Subacromial impingement and posture

Narrative Review

Subacromial impingement and posture Adam Phillips Rufa Physical Therapy Education, College of Health Professions, SUNY Upstate Medical University, Syracuse, NY, USA Background: It has been suggested that dysfunctional posture of the scapula, thoracic spine, and cervical spine is associated with the development of subacromial impingement syndrome (SIS). Objectives: This paper explores the current literature describing the association between static posture and SIS. Major findings: Thirty-one articles were included in this review. Nine of these articles looked specifically at the resting posture in subjects with SIS. All nine of these studies found no correlation between static posture and SIS. Fifteen articles included subjects without SIS and/or non-self-selected resting posture. Of those, nine provided support for the plausibility of a relationship between SIS and posture. Seven of the studies included in this review examined treatment aimed at posture. Only one looked specifically at the effects of postural correction in subjects with SIS and found that it improved pain-free shoulder range of motion (ROM) but there was no change in pain intensity. Three out of four studies, which examined the ability of stretching and strengthening exercise to change posture, found that exercise can have an effect on posture. Several limitations were present in the articles that reduce the strength of the conclusions. These include the heterogeneity of the SIS diagnosis, limited information on subject variables, complexity of measuring posture, lack of blinding, and limited reporting of power analysis. Conclusion: This review has highlighted the current lack of strong evidence to support an association between posture and SIS. The evidence examining this association is plagued by poorly defined diagnostic criteria, wide sample variation, and poor statistical power. Clinicians should be mindful of the current evidence pointed out in this review when considering using posture as a diagnostic and interventional strategy for patients with SIS. Keywords: Exercise, Glenohumeral joint, Physical therapy, Shoulder orientation, Posture, Rotator cuff

Subacromial impingement syndrome (SIS) is a term used to describe shoulder pain and dysfunction, which involves the structures of the subacromial space and is most pronounced with overhead activities. SIS has been described by several authors dating back to 1852.1 One of the first detailed descriptions of the condition was published by Neer in 1972.2,3 He described irritation and damage to the supraspinatus, biceps tendon, and/or subacromial bursa as a direct result of mechanical pressure from the anterior acromion and coracoacromial ligament. Neer went on to develop a classification system that divided SIS into three distinct stages based on the severity of tissue damage.3 Despite significant research since this early description by Neer, the exact mechanism of SIS remains unclear. Factors such as acromion shape,2,4,5 ligamentous structure,1,6 glenoid anatomy,7–9 rotator cuff pathology,10,11 scapular kinematics,12–14 muscle length,15 and motor control15,16 have all been suggested as possible contributing factors

Correspondence to: Adam Phillips Rufa, Physical Therapy Education, SUNY Upstate Medical University, 750 East Adams Street, Rm. 2229 Silverman Hall, Syracuse, NY 13210, USA. Email: [email protected]

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to the development of SIS. This uncertainty highlights the complexity of the disorder and suggests that SIS may represent a group of conditions that have numerous etiologies. Based on the challenges described above, it is not surprising that several treatment strategies have been proposed for the management of SIS. These interventions range from invasive procedures such as surgery and injections to more conservative treatments like medication, manual therapy, and exercise. These treatment options have produced varying results and the optimal intervention remains unclear.17–20 A common theme among many of the proposed treatment strategies is a focus on increasing the subacromial space. As suggested by the hypothesis of subacromial pinching, increasing the subacromial space should reduce compression on the rotator cuff, subacrominal bursa, and biceps tendon resulting in less pain and improved function. Surgeries such as acromioplasty and bursectomy are commonly utilized in an attempt to increase the available space by removing soft tissue and/or bone.17 Non-surgical strategies such as exercise and manual therapy also aim to maximize subacromial space by changing

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the position, normalizing muscle function, and/or correcting movement patterns of the shoulder complex. Several authors have implicated faulty cervical, thoracic, and scapular static posture in the development of SIS.15,21–24 This connection between posture and musculoskeletal dysfunction was popularized by Henry and Kendall. They wrote a pivotal book on posture in 1952 aptly named ‘Posture and Pain’.23 In this book, they wrote ‘postural faults which persist may cause discomfort, pain or deformity’.23 The proposed goal was to correct postural faults and promote adoption of a ‘standard posture’. This standard posture was thought to be the optimal position, which resulted in the least amount of stress and strain on the body.23,25 Since the publication of this book, it has been suggested that several disorders, including SIS, have a postural component.15 Factors such as resting scapular, cervical, and thoracic position are believed to have an impact on the development and prognosis of SIS.15 Abnormal posture of these areas is thought to reduce the subacromial space and be associated with changes in biomechanics, which result in increased mechanical stress on subacromial structures.26 This hypothesized connection between SIS and posture seems to be accepted by many health care professionals and is commonly reported in text books.22,26–35 Despite the apparent acceptance of this relationship, no comprehensive literature reviews have been performed on the topic. This paper explores the current literature, which addresses the association between scapular, cervical, and thoracic static posture and SIS.

