Predicting Shoulder Fatigue for Long Durations Using Psychophysical Measures Obtained from Short Trials

Abstract

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.