Muscles within muscles: anatomical and functional segmentation of selected shoulder joint musculature

The objectives of this study were two-fold. Firstly we wished to investigate the segmental anatomy, geometry and gross innervation patterns of selected shoulder joint musculature (pectoralis major, latissimus dorsi and deltoid) to establish if these muscles could be anatomically subdivided according to predetermined criteria. We then wished to use this information to help determine if a segments activation pattern was reliant on an efficient line of action and a large moment arm for the intended movement. Surface electromyography was used for this purpose whilst manipulating the independent variable of contraction intensity (% MVC) during two shoulder joint flexion tasks. For the anatomical portion of the study the dissection of ten cadaveric shoulders revealed the deltoid to consist of seven segments whilst the pectoralis major and the latissimus dorsi were both ascribed six segments according to predetermined anatomical/functional criteria. Primary nerve branching was evident in each muscle but no apparent relationship existed between the designated anatomical segments and the primary nerve branches. Differences were evident in the geometry of each segment in regard to moment arms and orientations of each segments line of action. The functional portion of this study, which utilized miniature bipolar surface electrodes during the performance of a static shoulder joint flexion task at 75% and 25% MVC, showed significant differences (p<0.05) in the activation patterns of active segments between the two tasks. Specifically, a "drop out" of segments with smaller flexion moment arms and more diverging lines of action in comparison to the movement plane was evident for all subjects in the 25% MVC task in comparison to the 75% MVC task.