compare the acromio-humeral distance, three-dimensional scapular position, and shoulder pain during maximum isometric contractions in the empty and full can arm positions. Methods: Subjects with subacromial impingement syndrome (n = 28) and a matched control group without shoulder pain (n = 28) participated. Acromio-humeral distance, scapular/clavicular positions and shoulder pain 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 between the groups (P = 0.598). The empty can position resulted in greater scapular upward rotation (P b 0.001, difference=4.9°), clavicular elevation (Pb 0.001, difference=2.7°), clavicular protraction (P=0.001, difference= 2.5°) and less posterior tilt (Pb 0.001, difference=3.8°) than the full can position. No differences in the scapular positions were found between the groups. Positive correlations were seen between the scapular positions in the control and not in the subacromial impingement group. Interpretation: Our results did not show a difference in acromio-humeral distance between the arm positions or groups, indicating that the kinematic differences between the positions are not associated with altered acromio-humeral distance. The increased pain in the EC position might be due to the lack of an association amongst 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 diagnostic tests and as therapeutic exercises in rehabilitation programs for patients with rotator cuff disease. Specifically, the EC position (“Jobe test”) is used to assist in the diagnosis of injury to the supraspinatus muscle and is theorized to maximize the activation of the supraspinatus during exercise (Jobe and Moynes, 1982; Kelly et al., 1996; Park et al., 2005). Prior research has indicated that these tests do not differ in supraspinatus muscle activity; therefore one is not recommended over the other to activate the supraspinatus (Boettcher et al., 2009; Takeda et al., 2002). There may be other parameters that differ between the FC and EC positions that will preferentially direct the use of the two arm positions. ⁎ Corresponding author at: Department of Physical Medicine, Hunter Holmes McGuire VA Medical Center, 1201 Broad Rock Blvd, Richmond, VA 23249, USA. E-mail address: [email protected] (M.K. Timmons).

The EC and FC tests are performed by resisting isometric arm elevation in the scapular plane at 90° elevation; the tests differ in the position of the glenohumeral joint. The EC is performed in glenohumeral internal rotation (thumb pointing down) and the FC is performed in neutral glenohumeral rotation (thumb pointing up). The glenohumeral internal rotation in the EC may place the greater tuberosity of the humerus closer to the acromion, leading to a decrease in the volume of the subacromial space (SAS) and therefore increasing the risk for subacromial impingement 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 by the borders of the coracoacromial arch and the humeral head. The acromio-humeral distance (AHD) is the linear distance between inferior acromion and humerus. This distance is used to represent the width of the SAS outlet (Fig. 1) (Azzoni and Cabitza, 2004; Azzoni et al., 2004; Desmeules et al., 2004). The SAS outlet allows for the excursion of the supraspinatus tendon into the SAS. Patients with subacromial impingement syndrome (SAIS) have been shown to

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

Please cite this article as: Timmons, M.K., et al., Differences in scapular orientation, subacromial space and shoulder pain between the full can and empty can tests, Clin. Biomech. (2013), http://dx.doi.org/10.1016/j.clinbiomech.2013.01.015

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

increased scapular internal rotation as compared to the FC. Secondarily this investigation had the purpose to determine if scapular position and AHD in the two test positions differ between subjects with and without SAIS. 2. Methods This was a prospective cross-sectional controlled laboratory study. This study was approved by the Institutional Review Board at the investigator's university. Participating subjects reviewed and signed the informed consent, completed the intake questionnaires and underwent an eligibility examination. Next, subjects underwent study testing in both the FC and EC positions. 2.1. Subjects

