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Comparing Upper and Lower Serratus Anterior Activation in Different Manual Muscle Testing Positions

Tuesday, June 6, 2023 - 6 Likes

Brent Brookbush

Brent Brookbush

DPT, PT, MS, CPT, HMS, IMT

Research Review: Upper and Lower Portions of Serratus Anterior Have Different Activity Patterns

By Nicholas Rolnick SPT, MS, CSCS

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

Original Citation: Ekstrom RA, Bifulco KM, Lopau CJ, et al. (2004). Comparing the function of the upper and lower parts of serratus anterior muscle using surface electromyography. JOSPT. 34(5): 235-243. - ARTICLE

By Anatomography - en:Anatomography , https://commons.wikimedia.org/w/index.php?curid=22208655; Serratus Anterior - notice the approximate 10 digitations that originate from the ribs and insert onto the medial border of the scapula and inferior angle.

Why is this relevant?: The serratus anterior (SA) is an upward rotator and scapular protractor whose role is essential for full (180°) shoulder flexion. Due to SA 's broad origin on the ribs, its insertion along the entire anterior medial border of the scapula, and distinct groupings of muscle fibers, it has been hypothesized that the amount of activity of each these groupings may vary depending on action and scapular position. Due to the importance of SA in arm elevation, human movement professionals routinely examine this muscle for weakness. The authors of this study used surface EMG to examine the activity of each of these groupings under different manual muscle testing positions to determine the best position for maximal activation.

Study Summary

Study Design Prospective Single-Group Repeated-Measures Design
Level of Evidence Level II-2
Subject Demographics
    • Age: 24.6 years old (range, 22-33)
    • Gender: 11 males, 18 females
    • Methodology:
      • Cadaver Dissection to aid in determining Electrode Placement
        • SA was dissected on 2 cadavers to determine best locations for electrode placement
        • Upper part of SA - slips from ribs 1-4
        • Lower part of SA - slips below rib 4 that insert into the inferior angle

      • EMG Data Collection on Subjects
        • Electrode Placement
          • Upper part - Two electrodes placed over the third rib at the midpoint between anterior border of the latissimus dorsi (LD) and posterior border of pectoralis major (PM)
            • Compression of the skin and subcutaneous tissue was given to ensure direct placement over the third rib to eliminate/reduce crosstalk from nearby muscles (LD, PM)

          • Lower part - Two electrodes placed over the seventh rib; one posterior and one anterior to the midaxillary line
            • Compression of the skin and subcutaneous tissue was given to ensure direct placement over the seventh rib to eliminate/reduce crosstalk from nearby muscles (LD)

          • Reference electrode - One electrode placed over the spinous process of C7

        • Testing Procedure
          • Subjects were positioned in the various testing positions; position selection was determined by random drawing
          • Resistance was gradually applied until a maximum contraction of the muscle was obtained with the exception of the push-up plus test (which was performed without external applied resistance)
          • Each test was performed three times with a thirty second rest period between repetitions
          • EMG values for the upper and lower parts of SA were normalized, as a percentage of the highest maximum voluntary isometric contraction (MVIC) from the nine tests performed for each subject.  These tests were then compared as percentages of the MVIC.

        • Test Positions (direction of resistance) (position tested)
          • Push-up plus position (performed on table)
          • Shoulder flexed, adducted, externally rotated (performed in sitting)
          • Shoulder abducted to 125° in the plane of the scapula (performed in sitting)
          • Shoulder flexed to 125° and protracted (performed in sitting)
          • Shoulder abducted to 90° (adduction)  (performed in sitting)
          • Shoulder flexed to 125°  (performed in sitting)
          • Scapula protracted at 90° of shoulder flexion (performed in supine)
          • Shoulder horizontally adducted to end range and scapula protracted (performed in supine)
          • Shoulder horizontally adducted 45° and scapula protracted (performed in supine)

        • Test-retest Reliability
          • 10 subjects were re-tested using the same protocol after a fifteen minute rest period without removal of the original electrode placements

      • Statistical Analysis
        • Intraclass correlation coefficient (ICC) used to determine same-day test-retest reliability.

