Many industrial workplaces involve tasks that require work to be performed in overhead postures. Epidemiological evidence suggests that working in these unavoidable, awkward postures leads to development of shoulder fatigue, pain and several musculoskeletal disorders. The accumulation of localized muscle fatigue has been strongly associated with the development of work-related musculoskeletal injuries (Armstrong et al., 1993). In order to prevent injury, minimizing muscular fatigue during short-cycled, repetitive work through different work organization schemes has been suggested (Dempsey et al., 2010). Previous research has examined the interactive effect of altering contraction level, duty cycle and cycle times on shoulder muscle fatigue. However, isolation of one factor while maintaining a constant workload has not been examined for overhead work tasks. The purpose of the study was to determine whether cycle time affected the progression of fatigue at the shoulder since the postural load during overhead tasks is inherently fatiguing. Ten university aged females performed a task rotation between an intermittent overhead pressing task and a neutrally located assembly task. Four conditions were defined by cycle time (15s, 30s, 60s and 120s) and each cycle consisted of one complete rotation. In order to quantify the progression of fatigue over time, four dependant measures were systematically collected for all conditions until exhaustion or to a maximum of three hours. These included root mean square (RMS) amplitude and median power frequency (MdPF) calculated from surface electromyography of nine muscles surrounding the shoulder, static strength capability, and rating of perceived exertion. Endurance time was also included as a fifth measure of fatigue. Linear regression was used to determine the slope of static strength and perceived exertion over time, and magnitude changes over normalized time were calculated for EMG measures. For all dependant measures, repeated measures ANOVA were used to identify significant differences across conditions. As the only independent factor investigated, cycle time influenced two out of the five dependent measures. Conditions induced differences in endurance time (F[3,24]=3.96, p=0.02) and RMS amplitude of the middle (F[24,189]=3.10, p<0.0001) and posterior deltoid (F[24,189]=2.52, p=0.0003). Performing overhead work in long cycles (120s) induced a shorter average endurance time (118.67min), and the shortest cycle time (15s) resulted in a longer average endurance time (152.44min). Over time, the rate of increase in RMS amplitude of both deltoid muscles was higher when working at the longest cycle time (120s). Although six muscles showed an indication of fatigue through significant decreases in MdPF in at least one condition, cycle time did not affect MdPF over time for any muscle examined. Similarly, the rate of static strength capability and rating of perceived exertion over time were not affected by cycle time. Two of five measures indicated that cycle time played a significant role in fatigue progression, making its effectiveness as a work organizational method for overhead work tasks unclear. Results indicate that that intermittent overhead work should be performed in shorter cycles to reduce the risk of shoulder injury. Identifying additional effects of cycle time on fatigue measures through increasing statistical power would provide ergonomists with more confidence in recommending this organizational strategy to mitigate the risk of musculoskeletal injury.

Many industrial workplaces involve tasks that require work to be performed in overhead postures. Epidemiological evidence suggests that working in these unavoidable, awkward postures leads to development of shoulder fatigue, pain and several musculoskeletal disorders. The accumulation of localized muscle fatigue has been strongly associated with the development of work-related musculoskeletal injuries (Armstrong et al., 1993). In order to prevent injury, minimizing muscular fatigue during short-cycled, repetitive work through different work organization schemes has been suggested (Dempsey et al., 2010). Previous research has examined the interactive effect of altering contraction level, duty cycle and cycle times on shoulder muscle fatigue. However, isolation of one factor while maintaining a constant workload has not been examined for overhead work tasks. The purpose of the study was to determine whether cycle time affected the progression of fatigue at the shoulder since the postural load during overhead tasks is inherently fatiguing. Ten university aged females performed a task rotation between an intermittent overhead pressing task and a neutrally located assembly task. Four conditions were defined by cycle time (15s, 30s, 60s and 120s) and each cycle consisted of one complete rotation. In order to quantify the progression of fatigue over time, four dependant measures were systematically collected for all conditions until exhaustion or to a maximum of three hours. These included root mean square (RMS) amplitude and median power frequency (MdPF) calculated from surface electromyography of nine muscles surrounding the shoulder, static strength capability, and rating of perceived exertion. Endurance time was also included as a fifth measure of fatigue. Linear regression was used to determine the slope of static strength and perceived exertion over time, and magnitude changes over normalized time were calculated for EMG measures. For all dependant measures, repeated measures ANOVA were used to identify significant differences across conditions. As the only independent factor investigated, cycle time influenced two out of the five dependent measures. Conditions induced differences in endurance time (F[3,24]=3.96, p=0.02) and RMS amplitude of the middle (F[24,189]=3.10, p<0.0001) and posterior deltoid (F[24,189]=2.52, p=0.0003). Performing overhead work in long cycles (120s) induced a shorter average endurance time (118.67min), and the shortest cycle time (15s) resulted in a longer average endurance time (152.44min). Over time, the rate of increase in RMS amplitude of both deltoid muscles was higher when working at the longest cycle time (120s). Although six muscles showed an indication of fatigue through significant decreases in MdPF in at least one condition, cycle time did not affect MdPF over time for any muscle examined. Similarly, the rate of static strength capability and rating of perceived exertion over time were not affected by cycle time. Two of five measures indicated that cycle time played a significant role in fatigue progression, making its effectiveness as a work organizational method for overhead work tasks unclear. Results indicate that that intermittent overhead work should be performed in shorter cycles to reduce the risk of shoulder injury. Identifying additional effects of cycle time on fatigue measures through increasing statistical power would provide ergonomists with more confidence in recommending this organizational strategy to mitigate the risk of musculoskeletal injury.