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

have a decreased AHD measurement when compared to patients without SAIS (Graichen et al., 1999b; Hebert et al., 2003). Changes in the AHD measurement may be related to the changes in scapular motion 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 rotation have been theorized to cause extrinsic impingement of the rotator cuff tendons by decreasing the size of the subacromial space, (Ludewig and Reynolds, 2009; Michener et al., 2003; Timmons et al., 2012) conversely, increased upward rotation and posterior tilt of the scapula have been theorized to increase the subacromial space (McClure et al., 2006). Evidence indicates that limited scapular upward rotation mobility (Atalar et al., 2009), scapular dyskinesis (Silva et al., 2008), and scapular protraction (Solem-Bertoft et al., 1993) decrease the size of the subacromial space, while a position of increased scapular upward rotation, posterior tilt, (Seitz et al., 2011) and scapular retraction (Kalra et al., 2010; Solem-Bertoft et al., 1993) is associated with an increase in subacromial space. It is unclear if the FC and EC test positions adversely affect scapular kinematics, the subacromial outlet and increasing risk of shoulder pain. Improved understanding of the effects of positioning the arm in the FC and EC positions during resisted maximal isometric force production on subacromial space outlet and scapular kinematics will assist health care providers in use of the FC and EC positions. The purpose of this investigation was to compare the three-dimensional scapular position, AHD, and shoulder pain during maximum isometric contractions in the EC and FC arm positions. We hypothesized that during the EC there would be increased shoulder pain, decreased acromio-humeral distance, decreased scapular upward rotation and posterior tilt, and

Two groups of subjects were recruited to participate in this investigation, a control group not reporting shoulder pain (n=28) and a group with a clinical diagnosis of SAIS (n=28). Descriptive data is available in Table 1. The control group and SAIS group were matched based on age (within 5 years), sex, and shoulder tested (dominant or non-dominant side). Control group inclusion criteria were 18–65 years of age without shoulder pain in the previous 6 months. Control group subjects were excluded if they had positive finding on any of the SAIS tests (painful arc, pain or weakness with resisted external rotation, Neer, Hawkins, and Jobe tests) (Michener et al., 2009), a history of upper arm fracture, shoulder surgery, or shoulder pathology. The SAIS group inclusion criteria were pain with resisted arm elevation or external rotation as well as 3 of 5 positive SAIS tests (stated above). In order to assure that subjects did not have adhesive capsulitis; subjects were excluded from the SAIS group if they could not elevate their shoulder greater than 150° nor had a 50% limitation of passive shoulder range of motion in more than 2 planes of motion. Additional exclusion criteria included shoulder pain greater than 7/10, a history of fracture to the shoulder girdle, systemic musculoskeletal disease, shoulder surgery, or a positive clinical examination for a full thickness rotator cuff tear. 2.2. Procedures Subjects sat with their feet flat on the floor, and shoulder-width apart, and they were instructed to sit up straight with head facing forward. The subject's arm was positioned with the shoulder in 90° of elevation in the scapular plane. The scapular plane is defined as being rotated 40° anterior to the coronal plane. For the FC, the arm was placed in neutral rotation standardized by the thumb pointing towards the ceiling (Fig. 2A). The EC was standardized by the thumb pointing down towards the floor. Arm elevation and scapular plane angles were verified with a digital inclinometer (Acumar, Lafayette Instruments, Lafayette, IN, USA). During 2 trials, each in the FC and EC positions, the subject performed a 6 second maximal voluntary isometric contraction resisted against shoulder elevation. A minimum of a one minute rest was given between the 2 trials. During the isometric contraction, dependent variables were measured including 1 — shoulder elevation force measurements with hand-held dynamometer,

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

Age (years) Height (cm) Mass (kg) PENN pain PENN function PENN total

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

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

Mean

SD

Mean

SD

Mean difference

t

P value

37.9 172.8 74.1 29.3 58.8 97.1

14.3 11.4 15.1 1.0 3.2 4.2

38.7 174.8 82.5 19.9 42.9 67.1

13.4 9.1 16.1 4.6 6.9 10.5

0.9 1.9 7.7 9.4 15.9 30.0

0.221 0.691 1.89 −10.577 −11.784 −14.064

0.826 0.492 0.064 b0.001 b0.001 b0.001

Please cite this article as: Timmons, M.K., et al., Differences in scapular orientation, subacromial space and shoulder pain between the full can and empty can tests, Clin. Biomech. (2013), http://dx.doi.org/10.1016/j.clinbiomech.2013.01.015