Search Strategies An extensive electronic literature search was performed using Pubmed, Physiotherapy Evidence Database (PEDro), The Cochrane Library, Cumulative Index to Nursing and Allied Health (CINAHL), OVID, and Google Scholar (Mountain View, CA, USA)H databases. The search terms posture, SIS, shoulder, impingement, biomechanics, postural correction, exercise, intervention, scapula, thoracic, and cervical spine were used in various combinations. The search was restricted to studies performed on human subjects and published in English. The search included articles published on or before January 2013. After a preliminary scan of titles, 97 articles were identified for closer review (Fig. 1). Of those, 26 were excluded, because they were duplicates or the title indicated that they were not relevant to the review. This left 71 articles for closer inspection, including 17 dealing with the association between SIS and posture, 17 regarding shoulder biomechanics, 14 addressing measurement, and 23 examining treatment. Twenty-eight articles were eliminated after review of the abstract because

Subacromial impingement and posture

they were not relevant to the topic. A full text review was performed on the 43 remaining studies. From this review, six additional articles were identified from references and an additional 18 articles were excluded for lack of relevance. The remaining 31 articles were included in the review. Nine of these articles specifically looked at resting posture in subjects with SIS (Table 1). Fourteen articles provided information on the plausibility of a connection between SIS and posture by looking at non-self-selected resting posture and/or using subjects without a diagnosis of SIS (Table 2). The remaining eight articles examined the effects of treatment aimed at changing postural components (Table 3).

Scapula It has been suggested that particular patterns of resting scapular position are associated with shoulder pain.15 Individuals with greater downward rotation (DR), internal rotation (IR), anterior tilting, and protraction of the scapula are thought to be at greater risk for SIS.36 These positions bring the acromion anterior and inferior, which may reduce the acromiohumeral distance potentially increasing the pinching of subacromial structures.13,37–39 Despite its popularity, this proposed relationship between scapular resting position and SIS has not been found by the current literature.12,13,40–42 In 1999, Lukasiewicz et al. compared the scapular position of 20 asymptomatic subjects with 17 individuals diagnosed with SIS, at rest, at 90u of abduction and at maximum abduction.42 A 3D electromechanical digitizer was used to measure the medial–lateral position, superior–inferior position, upward rotation, IR, and posterior tilt in sitting. They found between-group differences in scapular kinematics during shoulder elevation; however, there was no difference in the resting position of the scapula between the two groups.42 Another study, by Endo et al., used radiographs to assess scapular upward rotation, superior–inferior position and protraction of 27 subjects with chronic SIS.41 Images of the scapula were obtained, in sitting, at 0u, 45u, and 90u of abduction. They also found kinematic differences in subjects with SIS; however, resting posture of the scapula was similar between the affected and unaffected shoulders.41 McClure et al. studied the impact of a broader measure of scapular posture, forward shoulder posture, in 45 subjects with SIS.14 Subjects stood in a relaxed posture with their heels and back against a wall. A carpenter’s square was used to measure the perpendicular distance between the wall and the posterior angle of the acromion. No correlation between SIS and a forward shoulder posture was found. In fact, none of the identified studies that examined self-selected scapular resting


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Figure 1 Literature search results.

Table 1 Summary of the most relevant articles (subjects with SIS and self-selected resting post)

Article

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Subjects

Measurement method

Body part(s)

Measurement

Endo et al.41

27 with SIS, ipsilateral vs contralateral shoulder

Scapula

Radiograph

Rest, 45u and 90u of abduction

Finley et al.61

13 wheelchair users with SIS 10 asymptomatic wheelchair users

Thorax, scapula, humerus

Electromagnetic tracking

During wheelchair transfer


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Findings: general Ipsilateral shoulder had reduced UR at 90u, reduced axial rotation at 45u and 90u, and no difference in protraction at any point. Reduced thoracic flexion, increased scapular internal rotation, increased humeral internal rotation during transfer.

Findings: posture No difference in scapular UR, CE, or protraction at rest.

No difference in thoracic, scapular, or humeral position at rest.

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Table 1 Continued

Article

Measurement method

Subjects

Body part(s)

Measurement

Graichen et al.49

10 with SIS 10 asymptomatic

Subacromial space

Open MRI

Hebert et al.95

41 with SIS 39 asymptomatic shoulders

Subacromial space

Open MRI

Hebert et al.12

41 with SIS 39 asymptomatic shoulders

Scapula

3D motion capture system

Lewis et al.50


Cervical, thoracic

Lateral photographs, inclinometer

Ludewig and Cook13


Scapula, upper trap, lower trap, serratus anterior

Electromagnetic sensors

Humeral elevation in scapular plane with no load, 2.3 and 4.6 kg.

Lukasiewicz et al.42


Scapula

3D digitizer

Arm at side, scapular plane elevation to 90, and maximum elevation.

McClur et al.14


Scapula, glenohumeral joint

Goniometer, electromagnetic sensors, handheld dynamometer

Shoulder elevation in sagittal and scapular plane. Shoulder external rotation at 90u of abduction.