This study examined changes in task performance during performance of simulated overhead assembly task. The study objectives were to better understand the relationship between fatigue development during overhead work and task performance. This relationship is important, considering that performance changes, or decreases in task quality, have the potential for justifying and driving ergonomic changes that can help to improve worker safety. Sixteen people participated in eight experimental conditions (two levels of duty cycles, two work heights, and two hand positions). Four dependent measures based on endurance, subjective, objective, and physiological fatigue were used to quantify shoulder fatigue and were collected during the experiment. An overhead work task required participants to use a hand tool to strike targets at two reach distances above their head. Task performance was measured as a function of the closeness to the target center and the ability to apply a consistent force throughout the experiment. Data collected in this experiment is intended to provide a research basis for creating design guidelines that will help maximize efficiency and quality while reducing the likelihood of developing shoulder fatigue. / Master of Science

Localized muscular loads have in many cases replaced whole body loads in the current mechanized industry. In highly automated automobile industries, the prevalence of upper extremity musculoskeletal disorders is a matter of continuing concern. Overhead work has especially been noted for its association with shoulder related musculoskeletal disorders. Research aimed at determining causal relationships between overhead work and risk of injury has increasingly used localized muscle fatigue as an indirect or surrogate measure. In this study, localized muscle fatigue was used as a primary measure for studying the effects of workload level while performing overhead work. Subjective (ratings of perceived discomfort) measures of fatigue were collected and their predictive potential was investigated. Effect of personality type was also examined to account for any inter-individual differences in fatigue perception. While researchers have studied specific task conditions in controlled environments, the specific relationship between various risk factors and underlying injury mechanisms is largely unknown. Two main problems faced by researchers are limited resources and the large scope of potential ergonomic analyses. This study attempted to circumvent some of these limitations by examining the time-course of fatigue and the predictive potential of subjective measures. The feasibility of using shorter experimental durations to make deductions for a 2-hour work period was explored. Reductions in experimental duration means decreased experimental time, expenses and resources. Thus, in turn, the researcher can utilize available resources to study more factors and a more general scenario. Specifically, subjective measures of shoulder fatigue were used to determine the possibility of reducing experimental duration for an intermittent overhead task. A laboratory-simulated intermittent overhead task was designed based on observations made at an automotive assembly unit. For this study, two treatment conditions were tested consisting of different combinations of two tool masses and two duty cycles. The choice of the treatment conditions was made to simulate different task difficulty levels of occupational tasks and their effects on shoulder fatigue. Each experiment was conducted for 2 hours (a common duration in industries with job rotation) for these selected treatment conditions. Subjective measures of fatigue were collected to assess shoulder fatigue and relative acceptability of the overhead work. Any observed trends in the subjective fatigue measure were determined and tested using statistical and mathematical models to determine how best to represent their salient characteristics. Derived qualitative and quantitative measures were also used to estimate the maximal acceptable task durations using certain formalized assessment techniques. Results of this research suggest possible reductions in the experimental duration. Short (8 to 26 minute) trials were found to be sufficient to predict performance measures for 2 hours. Results also indicated a strong influence of task difficulty level on the predictive performance of subjective measures though personality type did not show very consistent trends. Various unique analysis techniques used to look at the psychophysical data may prove useful for further investigation into predictive verification. A generalized mathematical model, a type of approach, was also developed to represent changes in the psychophysical measures over time. This research can find both industrial and research applications where resources are constrained and using psychophysical measures is feasible. In the following report, details on this work are presented, including a description of the factors that inspired this study, an outline of the relevant literature, methodology, results and their implications. / Master of Science