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

A

B

3

2.5. Scapular position The 3-dimensional kinematics of the scapula, clavicle and humerus were measured with the 6 degree of freedom Polhemus 3Space Fastrak electromagnetic-based motion capture system (Polhemus, Colchester, VT, USA) integrated with Motion Monitor software (Innovative Sports Technologies Inc., Chicago IL, USA). Kinematic data were sampled at 30 Hz. The International Society of Biomechanics (ISB) protocol was followed for sensor placement, creation of anatomical coordinate systems, and Euler angle sequence of rotations (Wu et al., 2005). Electromagnetic sensors were placed and secured over the distal humerus with an elastic neoprene cuff, on the posterior lateral acromion with double sided tape, and over the third thoracic vertebra with a double sided tape. A fourth sensor was used to digitize bony landmarks for creation of local anatomical axis and coordinate systems. Twelve subjects from the SAIS group were asked to return within one week for a repeat kinematic testing in order to test the reliability of these measurements. The intra-class correlation coefficient (ICC) ranged between 0.93 and 0.97 for all scapular and clavicular motions. Measurement error for the scapular and clavicular positions was determined prior 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°), internal rotation (SEM=1.86°, MDC=2.62°), clavicular elevation (SEM=1.02°, MDC=1.44°) and clavicular protraction (SEM=1.43°, MDC=2.03°) at 90° of scapular plane elevation were all calculated.

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

2 — shoulder pain with the numeric pain rating scale, 3 — ultrasound imaging of the subacromial space for measurement of the acromiohumeral distance, and 4 — scapular position with 3-dimensional kinematic electromagnetic sensors. The EC and FC test positions were confirmed by calculating the glenohumeral internal/external rotation position in each test position during data processing. The means of the two trials for all dependent variables were used for data analysis. Kinematic and ultrasound data were collected during separate maximal voluntary isometric contraction (MVIC). 2.3. Shoulder scaption moment During each MVIC trial, the maximum force exerted was recorded using a hand held dynamometer (microFET3, Hoggan Health Industries, Draper, UT, USA) located immediately proximal to the hand. Force (N) was recorded, and the shoulder scaption net moment (Nm) was calculated by multiplying the arm scaption force by the arm length in meters. Joint moment was then normalized to the subject's body mass (kg). Handheld dynamometry for assessment of shoulder strength in symptomatic (Hayes et al., 2002) and healthy subjects (Agre et al., 1987) has shown excellent inter-rater and intra-rater reliability.

A diagnostic ultrasound unit (LogiQ e; GE Healthcare, Wisconsin, USA) with an adjustable 5.0–12.5 MHz frequency linear array transducer was used to capture ultrasound images of the subacromial space outlet. All ultrasound imaging was performed by the same examiner. The linear transducer frequency and focal image depth were adjusted for each subject in order to produce the best image of the subacromial space outlet. The position of the probe was standardized as previously described by Desmeules et al. (2004). Three coronal plane view images were collected with the transducer placed over the lateral acromion at its most anterior aspect (Fig. 2B). An image of the subacromial outlet for AHD measurement was saved when both the hyperechoic acromion and humeral head were clearly visualized on the screen of the ultrasound unit. The AHD was measured as the shortest distance between the humeral head and the lateral inferior tip of the acromion (Azzoni and Cabitza, 2004; Desmeules et al., 2004; Seitz and Michener, 2010). The AHD measurements were made using software embedded in the ultrasound unit. The AHD measurements from the 3 separate ultrasound images taken during the FC and EC positions were used for statistical analysis. From preliminary data the intra-rater test–retest reliability was calculated on n =9 subjects without shoulder pain. The ICC was calculated 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 2.4. Shoulder pain Subjects' shoulder pain was assessed in 2 ways. First, the subjects completed the PENN Shoulder Pain and Function questionnaire as part of the pretest screening. A second assessment of shoulder pain occurred during the shoulder testing session. Immediately following each contraction subjects were asked to rate the shoulder pain they experienced during the contraction on a numerical patient-reported 0–10 pain scales (NPRS). Subjects were shown the numeric pain scale with 0 equating to no pain at all and 10 equating to the most extreme pain imaginable.

Means and standard deviations were calculated for all demographic data and dependent variables. A 2 × 2 (group by arm position) repeated measure ANOVA was used to test for differences in dependent variables. Paired t-tests were used as post-hoc to determine statistical differences between levels of the independent variables when significant group main effects were found. The relationships between variables were determined using Pearson correlation. Statistical significance was determined a priori at P ≤ 0.05; all statistical analyses were completed using SPSS 19 statistical software (SPSS Inc., Chicago, IL, USA).