5 passive abduction positions (30u, 60u, 90u, 120u, and 150u) and 3 passive rotation positions at 90u of abduction (45u internal rotation, 45u external rotation) 5 active flexion positions (50u, 70u, 90u, 110u, and 130u) and 4 active abduction positions (79u, 80u, 90u, and 110u) Rest, 70u, 90u, and 110u of flexion and abduction Posture: cervical, thoracic, scapular motion: flexion and abduction

Findings: general

Findings: posture

SIS group had reduced subacromial space during loaded abduction.

No difference in subacromial space at rest.

SIS group had reduced subacromial space.

No difference in acromiohumeral distance at rest.

No difference

No difference in scapular UR, ER, or PT at rest. No difference in forward head, forward shoulder, thoracic kyphosis, or scapular protraction at rest. No difference in scapular UR, ER, PT, or humeral lateral rotation at rest.

No relationship between FHP and SIS was observed. SIS group had increased scapular UR, ant. Tipping, medial rotation, increased upper and lower trap EMG, decreased serratus anterior EMG with loading. SIS group had increased superior scapular translation and reduced posterior tilting. SIS group had increased scapular UR, increased clavicular elevation, increased posterior tilt, and clavicular retraction. SIS group had less ROM and reduced force production.

No difference in scapular UR, ER, PT, CE, or protraction at rest.

No difference in scapular UR, PT, ER, CE, and protraction at rest.

UR: upward rotation, PT: posterior tilt, ER: external rotation, CE: clavicular elevation, MRI: magnetic resonance imaging, EMG: electromyography, FHP: forward head positioning, SIS: subacromial impingement syndrome.


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Table 2 Summary of related articles (subjects without SIS and/or non-self-selected resting posture)

Article

Body part(s)

Measurement method

Measurement

Findings: general

Borstad and 50 asymptomatic, Ludewig44 25 with short, and 25 with long pectoralis minors

Scapula, humerus and trunk

Electromagnetic motion capture system

30u, 60u, 90u, and 120u of arm elevation

Borstad39

50 asymptomatic, 25 with short and 25 with long pectoralis minors

Scapula, pectoralis minor, thoracic spine

Electromagnetic motion capture system and plastic ruler

Relaxed standing position and supine

Bullock et al.55

28 subjects with SIS

Shoulder, cervical and thoracic

Video analysis system, pain visual analog scale

Shoulder flexion was greater in the erect position. No change in pain intensity.

Culham and Peat53

57 females 50–85 years old, 23 with osteoporosis

Thoracic spine, shoulder, and scapula

3D Space Isotrack digitizer

Shoulder flexion in neutral erect sitting vs end range cervical and thoracic flexion Relaxed standing position

Crawford and Jull54

60 asymptomatic females

Thoracic Myrin goniometer, spine inclinometer and shoulder

Relaxed sitting, full thoracic extension

Greenfield et al.59

30 subjects with shoulder overuse injuries and 30 health subjects 88 subjects

Scapula, cervical and thoracic spine, shoulder Neck and thoracic spine, shoulder complex

Standing

Shoulder elevation is accompanied by 15u and 13u of shoulder elevation in young and older groups. This represents 50 and 70%, respectively, of total thoracic ROM. The symptomatic group had reduced passive humeral elevation.

GriegelMorris et al.60

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Radiograph measurements

Pain questionnaire, Standing visual inspection using plumb line

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Reduced scapular PT and reduced ER were noted in the short pectoralis group. Supine pectoralis minor measurement with ruler were not different between groups.

27 subjects had normal thoracic posture, 18 had thoracic kyphosis, and 12 had thoracolumbar kyphosis.

See static posture findings.

Findings: static posture Not reported

Short pectoralis minor was associated with reduced scapular ER, a shorter distance between sternal notch and coracoids process and increased thoracic kyphosis Not reported

Increased kyphosis was associated with increased humeral extension, reduced humeral abduction, reduced scapular ER and PT, and increased scapular protraction. No significant effect on scapular UR was noted. Not reported

The symptomatic group had an increased forward head position. 66% had forward head, 38% thoracic kyphosis, 73% rounded shoulder right, 66% rounded shoulder left. There was no relationship between shoulder pain and posture.

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Table 2 Continued

Article

Subjects

Body part(s)

Measurement method

Measurement

Kebaetse et al.52

34 healthy subjects

Thoracic spine, shoulder and scapula

Electromechanical digitizer

Erect vs slouched posture. 0u, 90u, and full shoulder elevation

Ludewig and Cook62

25 asymptomatic subjects

Neck and scapula

Electromechanical digitizer, EMG

Seated with 0u, 25u, and 50u of cervical flexion and 0u, 90u, and 140u of shoulder elevation.

