From Structure to Function: A Characterization of the Upper-Thoracic Erector Spinae

The upper back, centred about the 4th vertebrae of thoracic spine, is between the shoulder and low back where the scapular and spine muscles interface. In order to improve our understanding of the shoulder-low back relationship it is important to establish quantitative techniques and document general structure-function changes in this transition region. Therefore, the purpose was to assess ultrasound imaging (USI), maximum voluntary contractions (MVCs), and kinematic collection methods from the upper back, and to apply these methods to assess T4 erector spinae muscle (T4ES) structure and function. Five studies were produced that included a total of 86 university-aged participants. Study#1 assessed the reliability and accuracy of muscle thickness and pennation angle measured from USI compared to magnetic resonance imaging (MRI). Electromyography (EMG) MVC techniques (e.g. Active postures) targeting T4ES were tested in Study#2; while Study#3 analyzed the reliability and accuracy of measuring sagittal plane spine angles in Passive postures (Upright, Flex, Slump) from a flexible ruler (Flexi) compared to an optoelectronic motion capture (MoCap). Study#4 looked at the relationship between EMG and changes in T4ES thickness and pennation angles from USI during the same Active and Postures as Studies #2 and #3. Finally, Study#5 was an application pilot study into the function of T4ES during breathing tasks. Overall, USI and Flexi were reliable and comparable to the gold standards of MRI and MoCap, respectively, and the thoracic extension technique was recommended to normalize T4ES activation, though caution should be used when assessing overhead work due to possible crosstalk. The T4ES region showed complex changes in morphometry during the MVC (Active) techniques and Passive postures. However, distinguishable changes in morphometry were exhibited within the Passive postures, particularly Flex and Slump compared to Upright. Therefore, it was suggested that Flexi and USI could replace MoCap and EMG, if the intent were to analyze different components of static postures. This could be especially useful where access to/use of typical kinematic and EMG equipment may be lacking/inappropriate, but access to USI and Flexi are more readily available and feasible. Thus, allowing for much needed upper back quantification in clinical and industrial settings.