Please cite this article as: Timmons, M.K., et al., Differences in scapular orientation, subacromial space and shoulder pain between the full can and empty can tests, Clin. Biomech. (2013), http://dx.doi.org/10.1016/j.clinbiomech.2013.01.015

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

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

Scaption moment (Nm/kg) AHD (mm) Scapular upward rotation (degrees) Scapular internal rotation (degrees) Scapular posterior tilt (degrees) Clavicular elevation (degrees) Clavicular protraction (degrees)

Empty can

Mean

SD

Mean

SD

Mean difference

F value

P value

1.74 8.6 25.3 −32.5 −7.0 19.6 −20.5

0.66 1.7 15.6 7.6 7.5 5.1 5.1

1.51 8.4 30.2 −33.1 −10.8 22.3 −23.0

0.57 1.6 13.9 7.3 7.4 6.7 6.6

0.23 0.2 −4.9 0.6 3.8 −2.7 2.5

13.166 1.034 31.430 1.120 66.053 15.948 12.811

0.001 0.314 b0.001 0.295 b0.001 b0.001 0.001

3. Results The PENN shoulder pain and function scores were higher in the control group than the SAIS group, indicating greater impairment among the subjects with SAIS (Table 1). Glenohumeral internal/external rotation position was significantly different (t= 21.120, P b 0.001) between the 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, P b 0.001; mean difference = 0.76) and produced a lower shoulder scaption moment in the EC (F(54,1) = 13.166, P = 0.001; mean difference = 0.23 Nm/kg). The group by arm position interaction for shoulder pain was significant (F(54,1) =13.234, Pb 0.001). Control subjects reported no difference (P=0.355) in shoulder pain between the two test positions, the pain reported during the maximum contraction did not differ from zero (t=1.441, P=0.161). Subjects in the SAIS group reported greater pain in the EC position than the FC position (t= −4.387, P b 0.001; mean difference = 1.4). Scaption moment 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 and group are available in Tables 2 and 3. The difference in the AHD between 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 between 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 positions by arm position and group are available in Tables 2 and 3. There were significant differences in scapular and clavicular positions between the FC and EC positions. In the FC position, subjects had less upward rotation (UR) (F(54,1) = 31.43, P b 0.001; mean difference= 4.9°), greater posterior tilt (F(54,1) = 66.053, P b 0.001; mean difference= 3.8°), less clavicular elevation (F(54,1) = 15.948, P b 0.001; 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

position. No differences were seen between EC and FC positions (F(54,1) = 0.013, P = 0.91) in scapular internal rotation. Differences between the control and SAIS groups and the two-way interaction effects did not reach statistical significance for all scapular and clavicular positions. The significant differences found for upward rotation, posterior tilt, clavicular elevation and clavicular protraction were all greater than the SEM and MDC. Correlations between the kinematic measurements are presented in Table 4. There was not a significant correlation between reported pain during the MVIC and any of the scapular and clavicular positions. In FC position significant positive correlations of moderate strength were found 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 correlation was found between UR and protraction (PRO) (r= −0.462, P b 0.001). The data was separated by group and the correlation analysis was performed on the separated data in order to determine differences between groups. In the control group the correlations between UR and IR (r= 0.471) and UR and PT (r= 0.467) in the FC position were strengthened. In the control group a positive correlation was found between 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 positions was not statistically significant. 4. Discussion It is theorized that shoulder pain during arm elevation in patients with rotator cuff disease results from the compression of the rotator cuff due to the narrowing of the SAS (Ludewig and Reynolds, 2009; Michener et al., 2003). In the current study, subjects in the SAIS group had greater shoulder pain during the MVIC with their arm in the EC position than in the FC. We hypothesized that the AHD would be narrower in the EC position when compared to the FC position; however, we did not find differences in the AHD between the EC and FC arm positions. We also did not find differences in AHD between those with and without SAIS. Other investigations have had mixed results, some showing significantly smaller AHD in subjects with shoulder pain, (De Wilde et al., 2003; Graichen et al., 1999a) while others have shown no difference in AHD between subjects with and without shoulder pain (Hardy et al., 1986). These earlier studies did not measure AHD during an MVIC, as was done in the current study. Thompson et al. (2011) found a decrease in AHD during a loaded dynamic arm elevation and White