Seitz et al.51 45 asymptomatic professional baseball pitchers

Scapula, thoracic spine and shoulder

Electromagnetic tracking system

Relaxed standing and bilateral maximal shoulder flexion holding 2.3 kgs Normal and relaxed sitting

Straker et al.58

1597 adolescents

7 spinal angles

Photographic measurements, neck and shoulder pain questionnaire

Straker et al.57

1593 14-year olds

Cervical, thoracic, lumbar spine

Photographic measurements, neck and shoulder pain questionnaire

Straker et al.56

1483 adolescents

Cervical, thoracic, lumbar spine

Photographic Normal and measurements, relaxed sitting neck and shoulder pain questionnaire, and computer use questionnaire

Thigpen et al.63

80 asymptomatic subjects

Cervical spine and shoulder

Electromagnetic tracking system, EMG

Normal and relaxed sitting

Weighted (3% body weight) shoulder flexion, forward overhead reaching.

Findings: general Between 90u and maximal elevation, scapular PT was reduced, UR was reduced, reduced ER, maximal shoulder elevation was reduced, and muscle strength was reduced in the slouched posture. The scapular tipping angle was increased at 90u and 140u of elevation when the neck was flexed.

Scapular motion was similar between throwing and non-throwing arm. Life, month, and point prevalence for neck/shoulder pain were 47, 29, and 5%, respectively. 5.3% reported neck and shoulder pain. Female subjects had greater incidence of pain and sat with a more erect posture. Females had increased neck and shoulder pain, spent less time on computers and sat more upright. Computer use was associated with pain. Forward head and rounded shoulder posture was associated with reduced scapular ER and reduced PT during elevation tasks.

Findings: static posture Slouched posture was associated with increased scapular elevation at rest.

Scapular UR reduced from 0u–50u of cervical flexion. Lower trapezius EMG was increased at 0u and 140u of arm elevation with 50u of cervical flexion. Throwing arms had greater scapula UR, greater PT, and greater scapular ER when compared to non-throwing arm. There was no correlation between posture and the presence of neck/ shoulder pain.

Neck and shoulder pain was related to increased lumbar lordosis.

There was no correlation between posture and the presence of neck/ shoulder pain.

Not reported

UR: scapular upward rotation, PT: scapular posterior tilt, ER: scapular external rotation, CE: clavicular elevation, EMG: electromyography, ROM: range of motion, SIS: subacromial impingement syndrome.


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Table 3 Summary of treatment articles Article

Subjects

Intervention type

Bernaards 466 computer et al.65 workers with frequent or long-term neck and upper extremity symptoms.

Intervention details

Outcomes

Results

Education including posture, workplace adjustment, breaks, coping with work demands, and physical activity. Performance of 4 exercises with and without conscious correction of scapular position.

6 group meetings over a 6-month period

Degree of recovery, pain intensity, work disability, amount of symptoms.

Work behavioral education, including postural education, resulted in reduced neck/shoulder pain. There was no benefit from addition of activity education.

Immediate

Effects on UT, MT, LT, and relative UT/MT and UT/LT activation as measured by EMG.

3 sets of 12 repetitions for strengthening and 3 stretching exercises held for 30 seconds performed 4 times per week for 10 weeks. 15 minutes 3 times a week for 6 weeks

Cervical flexion range of motion, 6 postural measures, exercise compliance, physical activity questionnaire, and a posture beliefs questionnaire.

Conscious scapular position correction resulted in increased trapezius activation for prone extension and side lying external rotation. One out of the 6 postural measures (shoulder to pelvis angle) changed in treatment vs control group and neck flexion range increased in treatment group. Treatment group demonstrated a reduced forward shoulder posture.

Immediate with 1 hour wash out period.

Cervical, thoracic and scapular posture; shoulder range and pain.

De Mey et al.67

30 asymptomatic overhead athletes with scapular dyskinesis.

Harman et al.77

17 controls and 23 subjects with forward head posture

Neck extensor and pectoralis major stretches, deep neck flexor, and shoulder retractor strengthening.

Kluemper et al.78

39 competitive swimmers

Lewis et al.64

60 asymptomatic and 60 subjects with SIS

Partner stretching of anterior shoulder muscles and posterior shoulder muscle strengthening. Thoracic and scapular taping.

Roddey et al.79

38 healthy subjects

Pectoralis stretching

3 repetitions of 30 second holds 1 time a day for 14 days.

Forward head and round shoulder posture.

Wang 199980

20 asymptomatic subjects with forward shoulder posture

Stretching of pectoral muscles and strengthening of glenohumeral abductors/external rotators and scapular retractors/elevators.

Thoracic spine posture, scapular position and strength

Wegner 201066

20 controls and 18 subjects with neck pain

Postural correction

Stretching was initially performed as one set of 10 repetitions held for 10 seconds with progression every 2 weeks for 6 weeks. Strengthening was initially performed as one set of 10 repetitions with progression every 2 weeks for 6 weeks. Cues for postural correction of the lumbar spine and scapula

Shoulder girdle posture

Base line differences and effects of postural changes on UT, MT, and LT as measured by EMG

Taping was associated with postural changes, increased pain-free shoulder motion, but was not associated with reduced pain intensity. There was no change in posture between the control group and individuals with mild forward head and rounded shoulder posture. There was a change in posture for subjects with moderate postural faults. Exercise resulted in increased strength, reduced upper thoracic kyphosis at rest and no change in resting scapular position.