      • Inclusion Criteria: N/A
      • Exclusion Criteria: Shoulder problems (tendonitis, adhesive capsulitis, instability, impingement)

Outcome Measures
  •  EMG Activation of the Upper and Lower SA as a Percentage of MVIC, ICC Values for test-retest reliability
Results
Upper SALower SA
%MVICICC%MVICICCP
Push-up plus position78 +/- 24.9672 +/- 17.950.12
Shoulder flexed, adducted, externally rotated68 +/- 21.9773 +/-26.970.43
Shoulder abducted to 125° in plane of scapula65 +/- 23.9781 +/- 16.860.01*
Shoulder flexed to 125° and protracted61 +/- 27.8069 +/- 19.940.10
Shoulder abducted to 90°60 +/- 26.8572 +/- 24.870.01
Shoulder flexed to 125°60 +/- 22.8683 +/- 17.900.001
Scapula protracted at 90° of shoulder flexion54 +/- 27.8847 +/- 29.910.20
Shoulder horizontally adducted to end range and scapula protracted40 +/- 21.8032 +/- 18.820.12
Shoulder horizontally adducted 45° and scapula protracted39 +/- 20.8736 +/- 24.880.39

Findings of note:

  • Shoulder abducted to 125° in the plane of the scapula, shoulder abducted to 90°, and shoulder flexed to 125° showed significantly greater activation in the lower part of SA than the upper part of SA (p < 0.05)

Cadaver Results:

  • Each SA dissected had 10 slips from the ribs
  • Upper part of SA : Superior 4 slips formed a continuous sheet of muscle which inserted onto the medial border of the scapula
  • Lower part of SA : Consisted of the remaining inferior slips that inserted onto the inferior angle of the scapula

Conclusions

  • The upper part of SA is best suited for scapular protraction, whereas the lower part of SA is more suited for upward rotation.

Conclusions of the Researchers

  • Upward rotation facilitates lower SA activity to a greater degree than upper SA activity. Muscle tests with scapular protraction increased upper SA activity but never to a degree that was significantly greater than the lower SA .
  • Muscle testing positions can be used to emphasize different portions of the SA based on relative scapular protraction/upward rotation.

Cadaver image of the serratus anterior&#39;s origin
Caption: Cadaver image of the serratus anterior&#39;s origin

Cadaver image of the serratus anterior's origin

Review & Commentary: The current study employed a simple methodology using surface electrodes on the upper and lower parts of serratus anterior (SA) to determine manual muscle testing (MMT) positions that elicited highest the activation of each segment. The strength of the study lie in its simplistic design and pre-study cadaver dissection to determine optimal placement for the electrodes. The authors sought to determine if commonly used MMT positions designed to test the SA actually elicit high levels of activation, and further, if the proposed functional components of the SA differ in their activity based on the testing position. The authors used common SA MMT testing positions based on the available literature, and by randomizing test positions and providing adequate rest to the participants between trials, performed an experimental protocol that minimized chance of fatigue influencing results. The cadaver dissection served to isolate the particular fiber bundles of interest and allowed for greater accuracy in electrode placement. The authors minimized crosstalk of latissimus dorsi and pectoralis major by manual placement and pressure of the surface electrodes into the third rib. The authors reported that the pressure over the upper electrode increased upper SA activity recording by 60%. Further, the authors conducted a reliability study on 10 of the subjects to ensure reproducible results. Finally, the authors used varying positions for their tests, including on all fours (with the prone push-up with a plus), supine (for fully protracted and 45° horizontally adducted, and two others), and seated (for shoulder flexed to 125° and four others), allowing comparisons to be made.

There were inherent weaknesses in the study design that must be taken into consideration before results are applied to practice. First, the methodology calculated maximum voluntary isometric contraction (MVIC) for each of the parts of SA by using the testing position that elicited maximum activation. That is, for each subject, the authors used the testing position that generated maximum activity and used that to normalize EMG data; all other tests were a percentage of that testing position. Of note, 6 of the 9 testing positions elicited different MVIC values from the subjects, indicating substantial variability. The authors hypothesized that the variance could be due to anatomical differences (some individuals only have 7 muscle slips versus 10 muscle slips, possibly leading to greater efficiency in performing work) and the SA 's shared role in upward rotation with the upper trapezius . Placement of the electrodes on the subjects required a second individual which could limit its applicability to other settings. Finally, the study was performed on healthy individuals, so caution is warranted before applying to injured populations.