Table 3 Means and standard deviations for scaption moment, acromio-humeral distance and scapular and clavicular positions by group collapsed across arm position. Control

Scaption moment (Nm/kg) AHD (mm) Scapular upward rotation (degrees) Scapular internal rotation (degrees) Scapular posterior tilt (degrees) Clavicular elevation (degrees) Clavicular protraction (degrees)

SAIS

Mean

SD

Mean

SD

Mean difference

F value

P value

1.83 8.4 28.0 −33.1 −8.0 20.6 −21.8

0.61 1.8 14.9 8.2 7.5 5.3 5.6

1.43 8.6 28.1 −31.4 −9.8 21.4 −21.7

0.54 1.6 15.8 8.1 7.4 6.3 6.1

0.4 −0.2 −0.1 −1.7 1.8 −0.8 −0.1

7.691 0.281 0.001 0.751 0.838 0.290 0.003

0.008 0.598 0.973 0.390 0.364 0.593 0.954

Please cite this article as: Timmons, M.K., et al., Differences in scapular orientation, subacromial space and shoulder pain between the full can and empty can tests, Clin. Biomech. (2013), http://dx.doi.org/10.1016/j.clinbiomech.2013.01.015

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

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Table 4 Scapular and clavicular positions correlation coefficients.

ALL Subjects UR UR IR PT ELE PRO

IR

SAIS Subjects

PT

ELE

PRO

UR

IR

PT

EC

0.124

0.196

-0.042

-0.381*

0.051

0.154

FC

0.343*

0.267*

0.248

-0.462*

0.209

0.134

Control Subjects ELE

PRO

-0.004

-0.427*

0.180

0.253

-0.011

-0.324

-0.466*

0.471*

0.467*

-0.069

-0.480*

0.515*

UR

IR

PT

ELE

PRO

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

0.004

0.100

-0.117

0.548*

-0.112

-0.289

-0.277

0.009

0.514*

0.018

0.013

-0.436*

-0.069

-0.089

-0.113

-0.059

-0.427*

-0.038

-0.238

0.548*

-0.324

-0.619*

-0.021

0.009

-0.466*

-0.373*

0.038

-0.436

-0.480*

-0.615*

-0.281

-0.059

EC

-0.042

-0.084

-0.180

FC

0.248

-0.018

-0.077

EC

-0.381*

-0.310*

-0.143

FC

-0.462*

-0.511*

-0.137

0.361* -0.218 0.361* -0.218

⁎P b 0.05.

et al. (2011) reported a decrease in AHD during an MVIC shoulder external rotation. It is likely that during maximal muscle activity, the scapula is positioned such that the SAS would be narrowed and compression of the rotator cuff is increased. We hypothesized that in the EC position the scapula would be positioned in a manner that is believed to be associated with rotator cuff impingement, i.e., less PT, less scapular UR and greater scapular IR. In the EC position, we found that the scapula was in less PT, a position that should produce a decrease in AHD. We also found an increase in scapular UR in the EC position, which should increase the AHD. It is likely that the decrease in the AHD that resulted from the decreased scapular PT was offset by the increase in AHD that resulted from the increase in scapular UR. This might explain why we found no difference in AHD seen between the FC and EC positions. The secondary purpose of this investigation was to determine if differences in the AHD and scapular position measurements seen between the EC and FC would be greater in subjects with SAIS. The SAIS group had greater pain during the MVIC in the EC position than the FC. We also found that the control and SAIS groups produced lower shoulder scaption moment in the EC position than in the FC. The decrease in scaption moment in the EC position was greater in the control group than the SAIS group, so it appears that the reduction in shoulder scaption moment was due more to the change in arm position than the increase in shoulder pain. The results of this investigation did not show any differences in scapular position between the control and SAIS groups. Correlation analysis revealed relationships between the scapular positions that could explain why we did not find a difference in the AHD between arm positions and groups. The positive correlations found between UR and IR, and UR and PT suggest that subjects with greater scapular UR also showed greater PT and IR. The positive correlation between IR and PT suggests that subjects with greater IR also had greater PT. Increasing IR is reported to decrease the AHD while increased PT increases the AHD. These correlations were not seen in the SAIS group. These relationships suggest that the combined scapular motions as opposed to a deficiency of a single motion contribute to the reduction in rotator cuff compression and that differing strategies might exist in order to reduce the compression. It is possible that patients with SAIS might not be able to produce the complex combination of motions necessary to maintain the dimensions of the SAS. The SAIS group did have smaller AHD than the control group even though the difference did not reach statistical significance, and the association seen between PT and IR in the control group might help the subject without SAIS to reduce compression of the rotator cuff, while the dissociation of PT and IR seen in the SAIS group might contribute to the pain reported during the maximal contractions.

