Variable Roles of the Upper and Lower Portions of the Subscapularis

Research Review: The Variable Roles of the Upper and Lower Subscapularis During Shoulder Motion

By Erik Korzen DC, NASM-CES, Acupuncturist

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

Original Citation: Wickham, J., Pizzari, T., Balster, S., Ganderton, C., Watson, L. (2013). The variable roles of the upper and lower subscapularis during shoulder motion. Clinical Biomechanics, 29: 885-891. Full Article

Why is this relevant?: The subscapularis may play a variety of roles in shoulder motion including internal rotation, compression, inferior glide, adduction, and anterior stabilization. It has been hypothesized that the subscapularis has 2 separate sets of fibers (upper and lower), with separate neural innervations, which may imply that the 2 sets of fibers may not activate in unison. This study compared the recruitment of upper and lower divisions of the subscapularis muscle during 4 shoulder movements - abduction, flexion, internal rotation (IR) and external rotation (ER). The researchers utilized intramuscular (fine wire) electrodes in the dominant throwing arm of asymptomatic subjects. The findings of this study imply that the lower portion of the subscapularis muscle has higher activity during shoulder elevation (flexion, scaption, abduction), but not during internal rotation.

Caption: By Anatomography - en:Anatomography (setting page of this image), CC BY-SA 2.1 jp, https://commons.wikimedia.org/w/index.php?curid=22855541

By Anatomography - en:Anatomography (setting page of this image), CC BY-SA 2.1 jp, https://commons.wikimedia.org/w/index.php?curid=22855541

Study Summary

Study Design	Case Control Study
Level of Evidence	Level of Evidence VI - Evidence from a single descriptive or qualitative study
Subject Demographics	Subject Demographics - 24 healthy subjects from university population (11 females, 13 males); Mean Age: 23.6 years +/- 5.3 Exclusion Criteria - Past history of shoulder pain, injury or surgery
Outcome Measures	2 bipolar fine wire intramuscular electrodes (EMG) were inserted with Ultrasound imaging guidance into upper/lower subscapularis. An accelerometer was placed on the subjects wrist to determine the start and finish positions of the movements. The 4 shoulder movements performed were: Abduction Flexion Internal Rotation (IR) External Rotation (ER) Participants performed 10 practice repetitions of each of the 4 exercises with a 30 second rest between each repetition, reducing the effects of fatigue. The order of exercises was randomized. Abduction and Flexion were performed holding a light weight (25% of maximum voluntary isometric contraction (MVIC) at 90 degrees abduction). Abduction and flexion were performed from anatomic position. ER and IR were performed in a seated position with the upper extremity in 90 degrees of shoulder abduction and 90 degrees of elbow flexion. The subject was resisted by a dynamometer for each movement at velocity of 90 degrees/second from neutral to full ER and from full ER to neutral for IR trials. Participants also performed 6 MVICs: abduction at 90 degrees of shoulder elevation in frontal plane, flexion at 90 degrees of shoulder flexion in sagittal plane, IR at 90 degrees of shoulder elevation with neutral rotation, ER at 90 degrees of shoulder elevation with neutral rotation, horizontal flexion at 90 degrees of shoulder elevation in the frontal plane and retraction seated. Each MVIC was held for 5 seconds and a total of 3 MVICs were performed for each movement with a 3 minute rest between contractions. The MVIC for each movement was determined using the highest average intensity from a 600 millisecond window, taken from the middle of each recordings.
Results	The lower subscapularis activated at a higher overall level than the upper subscapularis during abduction, flexion and ER. However, significant differences were only observed during abduction. Extension, IR and horizontal flexion MVICs produced the greatest muscle activity in the upper and lower portions of subscapularis.
Conclusions	In the population tested, the lower subscapularis muscle had higher activity during abduction and flexion. Temporally, the lower subscapularis activated significantly earlier than the upper portion during abduction. This study demonstrates that although the subscapularis muscle is identified and discussed as 1 contiguous muscle, the differences in activation between the upper and lower portions may imply that they play varied roles relative to shoulder motion.
Conclusions of the Researchers	The results demonstrated that the function of the upper and lower portions of subscapularis differ, depending on the movement being performed. The increase in the lower subscapularis during elevation movements (abduction and flexion) may imply a role in resisting superior shearing forces of the delotid muscle. According to a previous study by Halder, et al. (1), the inferior portion of the subscapularis would facilitate humeral head depression when compared to the upper portion. Based on cadaveric studies it appears that the superior portion of the subscapularis is thicker and has a larger abduction moment arm, leading to greater abduction torque. The inferior portion of the subscapularis muscle acts on a smaller moment arm, which could enhance the dynamic stability of the glenohumeral joint. During the MVICs both portions of the subscapularis were significantly active during IR and horizontal flexion, contributing to the overall notion that subscapularis is a main mover for these motions. Finally, the lower subscapularis appears to function significantly during any movement that involves shoulder elevation.