Subjects with neck pain had greater MT and less LT activation at baseline. These differences in EMG resolved with postural correction.

UT: upper trapezius; MT: middle trapezius; LT: lower trapezius; EMG: electromyography; SIS: subacromial impingement syndrome.

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posture in subjects with SIS found a postural pattern that was associated with the condition (Table 1). This failure to identify a consistent positional difference between the scapula of asymptomatic individuals and those with SIS is not surprising based on the significant postural variations that occur in the population.43 In a review article, Struyf et al. reported that scapulothoracic resting angle varied from 5.4u of upward rotation to 5.3u of DR, IR angles ranged from 26.5u to 41.1u, and anterior tilt angles ranged from 2u to 15.9u.40 This review indicates that humans may not have a scapular ‘standard posture’ as proposed by Kendall et al.23 It has been suggested that the link between posture and SIS may be elusive because it is an indirect rather than a direct relationship.15,39,44 Sahrmann suggests that posture is associated with movement dysfunctions, which result in excessive stress on the shoulder and can lead to SIS.15,45 Based on this rationale, one would expect to observe a difference in biomechanics between patients with SIS and asymptomatic individuals. Furthermore, an individual’s resting scapular posture should be correlated with these movement dysfunctions. Several studies have shown a difference in biomechanics between symptomatic and asymptomatic individuals.46–48 During shoulder elevation, subjects with SIS have been found to have decreased scapular upward rotation,13,14,41 increased scapular anterior tilting,13,41,42 increased clavicular retraction,14 increased posterior tilting,14 increased upper and lower trapezius electromyography (EMG) activity,13 decreased serratus anterior EMG activity,13 and greater subacrominal space reduction with muscle activation.49 However, these kinematic and kinetic patterns found in patients with SIS were not associated with specific postural patterns.14,42,49,50 In a study of professional baseball pitchers, Seitz et al., used a 3D electromagnetic tracking system to compare the scapular position and biomechanics of the throwing versus non-throwing arm.51 They reported that despite differences in resting scapular position, the pattern of scapular motion in these pitchers was similar in their throwing versus non-throwing arms.51 This suggests that scapular position may not predict the pattern of scapular motion in asymptomatic baseball pitchers but this may not generalize to other populations. In contrast to the Seitz study described above, Borstad found a relationship between posture, pectoralis length, and shoulder mechanics in asymptomatic subjects.39,44 In 2005, Borstad and Ludewig found that asymptomatic subjects with short pectoralis minors demonstrated scapular kinematic patterns similar to those found in patients with SIS.44 In a follow-up study, Borstad found a relationship between a short pectoralis and postural measures such as thoracic kyphosis and scapular IR.39 These


two studies suggest that pectoralis length is a link between faulty biomechanics and posture. This observed link provides plausibility for an indirect relationship between posture and SIS; however, this relationship has not been studied in a symptomatic population. Future studies specifically designed to assess this relationship in subjects with SIS are needed.

Thoracic Spine Thoracic spine posture has the potential to impact the biomechanics of the shoulder both directly and/or indirectly. Indirectly, the position of the thoracic spine may have a significant impact on the position and movement of the scapula via bony and muscular attachments. Increased thoracic kyphosis is associated with increased scapular anterior tilting, increased protraction, and increased scapular elevation at rest in healthy subjects52 and in osteoporotic women.52,53 During shoulder elevation, Kebaetse et al. found that greater thoracic flexion was associated with reduced scapular upward rotation (9.8u), increased scapular IR (2.9u), increased superior translation (2.4u), and reduced scapular posterior tilting (18.2u) from horizontal to full elevation.52 These results suggest a mechanism by which posture could impact SIS; however, subjects were asymptomatic and the slouched posture was not the resting, self-selected posture of the subjects. It is also unclear if the changes in scapular movement seen in this study are consistent with those found in patients with SIS.48 As indicated above, thoracic spine position has the potential to directly impact the biomechanics of the shoulder. In a study of 60 females, Crawford and Jull found that full bilateral shoulder elevation was accompanied by an average of 15u of thoracic extension in 18–30-year olds and 13u in 50–75-year olds.54 These findings were supported by both Bullock et al. and Kebaetse et al. who found a 12.7u and 23.6u reduction in shoulder elevation when the thoracic spine was kept in a flexed position versus an erect sitting posture.52,55 Based on these findings, individuals with increased thoracic kyphosis posture and subsequent reduction in thoracic extension may have greater stress on their shoulders with elevation. This increased stress could result in greater subacromial pressure resulting in or contributing to SIS. Despite the biomechanical plausibility established by the articles discussed above, studies that examined the thoracic resting posture of symptomatic subjects did not find an association.56–61 Greenfield et al. used radiographs to compare the posture of 30 subjects with shoulder overuse injuries to 30 asymptomatic subjects and found no relationship between thoracic posture and pain.59 In a series of studies on a large group of Australian 14-year olds, Straker et al.