Why is this study important?

The current study supports the notion that the SA consists of at least two distinct sets of muscle fibers, that may activate at different intensities depending position, and whether upward rotation or scapular protraction is the desired motion. Specifically, the study suggests that the upper part of SA is more active during scapular protraction while the lower part is more active during upward rotation. It should be noted that these differences are small, as activation levels of the upper part of SA never increased to levels significantly different than the lower part of the SA .

How does it affect practice?

Although future research is warranted, protraction of the scapula may be a better position for activating and/or testing the lower fibers (push-up plus elicited the highest activity). Whereas as, upward rotation may be advised for greater activation and/or testing of the lower fiber, this includes resistance to shoulder abduction to 125° in the plane of the scapula (65% and 81%), shoulder abducted to 90° (60% and 72%), and shoulder flexed to 125° (60% and 83%). Testing positions that stressed both upper and lower parts equally include the push-up plus position (78% and 72% upper and lower SA , respectively) and the shoulder flexed, adducted, and externally rotated (68% and 73%) (protraction through elbows); keep in mind, that these positions may also recruit the pectoralis minor .

Based on common dysfunctions of the upper body (loss of optimal upward rotation) and the results of this study, we may presume that weakness/inhibition upper fibers of the SA is more common than weakness/inhibition of the lower fibers of the SA .

How does it relate to Brookbush Institute Content?

The Brookbush Institute predictive model of Upper Body Dysfunction (UBD) , proposes that the scapula commonly exhibits excessive downward rotation, anterior tipping and internal rotation, implicating the serratus anterior as long and under-active. On the overhead squat assessment this is correlated with the signs "Arms Fall " and "Shoulders Elevate ". Due to the vital role this muscle plays in providing axio-scapulo-humeral stability and its propensity for under-activity, it is recommended that serratus anterior activation is added to rehabilitation, fitness and performance programs for those who exhibit signs of UBD , and potentially as part of an integrated warm-up before all moderate to high intensity upper body activity. For those exhibiting UBD , it is also recommended that commonly overactive synergists are released (pectoralis minor and subscapularis ), and stiff joints are mobilized (thoracic spine, shoulder ), and that further activation work may be necessary for optimal upper body motion to be restored. Specifically, activation exercise for the external rotators should proceed serratus anterior isolated activation , and Integration of appropriate lower/upper trapezius activation should follow to facilitate normal scapulohumeral rhythm and restoration of length/tension relationships of the scapular musculature.

Due to the proposed loss of upward rotation and synergistic dominance of the pectoralis minor in those exhibiting UBD , the Brookbush Institute uses a variation serratus anterior manual muscle testing (SAMMT) that resembles the test used in this study, "shoulder flexed to 125° and protracted". This test generated approximately 61% MVIC for the lower serratus anterior and and 69% MVIC for the upper part of the serratus anterior . This may imply that weakness/inhibition of the upper fibers of the serratus anterior are affected to a greater degree than the lower fibers in those exhibiting UBD . The SAMMT used by the Brookbush Institute varies from the research position, as the client/patient in tested in various degrees of shoulder flexion, from 120° to 180°, while looking for weakness and/or compensations (anterior tipping and/or elevation of the scapula) which may suggest synergistic dominance of the pectoralis minor , levator scapulae and upper trapezius . SAMMT results may influence intervention selection as prescribed above.

The following videos include the serratus anterior MMT test, isolated activation exercises for serratus anterior, as well as releases for commonly overactive muscles inhibiting proper serratus activity.

Brookbush Institute Videos

Serratus Anterior MMT for An Active Population

Serratus Anterior Isolated Activation:

Serratus Anterior Isolated Activation Progressions (Sahrmann Activation with Brookbush/Fluegel Modifications):

Pectoralis Minor SA Static Release:

Subscapularis SA Static Release:

© 2016 Brent Brookbush

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