It would seem logical that while experiencing pain, subjects would attempt to configure the shoulder girdle in a manner that would minimize their pain. The options available to the subject would be to change the orientation of the scapula in order to open up the SAS outlet and reduce compression of the rotator cuff tendons or to decrease the length of the rotator cuff tendons in order to reduce the internal strain of the tendon. The differences between the groups in the correlation between the scapular positions suggest that patients with SAIS are not able to make the adjustment in order to maintain the width of the SAS outlet. The SAIS group produced a lower scaption moment than the control group in both EC and FC positions. Patients with rotator cuff tendinopathy have been shown to produce significantly lower shoulder isometric, isokinetic, concentric and eccentric torques (Leroux et al., 1994; MacDermid et al., 2004; Tyler et al., 2005). The increase in pain experienced by the SAIS group during the MVIC in the EC position may 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 the scapula, but also may be due to the intrinsic deficits of the rotator cuff muscles producing a superior translation of the humeral head into the SAS (Chen et al., 1999; Deutsch et al., 1996; MacDermid et al., 2004; Royer et al., 2009). The results of this investigation, however, do not suggest that the increase in shoulder pain in the EC position was due to an increase in tendon compression in the SAS. It remains to be explained why the subjects of the shoulder pain group reported the greater pain in the EC position. During testing subjects' arms were positioned at 90 degrees relative to their trunk for both EC and FC tests, a position obtained by a combination of scapular upward rotation and glenohumeral abduction. If subjects had greater scapular UR, then they would have less glenohumeral abduction. We saw greater scapular UR in the EC position than in the FC position. With the greater scapular UR in the EC position, glenohumeral abduction contributed less to the elevation of the arm. The decrease in glenohumeral abduction would expose more of the rotator cuff tendons to compression in the subacromial outlet (Bey et al., 2008; Flatow et al., 1994). Excursion of the rotator cuff is increased with increased glenohumeral internal rotation, as in the EC arm position (Hughes and An, 1996; Nakajima et al., 1999). We did not see difference in the AHD between the EC and FC positions, but in the EC position, there could be more rotator cuff tendon in the subacromial outlet due to the lower scapular upward rotation. There is an evidence that patients with RCD have increased supraspinatus tendon thickness of 1.5 mm. Other authors (Cholewinski et al., 2008) have found a decrease in tendon thickness of 1.1 mm in tendons in the shoulders of patients with rotator cuff disease (RCD). The thickness of the supraspinatus tendon was not measured as part of this investigation,

Please cite this article as: Timmons, M.K., et al., Differences in scapular orientation, subacromial space and shoulder pain between the full can and empty can tests, Clin. Biomech. (2013), http://dx.doi.org/10.1016/j.clinbiomech.2013.01.015

