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Tuesday, June 6, 2023

Altered Scapular Kinematics and Muscle Recruitment in Overhead Workers with Impingement Syndrome

Brent Brookbush

Brent Brookbush


Research Review: Altered scapular kinematics and muscle recruitment in overhead workers with impingement syndrome

By Stefanie DiCarrado DPT, PT, NASM CPT, CES, PES

Edited by Brent Brookbush DPT, PT, COMT, MS, PES, CES, CSCS, ACSM H/FS

Original Citation: Ludewig P.M., Cook, T.M. (2000) Alterations in shoulder kinematics and associated muscle activity in people with symptoms of shoulder impingement.

Physical Therapy. 80 (3) 276-291 - ARTICLE

The synergy required for scapular upward rotation

Why is this relevant?: Understanding changes in joint motion and muscle recruitment in those with symptoms of shoulder impingement may provide crucial information for practitioners responsible for caring for these individuals. This study specifically evaluated altered scapular movement patterns and trapezius and serratus anterior activation patterns in overhead manual workers (construction workers and carpenters) with and without impingement. The findings of this study may have implications relative to examination, assessment and treatment.

Study Summary

Study Design Controlled experimental
Level of Evidence Level III: Evidence from well-designed controlled trials without randomization
Subject Demographics
  • Age (years):
    • Impingement Group (IG): 39.7 + 12.0
    • Control Group (CG): 39.9 + 13.3

  • Gender:
    • IG: 26 male
    • CG: 26 male

  • Characteristics:
    • IG:
      • Height: 1.81 + 0.06
      • Weight: 90.9 + 14.0

    • CG:
      • Height: 1.80 + 0.08
      • Weight: 85.7 + 12.7

    • Construction workers (31 sheet metal workers, 21 carpenters)
    • Resting scapular position relative to the trunk and humerus did not vary between groups

  • Inclusion Criteria:
    • IG: Shoulder pain > 1 week in proximal anterolateral shoulder region, positive impingement sign, a painful arc (60°–120°), tenderness to palpation around greater tuberosity, acromion, or rotator cuff tendons, and > 130° shoulder abduction
    • CG: No shoulder pain

  • Exclusion Criteria:
    • IG: Reproduction of symptoms during cervical screen (active and resisted range of motion, over-pressure, quadrant test), abnormal results on thoracic outlet tests, numbness or tingling in the upper extremity, or onset due to traumatic injury, joint dislocation (glenohumeral (GH) or acromioclavicular (AC)), history of shoulder surgery
    • CG:  < 1 year employment involving overhead work, less than 150 degrees of shoulder flexion or abduction, less than normal medial/lateral (GH) rotation, history of pain, trauma, or joint dislocation (GH or AC joints)

Outcome Measures
  • CG: general pattern of scapular upward rotation and posterior tipping with glenohumeral external rotation
    • 8% demonstrated decreased posterior tipping

  • IG: general pattern of upward rotation with decreased posterior tipping
    • 31% demonstrated decreased posterior tipping

  • EMG % of MVIC (IG group only)
    • SA
      • 9% decreased (averaged) for all conditions

    • UT: increased for all conditions
    • LT
      • 61°-90°: 13% increased activity
      • 91°-120°: 17% increased activity

  • 3D scapular kinematics
    • Upward rotation
      • 60° elevation: decreased in IG
      • 90° elevation: no difference between groups
      • 120° elevation: no difference between groups
      • No difference with weight

    • Anterior Tipping
      • 60° elevation: no difference between groups
      • 90° elevation: no difference between groups
      • 120° elevation: 5.8° greater in IG
      • No difference with weight between groups

    • Medial Rotation
      • No difference with elevation between groups
      • 2.3 kg: 5.2° greater in IG
      • 4.6 kg: 4.4° greater in IG

  • 3D Humeral kinematics
    • External rotation: no difference among groups for any condition

ConclusionsIndividuals with signs of subacromial impingement syndrome demonstrate decreased scapular upward rotation early through the range and increased scapular anterior tipping as elevation increased.  Coupled with those mechanics is decreased serratus anterior and increased upper trapezius and lower trapezius activity as compared to controls.  The increase in upper and lower trapezius activity may be compensatory due to decreased serratus anterior activity.
Conclusions of the Researchers It is important to address scapular kinematics and SA muscle function when creating a rehabilitation program for individuals with shoulder impingement as these muscles demonstrate dysfunction and alter normal mechanics during arm elevation.

By User:Mikael Häggström - Image:Gray409.png, Public Domain, https://commons.wikimedia.org/w/index.php?curid=2892742
Caption: By User:Mikael Häggström - Image:Gray409.png, Public Domain, https://commons.wikimedia.org/w/index.php?curid=2892742

Upward rotators right, downward rotators left. By User:Mikael Häggström - Image:Gray409.png, Public Domain, https://commons.wikimedia.org/w/index.php?curid=2892742

Review & Commentary:

This study is unique in its evaluation of arm elevation, the affect of shoulder pain and signs of impingement syndrome on scapular mechanics and muscle activity, and the use of a specific population prone to overhead activity and over-use injuries (construction workers and carpenters). The authors recreated a scenario similar to motions commonly used during work conditions by limiting elevation to 120° and using the dominant arm. Additionally they used weights that mimick the weights of tools the subjects may use throughout a typical work day.

The authors provided a detailed description of the study objectives and hypotheses along with a sufficient review of the current literature surrounding the topic of subacromial impingement syndrome (SIS). They listed previous studies that proposed altered scapular kinematics and muscle activity in those with SIS but explained that EMG evidence at the time of publication was limited and/or did not examine a specific population with SIS. One strength of this study is their specificity in subject selection, involving individuals whose occupations require overhead work. The authors created a strong methodology that assessed not only arm elevation, but the increase stress of added weight; as individuals performing over head work often hold tools overhead which may increase the demands on the shoulder and shoulder girdle.