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also looked at the relationship between posture and neck/shoulder pain, using standardized photographic measurements and questionnaires.56–58 They found differences in pain (greater prevalence in females) and sitting posture based on gender; however, there was no association between sitting posture and neck/ shoulder pain.56–58 Using plumb line observations and pain questionnaires, Griegel-Morris et al. did find a correlation between increased kyphosis and interscapular pain; however, there was no relationship with shoulder pain. Their analysis also revealed no relationship between the severity of postural abnormality and the severity and frequency of pain.60 Bullock et al. found a reduction in shoulder range of motion (ROM) (17.67u) when subjects with SIS sat in a slouched position, but there was no change in pain intensity between an upright and slouched posture.55 The change in ROM seen between a slouched and erect posture is not surprising based on the findings discussed above that thoracic extension contributes to arm elevation. These studies question the relationship between SIS and posture; however, the subjects in the Greenfield et al.,59 Griegel-Morris et al.,60 and Straker et al.56–58 studies were broadly classified as having shoulder pain and did not clearly have SIS and the Bullock et al.55 study did not look at self-selected resting posture. Further studies examining the effects of resting self-selected thoracic posture in subjects with a clear diagnosis of SIS are needed.

Cervical Spine It has been suggested that a forward head positioning (FHP) of the cervical spine, usually described as lower cervical flexion and upper cervical extension, changes the relative length and activity of muscles, which attach to both the cervical spine and shoulder girdle.62 These changes in the muscle are thought to influence scapular position and mechanics, contributing to the development of SIS. Several studies, looking at symptomatic subjects, have failed to find a connection between FHP and SIS.50,56,57,59,60 For example, Lewis et al. used photographs of 120 subjects, 60 asymptomatic, and 60 diagnosed with SIS, to determine if FHP was associated with thoracic posture, scapular position, glenohumeral elevation, and pain.50 In agreement with previously discussed postural studies,40,57–60 Lewis found large postural variations with no distinct patterns and no association between FHP and SIS. Studies performed by Ludewig and Cook62 and Thigpen et al.63 do provide support for a connection between cervical position and scapular kinematics in asymptomatic subjects. Thigpen found that subjects with a forward head and rounded shoulder posture had altered scapular kinematics. Some of these changes

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were similar to those seen in patients with shoulder pain (increased anterior tipping);13,41,42 others were not consistent (increased scapular upward rotation).13,14,41 Ludewig and Cook examined the effects of varying cervical flexion angles (0u, 25u, and 50u) on scapular orientation and muscle activation at three different arm positions (0u, 90u, and 140u of elevation in the scapular plane) in 26 asymptomatic subjects.62 They found that mean scapular UR resting posture reduced by 1.4u when cervical flexion was increased from 0u to 50u. The mean difference in scapular tipping angle increased at higher levels of elevation when greater cervical flexion was present. These differences in scapular motion, although small, do support the notion that cervical position can affect the mechanics of the scapula. They tested cervical forward flexion however, which may affect the position of the scapula differently than FHP. As with previously discussed studies looking at asymptomatic subjects, these studies establish underlying plausibility for a connection between SIS and posture; however, as described above, this relationship has not been seen in symptomatic subjects.

Treatment Despite the lack of evidence for a connection between posture and SIS, postural faults are often a focus of conservative management.32,34 These conservative interventions can be divided into two broad categories. The first category is made up of interventions, which directly target posture with education, manual feedback, and external devices such as braces, taping, and workplace design. The second broad category consists of exercises, which focus on postural muscles. In this section, we will examine the evidence for interventions, which fall into these broad categories. Interventions focused on directly changing the posture of patients with SIS can be further divided into active and passive treatments. Passive treatments include bracing, taping, and passive positioning, while active interventions often focus on education and cuing. An extensive review of the literature produced only one study that specifically looked at the effectiveness of passive postural correction treatments in patients with SIS. In this study, Lewis et al. examined shoulder flexion and scapular plane abduction in 60 subjects with SIS and 60 asymptomatic subjects.64 Subjects performed shoulder elevation with no tape, with placebo taping, with tape aimed at increasing thoracic extension and increasing scapular retraction, and with depression posterior tilt. Significant increases in pain-free shoulder flexion (16.2u) and abduction (14.7u) ROM occurred in the subjects with SIS when the postural taping was performed. There was no change, however, in the overall intensity of pain felt with shoulder elevation. This study does provide support for the hypothesis that postural correction can have a positive impact in