Commentary:

There were many strong components to the methodology of this research. The authors obtained 24 healthy, young subjects for this study with a mean age of 23.6 years. The authors utilized intramuscular electrodes, instead of surface electrodes, to examine the myo-electric activity of the subscapularis . The researchers used 4 applicable shoulder movements in addition to assessing the the maximal voluntary contractions (MVIC) of common motions. Further, the use of an accelerometer provided the researchers with the ability to determine the standardize motion.

This research also had limitations. The current study had subjects that were young and healthy, which may limit transferability to an older or injured population. The placement of the intramuscular electrodes were performed using standard research method; however, varying methods for intramuscular placement in the subscapularis muscles exist. The anatomical and functional distinction of the portions of the subscapularis is complex, as studies have suggested varied organization of this muscle. Last, with the prevalence of rotator cuff injuries in the general population, future studies would benefit from including a subgroup of subjects with a diagnosed rotator cuff pathology.

Why is this study important?

This study provides evidence that the superior and inferior portions of the subscapularis muscle may function in distinctly different ways, and should be treated separately. While the subscapularis muscle is traditionally considered an internal rotator, this study demonstrates differing levels of activity in the inferior and superior portion of this large muscle depending on the motion of the shoulder. As the researchers discovered, the lower subscapularis had higher activity levels during abduction, flexion and ER when compared to the upper portion. Hypothetically, the dual innervation of the subscapularis muscle may indicate that there are in fact 2 distinct muscles located in the subscapular fossa that anatomists have traditionally grouped together. This finding implies that while anatomy may visually support this notion via cadaveric studies, the true function of muscles is more complex. This research highlights that recruitment of the lower subscapularis occurs in any movement involving shoulder elevation, and that the upper and lower portions of the subscapularis may need to be treated differently.

How does it affect practice?

While it is important to employ various exercises to recruit muscles of the shoulder complex, this study provides a link between specific movements and portions of the subscapularis muscle. While it is traditionally considered an internal rotator of the glenohumeral joint, the lines of pull during various shoulder movements results in significantly different muscle activity levels. The relatively large muscle belly, the dual innervation (upper and lower subscapularis nerves) and the findings of this study indicate that our anatomical classification of this muscle may be incorrect. Extrapolation of this information allows the movement professional to combine the information in this study with already established knowledge to train the subscapularis muscle in a much more comprehensive fashion. This includes realizing that the lower portion of the subscapularis muscle could play a role in humeral head stability, thus overall glenohumeral joint stability. A main outcome of this study is that the lower subscapularis has higher activity levels during shoulder elevation movements (abduction, flexion, scaption). This information is significant for individuals with a possible injury to the subscapularis or in an individual performing regular exercises that involve shoulder elevation. Movement professionals interested in improving shoulder complex stability should be aware of the role the lower subscapularis plays in shoulder elevation, and consider its relative strength, activity level and extensibility when incorporating shoulder elevation movements in an exercise program.

How does it relate to Brookbush Institute Content?

The compensatory pattern described in the predictive model of Upper Body Dysfunction (UBD) are attributed to alterations in motion of the thoracic spine, scapula and shoulder, and identify the subscapularis as having a propensity to become short and over-active. Further consideration will need to be given to this identification, as well as the techniques implied, to determine whether the upper and lower portions of the subscapularis should be treated with different techniques. This study does provide evidence that shoulder flexion and abduction may be affected by the lower portions of the subscapularis and that limitations in these motions, as well as, the sign "Arms Fall " during the overhead squat may imply changes in length and activity. The videos below demonstrate useful assessment procedures as well as activation and integration techniques for specific muscles.

Overhead Squat Assessment: Arms Fall Breakdown

Apley's Scratch Test

Shoulder External Rotation Goniometry:

Shoulder Flexion Goniometry:

Subscapularis Release (Self-administered):

Subscapularis (and Pec) Stretch:

Bibliography

Halder, A; Zobitz, M E; Schultz, F; An, K N. Structural properties of the subscapularis tendon. Journal of Orthopaedic Research18.5 (Sep 2000): 829-34.

Questions, comments, and criticisms are welcomed and encouraged -

Variable Roles of the Upper and Lower Portions of the Subscapularis

Research Review: The Variable Roles of the Upper and Lower Subscapularis During Shoulder Motion

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