Other strengths include standardization of testing and measurement protocols and the attention to detail throughout. The authors created phases of arm elevation and used timing to improve the interpretability of the EMG data, as both muscle length and speed of contraction can influence readings. The researchers used a metronome to ensure a similar speed of motion among subjects, and verified electrode placement by having subjects perform maximal voluntary isometric contractions (MVIC). Subjects practiced the appropriate arm motions to ensure proper speed. Randomization of loading conditions eliminated any training effect or muscle fatigue which would confound data. Subjects rested for 2-3 minutes between practice and testing conditions; resting longer if needed, further eliminating the affect muscle fatigue may have on data. To increase data validity, 5 subjects returned the following day to repeat the testing and ensure reliability of the testing protocol. The authors used a local coordinate system for kinematic analysis to minimize any skin motion artifacts picked up by the 3D motion sensors.

The amount of detail provided in the published article allows for a better understanding of the methodology and enhances reproducibility of the study. The authors clearly described any equipment used, electrode placement, settings, and even included diagrams. Maximal voluntary contraction data was collected using standard manual muscle testing positions as described by Kendal et. al. (1) and any variations were explained and cited. The authors cited previous research to validate the use of certain equipment, sensory placements, and settings.

The authors discussed the following study limitations: possibility of crosstalk with surface electrodes, influence of pain on MVIC, and lack of specific pathoanatomical analysis. Selective electrode placement helped reduce risk of crosstalk. To eliminate the effect of pain on MVIC, the researchers questioned each subject on pain levels and did not find a reason to be concerned; as only 5 subjects reported feeling mild pain or discomfort (MVICs were recorded in midrange). The researchers made no attempt to distinguish the actual tissue being damaged, as there are several anatomical structures that have been implicated as potential sites of tissue damage and pain due to SIS, and this study sought to evaluate only the mechanical component of SIS. It is worth considering that the consistent results noted in this study in individuals with SIS may indicate that the specific anatomical structure damaged is of little importance when designing an exercise program.

Last, it is worth noting that this population chose to continue working despite their shoulder pain, resulting in data that is not representative of muscle activity and kinematics that lead to SIS or an immediate result of tissue damage, but is representative of general compensation patterns developed by the individual to "work around their pain." The subjects of this study reported relatively mild symptoms and low levels of disability related to SIS. It is possible that increased pain and disability may result in different or more substantial alterations in kinematics or muscle recruitment strategies.

Why is this study important?

This study is important because it provides a comparison of scapular mechanics and muscle activation between those individuals who have symptoms of shoulder impingement syndrome (SIS) and those who do not. The group selected for this is of particular interest, as "overhead workers" (carpenters and construction workers) place a considerable amount of stress on the shoulder and shoulder girdle with arms higher than head level.

How does it affect practice?

This study revealed that those with SIS are prone to excessive anterior tipping, inadequate upward rotation, decreased activity of the serratus anterior , and a compensatory increase in trapezius activity. This information may imply that scapular motion and muscle strength should be assessed, and that techniques for addressing these changes in motion should be part on a comprehensive intervention to resolve SIS.

How does it relate to Brookbush Institute Content?

The predictive model of Upper body dysfunction (UBD) , as described by the Brookbush Institute describes similar changes in scapular mechanics and muscle activity as described in this study. This includes excessive anterior tipping and downward rotation of the scapula, decreased serratus anterior activity and increased activity of the upper trapezius . The increased activity of the lower trapezius noted in this study, differs from the Brookbush Institute's predictive model of UBD , but the increased activity may be a compensatory, rather than an indication of length change or muscle imbalance. Further consideration and research is needed.

Further, in the UBD model, the upper trapezius is a muscle that is hypothesized to be either under-active or over-active, as it will upwardly rotate and anteriorly tips the scapula. We hypothesize that over-activity may be due to serratus anterior under-activity and an increase in anterior tipper activity (levator scapulae , pectoralis minor and upper trapezius ) in an attempt to stabilize and elevate the scapula during arm elevation. This notion is supported by this study and noted by the authors in their discussion. All short/overactive muscles should be treated with release and lengthening techniques, and under-active muscles treated with isolated activation techniques. The UT , as it falls under both categories, is best addressed with release only techniques.Integrated exercises following release, mobilization, lengthening, and activation exercises will aid in optimal synergistic recruitment during functional tasks.The videos below describe and demonstrate proper release, lengthening, and activation techniques along with an integrated functional movement pattern.

Note: The Brookbush Institute's predictive model of Upper Body Dysfunction expands on these findings by considering all muscles crossing the shoulder, scapula and thoracic spine, arthrokinematic dyskinesis of all joints involved, fascial restrictions, and potential for peripheral nerve restriction.

Upper Trap SA Static Release

Pectoralis Minor SA Static Release

Levator Scapulae SA Static Release

Rhomboid SA Static Release

Levator Scapulae SA Static Stretch

Pectoralis Major and Minor SA Static Stretch

Serratus Anterior Isolated Activation

Serratus Anterior Reactive Activation


  1. Florence Peterson Kendall, Elizabeth Kendall McCreary, Patricia Geise Provance, Mary McIntyre Rodgers, William Anthony Romani_, Muscles: Testing and Function with Posture and Pain: Fifth Edition © 2005 Lippincott Williams & Wilkins_

© 2016 Brent Brookbush

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