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patients with SIS. It was difficult, however, to determine from the description of the testing procedure if the increased ROM was derived from the glenohumeral joint or from trunk motion. Shoulder elevation was measured using an inclinometer at the deltoid insertion, and it was not clear if attempts were made to differentiate between glenohumeral, scapular, or trunk contributions to the total arm elevation. Therefore, it is possible that the improved ROM with the tape was primarily a product of increased extension through the thoracic spine,54,55 lumbar spine, or hips. Active correction of static posture is often attempted through educating and cueing patients to maintain greater thoracic extension, retracted scapula, and a retracted cervical spine. As with passive correction of posture, there are very few studies examining the effectiveness of this intervention category. In 2007, Bernaards et al. had office workers with upper extremity and cervical pain attend six educational sessions, which included postural education.65 They found a significant reduction in pain, and no reduction in work disability with the education group. It was not clear if any of the subjects had SIS, and it is possible that other aspects of the education (workplace adjustments, breaks, and work demand coping strategies) were the cause of the improvement in pain scores at the 12 months follow-up. Two other studies examined the effects of active postural correction on the electromyographic activity of the trapezius muscles in patients with neck pain66 and in asymptomatic subjects with altered scapular resting position.67 It was found that active postural correction resulted in increased trapezius (upper, middle, and lower) activity when asymptomatic subjects performed two exercises,67 and it normalized middle trapezium and lower trapezium activity in patients with neck pain during a typing task.66 These studies support the notion that active postural correction can have an effect on muscles of the shoulder girdle. It is unclear, however, if these changes would occur in patients with SIS and if these alterations in EMG activity are lasting and clinically meaningful. The literature examining the effects of exercise that may indirectly impact posture in patients with SIS is much more abundant.19,20,68–73 It is proposed that exercises which strengthen and stretch specific muscles can correct imbalances that lead to faulty posture and mechanics.74,75 A muscle that is used more than its antagonist will theoretically become stronger and shorter and the antagonist will become lengthened and weakened. This imbalance could then lead to changes in posture that, as stated previously, are believed to be associated with musculoskeletal disorders. Based on this hypothesis,74 exercises that strengthen weak and elongated muscles and stretch


shortened muscles are often used in hopes of changing posture. Several studies have been performed on patients with SIS that look at the effectiveness of exercises that target muscles commonly associated with the aforementioned theory of muscle imbalances. This literature includes several systematic reviews, which have examined conservative treatment for SIS, including exercise targeting postural muscles.19,20,68–73,76 These studies provide support, although limited, for the effectiveness of exercise in patients with SIS.20,68,71–73 They were not officially included in the article count for this review because although they included exercises that may impact posture, none of these articles specifically looked at the impact of these exercises on posture. As a result, it is not clear if posture changed in the subjects, and the mechanism behind any improvement seen after performing these exercises remains unknown. Positive outcomes could be a result of numerous factors, including biomechanical changes after exercise, general pain modulation effects of exercise, placebo, postural changes, or a combination of several factors. Although no articles were found that directly assess the relationship between postural changes and outcomes in subjects with SIS treated with strengthening and stretching exercises, a few studies were identified that explore the ability of these exercises to change posture.77–79 For example, Kluemper et al. found that a 6-week program of anterior shoulder muscle stretching and posterior shoulder muscle strengthening resulted in reduced forward shoulder position in competitive swimmers.78 Similar results were found in two other studies, which examined the effects of exercise and/or stretching on the posture of pain-free volunteers.77,79 Although these studies performed by Harman et al.77 and Roddey et al.79 did find changes in posture, only one out of six postural measures improved over the control group in the Harman study, and only subjects classified as having a moderate postural abnormality in the Roddey study showed improvement. In all three of these studies, it is unclear if the postural changes found were clinically meaningful, or if these postural changes were long lasting. In contrast, Wang et al. found no changes in resting scapular posture in 20 asymptomatic subjects with a forward shoulder posture who performed stretching and strengthening exercises three times a week for 6 weeks.80 It is difficult to interpret the validity of these results because it was not clear if exercise compliance was tracked and there was no control group. Based on the limited number of studies and methodology issues, it is difficult to determine if strengthening and stretching exercises can have a meaningful impact on posture.


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Discussion It is commonly believed that the static resting position of the scapula, thoracic spine, and neck are important aspects of SIS. The acceptance of this hypothesis is understandable based on the logical biomechanical arguments used to support the idea. SIS is traditionally thought to be a mechanical disorder, which involves pinching of subacromial tissue and changes in scapular biomechanics. Therefore, factors that could lead to increased pinching or changes in biomechanics are thought to have the potential to influence the disorder. For example, it has been found that an increased kyphotic posture81 and scapular protraction37 can both lead to reduced subacromial space. As a result, it is reasonable to hypothesize that postures such as these could play a role in SIS. It has also been shown that posture can have an effect on shoulder girdle muscles such as the upper trapezius, lower trapezius, and serratus anterior.62,63,82–85 Changes in activation of these muscles could influence the biomechanics of the scapula and shoulder contributing to SIS. Other studies, however, question the relevance of these postural and EMG changes. Several studies have found no correlation between posture and subacromial space,86–88 and a recent review highlighted the fact that muscular changes in subjects with SIS are complicated and studies have had mixed results.89 Several authors have questioned the hypothesis that SIS is a mechanical disorder. They point to the fact that conditions such as rotator cuff tendinosis and tears, which fall under the umbrella of SIS, are often not associated with abnormal pinching.90,91 They also highlight that contact between the subacromial structures and the acromion occurs in normal shoulders92,93 and is unlikely to cause dysfunction unless other factors are present.94 These arguments provide rationale for the possibility that posture and SIS are not connected. The goal of this review was to examine the literature, which explores the potential relationship between posture and SIS. This review demonstrates that the importance of static posture in the development and treatment of SIS has not been confirmed by the current literature. Of the nine identified studies that examined self-selected posture in patients with SIS, none showed a correlation between posture and SIS.12–14,41,42,49,50,61,95 Several studies that included subjects without a clear diagnosis of SIS, asymptomatic subjects and/or non-self-selected resting postures were also examined for this review. Although they did not directly address the relationship between SIS and posture, they were able to provide insight into the plausibility of a relationship. Several of these studies had mixed results; however, generally five supported no correlation51,56–58,60 and nine provided

