Upper extremity kinematic changes and shoulder muscle fatigue during a repetitive goal directed task
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Repetitive workplace tasks are associated with fatigue induced changes to shoulder muscular behavior, which can alter glenohumeral joint kinematics and lead to chronic injury. However, accessible and reliable methods to detect shoulder muscle fatigue in the workplace are scarce. The overall purpose of this thesis was twofold. First to determine if changes in upper extremity joint angle across individuals during a workplace emulative repetitive task exhibit features that may be visually identifiable, and second, to characterize the relationship between potentially visually identifiable changes in thoracohumeral elevation and traditional indicators of shoulder muscle fatigue. Twenty-seven, young healthy individuals performed a seated repetitive manuals materials handling task requiring them to lift and lower a weighted bottle between two target locations to exhaustion. During the last five lift motions of each 2-minute interval during the repetitive task, a symbolic motion structure representation (SMSR) algorithm was used to identify the basic spatial-temporal structure of the time series upper extremity joint angle data (i.e. torso, thoracohumeral, elbow and wrist), followed by measures of selected shoulder muscle electromyography (EMG) mean power frequency (MPF) and ratings of perceived fatigue and discomfort (RPF/RPD). Joint angle SMSRs characterize motion as a sequence of directional changes in joint angle time series data, which are easier to visually identify by ergonomists, in comparison to joint angle magnitudes. Changes in joint angle SMSRs occurred across upper extremity joints for most participants (at least 24 of 27) in this repetitive task. A weak positive linear relationship existed between the onset of changes in thoracohumeral elevation SMSR and the onset of shoulder muscle fatigue (as identified by a decline in the EMG MPF from the infraspinatus muscle) (R2 = 0.275, p = 0.02). Participants who varied the thoracohumeral elevation SMSR, in comparison to those who did not, exhibited a 7.45% greater decrease in anterior deltoid EMG MPF (p = 0.304), indicative of higher levels of muscle fatigue, throughout the repetitive task. In principle, the results of this thesis indicate that upper extremity kinematic changes in a repetitive task may be visually identifiable as directional changes in joint motion identified by the SMSR algorithm. The relationship between anterior deltoid muscle fatigue and variability in thoracohumeral elevation SMSRs throughout the repetitive task provide a link between potentially visually identifiable directional changes in thoracohumeral elevation joint motion (SMSR) and shoulder muscle fatigue accumulation. These initial findings can inform future research endeavors aimed at developing heuristic guidelines for visually identifying variations in thoracohumeral joint angles as a more accessible method to identify local shoulder muscle fatigue in ergonomics assessments.
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Rachel Whittaker (2017). Upper extremity kinematic changes and shoulder muscle fatigue during a repetitive goal directed task. UWSpace. http://hdl.handle.net/10012/11219