Examination of Healthy Neuromusculoskeletal Control of the Pectoralis Major Muscle
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The pectoralis major, a large, multipennate muscle, assists in shoulder complex mobility and stability. Although its highly intricate architectural properties allow it to contribute to many upper extremity tasks, its exact role in typical shoulder function is ambiguous. Due to this, the pectoralis major is typically classified as an ‘exercise’ muscle; its functional relevance to daily and occupational tasks dismissed, and its purpose in arm movements diminished. However, mounting evidence associates direct or indirect injury to this muscle to debilitating long-term arm disability. A more deliberate investigation of its role in typical shoulder function is paramount for developing targeted treatments, exercises, and rehabilitation protocols. Therefore, this dissertation aimed to establish critical foundational knowledge on regional pectoralis major neuromusculoskeletal control in males and females. Study 1 demonstrates that current electromyographic (EMG) methods misrepresent pectoralis major activation in several tasks and effort levels in healthy males. It proposes a holistic framework, combining high-density surface electromyography (HD-sEMG) and neural decoding. This framework allows for an investigation of the spatial distribution of whole pectoralis major activation, with in-depth insights into its neural and neuromuscular control. In study 2, methodological challenges in EMG acquisition from pectoralis major in females are addressed, demonstrating that HD-sEMG signals in the array overlaying the breast have low amplitudes and high mean power frequency. However, the acquisition of HD-sEMG signals from the top regions of the pectoralis major in females is achievable. Studies 3 and 4 evaluated the activity of the pectoralis major in healthy females. These studies showed predominantly middle sternocostal region involvement in adduction, internal rotation, and extension, while clavicular regions specifically contributed to flexion and horizontal adduction. Further, characterization of pectoralis major activation in males (Study 5 and 6) revealed lower sternocostal region involvement in tasks requiring adduction, internal rotation, and extension. All three regions assisted in flexion and horizontal adduction. Lastly, study 7 revealed high discharge rates of motor units at low effort levels and reliance on motor unit recruitment to increase force. Findings from this dissertation have broad implications in fundamental and clinical sciences. First, the scope of this work represents the first transformative step in understanding the role of pectoralis major in typical shoulder function. Second, it addresses several methodological limitations and challenges that currently limit the ability to investigate its intricate control. Lastly, current findings inform surgical procedures involving pectoralis major resection or disinsertion, rehabilitation or exercise protocols aimed at regional pectoralis major recovery, and fundamental studies, aimed at understanding the complexities of shoulder function. This dissertation’s outcomes collectively highlight the utility of examining the neuromusculoskeletal control of the pectoralis major and its significance in numerous tasks.
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Tea Lulic (2020). Examination of Healthy Neuromusculoskeletal Control of the Pectoralis Major Muscle. UWSpace. http://hdl.handle.net/10012/16480