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support for the plausibility of a connection between SIS and posture.39,44,52–55,59,62,63 Of these studies, five looked at subjects with shoulder pain but did not delineate between SIS and other conditions. Of these five articles looking at posture in subjects with shoulder pain, two supported a connection between shoulder pain and posture57,59 and three found no connection. Only one study was found that looked directly at the effects of postural correction on patients with SIS.64 This showed a positive response for ROM but no change in pain. Two other studies found positive results with interventions aimed at posture; however, neither study clearly looked at subjects with SIS65,66 and one included interventions other than postural correction.65 Three studies that examined the effects of interventions on resting posture in asymptomatic subjects found short-term postural changes,77–79 while a fourth found no significant changes in scapular posture.80 Despite the consistent results of the reviewed studies that examined posture in patients with SIS, several limitations in those studies reduce the strength of the conclusions, which can be drawn from them. First, it is important to note that SIS is a complex syndrome and difficult to study. The diagnosis of SIS is poorly defined and is thought to represent a heterogeneous group of disorders.91 The studies examined in this review used varying criteria for diagnosing SIS, did not attempt to sub-classify patients with SIS, and did not control for numerous potential confounding variables. For example, five of the included studies did not report functional status or pain levels,12,41,49,59,95 only three of the studies discussed the potential mechanism of injury,13,49,59 and detailed information regarding the duration of symptoms was provided in just three of the articles.12,14,95 It was also difficult to determine the activity level of subjects in the majority of the articles.12,14,41,42,49,50,59,95 It is possible that using specific variables, such as the ones mentioned previously, to reduce the heterogeneity of the diagnosis could have identified a sub-group of patients with SIS who have a postural component to their condition. Obtaining an accurate, reliable, and relevant measurement of posture is also very challenging. There is no accepted gold standard measurement tool for identifying posture and the minimal clinical important difference for postural measures is unknown. Factors such as postural sway, ability to relax, palpation errors, and measurement errors can all have an impact on the measurement of posture. There are also components of posture that span several regions of the body, making it difficult to accurately measure all aspects of posture. Although the studies covered in the review looked at many different aspects of posture

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using a variety of methods, it is possible that important postural aspects were missed. It is also concerning that only one of the nine studies reported that testers were blinded to the diagnosis of the subjects.12 This apparent lack of blinding increases the chances of bias in the measurements. The significant variability in postural measures between subjects is also a risk to the validity of the studies used in this review. These higher levels of variance increase the risk of making type two errors. This variability combined with low sample sizes (12050–2049 subjects) and a lack of power analysis in seven out of nine studies12,14,41,42,49,61,95 suggests that several of these studies could be underpowered. It is possible that a connection between posture and SIS was missed due to these limitations. When interpreting the results of this review, it is also important to consider the methodological limitations of this paper. One of the most significant limitations is that this article is not a systematic review. As a result, there were broad inclusion and exclusion criteria and a systematic means of article appraisal was not used. There was also no attempt made to pool the data and perform a meta-analysis. It is also possible that information contained in unpublished data, non-English articles and/or articles which were not identified due to search strategy limitations, could influence the results of this review. To clarify this important clinical question, further studies specifically designed to assess the relationship between static resting posture and SIS are needed. These studies should be adequately powered, include blinding and use reliable and valid methods of assessing posture. Attention should also be given to specific patient variables and pathoanatomical factors that could assist in more detailed classification of patients with SIS. Factors such as symptom duration, presence of tendonopathy or cuff tears, morphology of acromion, type of onset, degree of pain and disability, activity level, and degree of postural abnormality may assist in identifying a sub-group of patients with SIS who have a postural component to their condition. There is also a need for intervention studies that specifically look at the short- and long-term effects of postural interventions on resting posture, pain, and function.

Conclusion In conclusion, this review has highlighted the large variance in normal posture among the general population and the current lack of strong evidence to support an association between posture and SIS. Although posture is routinely considered in the management of patients with SIS, the evidence examining this association is plagued by poorly


defined diagnostic criteria, wide sample variation, and poor statistical power. Clinicians should be mindful of the current evidence pointed out in this review when considering using posture as a diagnostic and interventional strategy for patients with SIS.

Disclaimer Statements Contributors I would like to thank Chris Neville, Michelle Dolphin, and Gary Brooks for providing editing and content review. Funding No financial support was received for the development of this submission. Conflicts of interest The author has no affiliation or involvement with any organization that has a financial interest in the results of this manuscript. Ethics approval This article is a literature review and did not involve testing on human or nonhuman subjects.

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