Patterns Of Scapular Motion And Serratus Anterior Muscle Activity During Different Elevation Strategies
Ebaugh D, Kelley M

Introduction/Purpose:  Scapula dyskinesia is observed in patients with shoulder pathology and impairment of the serratus anterior muscle is believed to be a contributing factor.  We have noticed that scapula dyskinesia in patients with glenohumeral multidirectional instability (MDI) is influenced by how the arm is raised, and we believe this is in part due to different activation levels of the serratus anterior muscle.  The purpose of this pilot study was to investigate serratus anterior muscle activity levels and scapular upward rotation patterns during different arm elevation strategies.

Subjects:  Four subjects (3 male, 1 female, average age = 29.2) with MDI volunteered to participate.  Comparison data were obtained from 17 healthy subjects (9 male, 8 female, average age = 22.5) who participated in a separate study.

Methods:  The Polhemus 3Space Fastrak was used to collect 3D scapular kinematics, and surface electrodes were used to record the electromyographic (EMG) activity from the lower serratus anterior muscle during three different strategies of arm elevation.  For all three strategies subjects were asked to raise their arm (sagittal plane) as high as they could.  In the first strategy subjects were asked to raise their arm naturally.  For the second strategy subjects were asked to externally rotate their arm and then raise their arm.  For the third strategy, subjects were asked to externally rotate their arm and protract/elevate their shoulder, then raise their arm.

Findings:  For all strategies MDI subjects demonstrated less upward rotation when compared to healthy subjects with the exception of the beginning part of the motion for strategy three.  For the first strategy, upward rotation differences at 30°, 90°, and 115°,  of elevation were 8.6°, 23.8°,  and 24.0°,   respectively.  Upward rotation differences for strategies 2 and 3 were less than those noted in strategy one with the maximum difference being 8.1° .  Throughout the first strategy serratus anterior activation levels were lower for MDI subjects when compared to healthy subjects.  These differences ranged from 4.7% to 15.3%.  For the second and third strategies, serratus anterior activation levels for the MDI subjects exceeded those of healthy subjects ranging from 0.1% to 33.7%.

Conclusion/Clinical Relevance:  It appears that external rotation of the arm or a combination of shoulder protraction/elevation and external arm rotation result in increased amounts of scapular upward rotation and serratus anterior muscle activity during sagittal plane arm elevation.  This information may be useful for assessing and treating serratus anterior muscle dysfunction.  We plan to investigate this further with a larger scale study in a population of subjects with a variety of shoulder pathologies.