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

if the thickness of the rotator cuff tendon was greater in the SAIS group than in the control group, it is possible that the rotator cuff tendons might have been impinged even if AHD did not change. The ultrasound technique used to measure the width of the subacromial outlet would not allow us to measure changes to the other areas of the SAS, it is possible that the glenohumeral (GH) internal rotation in the EC position produced changes in the SAS underneath the acromion and thus not detected by the ultrasound imaging technique. The measurement of AHD was conducted under static conditions. It is likely that these measurements would be different under dynamic conditions as the SAS has been shown to decrease during active arm elevation (Desmeules et al., 2004; Thompson et al., 2011). Thigpen et al. (2006) reported that subjects without shoulder pain showed less scapular PT and greater IR during dynamic arm elevation in the EC position. In our investigation during static conditions, subjects with SAIS had less posterior tilt as in the Thigpen et al.'s (2006) study, but differed in the scapular IR and UR findings. The disparity in the findings between static and dynamic investigations may be due to the differences in muscle activity between the test conditions producing differences in the positioning of the scapula at lower arm elevation angles. 5. Conclusion In our investigation we did not see differences in the AHD between the EC and FC arm positions or between the groups. Subjects in the SAIS group had greater pain in the EC position that was likely not due to the compression of the rotator cuff tendon in the SAS outlet because both groups had similar AHD. We did see significant differences in scapular and clavicular kinematic data between the two arm positions. The differences in scapular orientation between the two arm positions did not seem to affect the AHD, this might be due to the association between scapular UR, IR and PT. The results of this investigation suggest that increased scapular UR is associated with increased IR and PT in order to reduce impingement of the rotator cuff tendons at the SAS outlet; we did not see this association in the SAIS group. The adaptation of greater scapular UR seen in the SAIS group might very well expose more of the rotator cuff to injury at the SAS outlet and increase the subjects reported pain. Further investigation is needed to determine if these adaptations are also seen during dynamic arm elevation in the EC and FC positions. References Agre, J.C., Magness, J.L., Hull, S.Z., Wright, K.C., Baxter, T.L., Patterson, R., et al., 1987. Strength testing with a portable dynamometer: reliability for upper and lower extremities. Arch. Phys. Med. Rehabil. 68 (7), 454–458 (available from: PM:3606371). Atalar, H., Yilmaz, C., Polat, O., Selek, H., Uras, I., Yanik, B., 2009. Restricted scapular mobility during arm abduction: implications for impingement syndrome. Acta Orthop. Belg. 75 (1), 19–24 (available from: PM:19358393). Azzoni, R., Cabitza, P., 2004. Sonographic versus radiographic measurement of the subacromial space width. Chir. Organi Mov. 89 (2), 143–150 (available from: PM:15645791). Azzoni, R., Cabitza, P., Parrini, M., 2004. Sonographic evaluation of subacromial space. Ultrasonics 42 (1–9), 683–687 (available from: PM:15047367). Bey, M.J., Kline, S.K., Zauel, R., Lock, T.R., Kolowich, P.A., 2008. Measuring dynamic invivo glenohumeral joint kinematics: technique and preliminary results. J. Biomech. 41 (3), 711–714 (available from: PM:17996874). Boettcher, C.E., Ginn, K.A., Cathers, I., 2009. The ‘empty can’ and ‘full can’ tests do not selectively activate supraspinatus. J. Sci. Med. Sport 12 (4), 435–439 (available from: PM:19054712). Chen, S.K., Simonian, P.T., Wickiewicz, T.L., Otis, J.C., Warren, R.F., 1999. Radiographic evaluation of glenohumeral kinematics: a muscle fatigue model. J. Shoulder Elbow Surg. 8 (1), 49–52 (available from: PM:10077797). Cholewinski, J.J., Kusz, D.J., Wojciechowski, P., Cielinski, L.S., Zoladz, M.P., 2008. Ultrasound measurement of rotator cuff thickness and acromio-humeral distance in the diagnosis of subacromial impingement syndrome of the shoulder. Knee Surg. Sports Traumatol. Arthrosc. 16 (4), 408–414 (available from: PM:18157491). De Wilde, L., Plasschaert, F., Berghs, B., Van, H.M., Verstraete, K., Verdonk, R., 2003. Quantified measurement of subacromial impingement. J. Shoulder Elbow Surg. 12 (4), 346–349 (available from: PM:12934028). Desmeules, F., Minville, L., Riederer, B., Cote, C.H., Fremont, P., 2004. Acromio-humeral distance variation measured by ultrasonography and its association with the

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Please cite this article as: Timmons, M.K., et al., Differences in scapular orientation, subacromial space and shoulder pain between the full can and empty can tests, Clin. Biomech. (2013), http://dx.doi.org/10.1016/j.clinbiomech.2013.01.015