A combined in vivo and in vitro approach to assess supraspinatus activation and tissue responses to arm elevation demands
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Rotator cuff degeneration affects a large portion of the human population, yet knowledge surrounding which loading scenarios allow transition from healthy to diseased states remains largely unresolved. Mechanistic progression of rotator cuff pathology often originates in the supraspinatus before cascading to other tissues, leading to substantial degeneration. Posture, loading and repetitive motions are known risk factors that exacerbate shoulder injury progression. This suggests a causal relationship between specific upper extremity task scenarios and degenerative rotator cuff loading. This thesis intentionally explored regional activations of the supraspinatus and accompanying tendon loading across a range of postures. The global objective was to evaluate how postural and task intensity differences alter tissue-level mechanical parameters in both in vivo muscular activation and in vitro tangent stiffness, hysteresis and optical stretch ratios. These findings combine in vivo muscular activation and physiologically relevant in vitro mechanical testing results through novel methods to better understand supraspinatus loading. Three experimental studies provided the means to achieve this global objective. In Vivo Examination of Supraspinatus Activation: The purposes of this study were 1) to document the interplay of anterior and posterior supraspinatus activations and 2) to describe the influences of posture and hand loads on anterior and posterior supraspinatus activations. Forty participants completed arm elevations in seven planes of elevation with three hand loads that were normalized to the individual’s maximal elevation force. Indwelling electromyography was collected from the anterior and posterior regions of supraspinatus. Hand load and elevation angle interacted to affect the anterior region activation in most planes of elevation by up to 41% of maximal activation, but these changes were less influential for the posterior portion. Activation patterns between the two regions suggest different functional roles of the supraspinatus portions; consistent levels of activation in the posterior supraspinatus may indicate this region is primarily a glenohumeral stabilizer, while the larger anterior region acts to achieve glenohumeral motion. This work represents the most comprehensive concurrent evaluation of these supraspinatus regions over a large set of planes of elevation, hand loads and humeral elevations, providing more holistic descriptions of supraspinatus activation in a critical arm movement. Comparing Surface Electromyography of Supraspinatus to Anterior and Posterior Indwelling Recordings: The purpose of this study was to compare anterior and posterior supraspinatus indwelling electromyography responses to a surface supraspinatus signal across a range of arm postures in order to develop relationships between these two recording methods. Forty participants completed arm elevations with altering hand loads and planes of elevation at a fixed cadence. Indwelling electromyography of the anterior and posterior supraspinatus as well as a surface recording of supraspinatus were collected. Bivariate regressions of anterior and posterior indwelling electrodes relative to the bipolar surface electrodes were used to determine relationships between these signals throughout the range of these humeral elevations. Differences between these predictions were modulated by plane of elevation, elevation angle, load intensity and sex of the participant, but no interactions existed. Surface signals underestimated indwelling activation recordings at low elevation angles, then overestimated as humeral elevation angle increased. Surface recordings underestimated indwelling signals by up to 15% in unloaded conditions, while overestimating the posterior region by up to 17% at the highest hand load intensity. In addition, surface signals overestimated posterior supraspinatus indwelling activity by 21%. This work greatly expands current knowledge surrounding relationships between these indwelling and surface signals, both in the inclusion of the indwelling posterior supraspinatus recordings and the expansion of arm postures examined. These findings indicate that relationships between the surface and indwelling signals are altered by plane of elevation, load and elevation angle, and the surface signal more closely predicts anterior region activity. Examining Changes of In Vitro Supraspinatus Mechanical Properties in a Rat Model: The purposes of this study were 1) to complete in vitro mechanical tissue testing in scenarios emulative of empirical muscular activation and postural conditions in an animal model, and 2) to determine the relative influences of arm posture and external loading levels on tissue responses. Forty-eight shoulders harvested from Sprague-Dawley rats were affixed into custom 3D printed mounting pots and placed into one of eight testing groups combining glenohumeral posture and load magnitude. Orientations represented four different postures observed in vivo, and applied tensile load within the animal model was scaled from human activation of the two supraspinatus regions collected from in vivo research for 1500 cycles. A three-way interaction between elevation angle, load magnitude and cycle number occurred for tangent stiffness within specific cycles, with increasing angles, loads and cycles increasing stiffness by up to 49% in some scenarios; differences in maximum and minimum displacement indicated elevated tissue responses in higher elevation angles. Interactions between elevation angle, load intensity and cycle number altered stretch ratio characteristics, with increased elevation angles, loads and cycles increasing stretch ratios, as well as differentiating articular and bursal side responses. Complex interactions between angle, load and cycle number suggest higher abduction angles, increased load magnitude and subsequent cycles generated increased tendon response characteristics. Novel thesis contributions: Multiple novel findings and contributions originated from this work. This dissertation has combined in vivo and in vitro methodologies to advance understanding of rotator cuff mechanics. This dissertation supports the notion that supraspinatus loading varies throughout the range of motion, and postural and external load variations alter tissue-level supraspinatus responses. Activations of the anterior and posterior regions of the supraspinatus were collected from the largest collection of postures to date and described activation differences between these regions. These EMG activations were used to assist in determining applied force load levels for mechanical testing, representing the first known attempt to generate force-controlled tensile loading using physiologically derived exposure levels for the supraspinatus. This work is also the first to maintain a functional glenohumeral unit to complete mechanical testing using postures representative of those observed in vivo to examine supraspinatus responses. General conclusions: Posture and load magnitude have distinct and noteworthy effects on supraspinatus, both in muscular activation and tendon responses. This research combined in vivo muscular activation with in vitro mechanical tissue testing to generate novel findings for rotator cuff loading; further work should continue to pair in vivo responses with mechanical tendon loading to generate physiologically relevant research scenarios throughout the range of humeral postures. This work has established that the supraspinatus is sensitive to scenario conditions, but continued expansion of our understanding of exposure aspects would help diagnose or anticipate overexposure.
Cite this version of the work
Alan Christian Cudlip (2020). A combined in vivo and in vitro approach to assess supraspinatus activation and tissue responses to arm elevation demands. UWSpace. http://hdl.handle.net/10012/15529