Browsing by Author "Stephens, John-Paul"
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Item An analysis of muscle fatigue due to complex tasks and its relation to the strain index(2009-06-02) Stephens, John-PaulThe Strain Index was originally designed to analyze mono-task jobs. An experiment using a grip dynamometer was used to simulate six multiple task jobs to study the effect of complex tasks on localized muscle fatigue and to evaluate six different models used to calcula te a Complex Strain Index score. These models included average Strain Index score, unadjusted summation, duration adjusted summation, complex equation, minimum intensity, and peak intensity. Two methods of calculating a continuous Strain Index score were also analyzed. Ratings of perceived exertion, hand and forearm fatigue and discomfort, Difficulty Rating, maximum voluntary contraction (MVC), and percent strength loss were recorded for each of the six treatments. Electromyography (EMG) was also recorded for the 24 subjects (12 males and females) who completed the experiment. The EMG signal was analyzed using root mean square (RMS), initial mean power frequency (IMnPF), and slope of the mean power frequency (MnPF). Each treatment, lasting one hour each, contained a primary exertion (Task 1) of either 10% or 40% MVC for three seconds and a secondary exertion (Task 2) of either 10% or 40% MVC for one or three seconds. Subjective variables linearly increased (R2 > 0.88) over the duration of the treatments and significantly differed between treatments (p < 0.05). Percent strength loss was the only variable with a gender effect (p < 0.05). RMS values did not indicate fatigue and were constant over each treatment, but were highly correlated with percent MVC. A significant difference was not found in IMnPF between pre and post treatment values or between treatments (p > 0.05). A significant difference was found for MnPF slope pre and post treatment, but no treatment effect was found (p > 0.05). The complex equation method of calculating a Strain Index score was the only model of the six evaluated that met all criteria for being an acceptable method of calculating a Complex Strain Index score. The two continuous methods presented for calculating a Strain Index score should not be used for job analysis until further research evaluates their reliability, validity, and critical scores for Hazard Classification.Item Test re-test repeatability of the strain index(Texas A&M University, 2004-09-30) Stephens, John-PaulThe Strain Index (SI) has repeatedly shown high levels of validity for differentiating between safe and hazardous tasks for the distal upper extremity (DUE). One limitation of the SI is the lack of reliability data. This study was designed to evaluate the test-retest repeatability of the SI. Fifteen raters, divided into five teams of three, were asked to use the SI to analyze 73 video AVI files of different job tasks; initially as individuals and then as teams. Several months later, raters were asked to repeat individual and team job task assessments. Raters were instructed to analyze tasks using five of six SI task variables, while the sixth was held constant. For three of these task variables, additional data was collected such as peak force and duration of job cycle. Test-retest repeatability was measured using Pearson's R, Spearman's rho, and tetrachoric correlation according to the nature of the variable. Spearman's rho values for individual and team task variable ratings ranged from 0.68 to 0.96 (0.88 average). Pearson's R for task variable data ranged from 0.76 to 0.99 for both teams and individuals with an average of 0.91. The Strain Index's rho values for individuals and teams were 0.70 and 0.84, respectively. For hazard classification, the tetrachoric correlation for individuals was 0.81 and 0.88 for teams. Results of this study support the conclusion that the Strain Index is repeatable when used by teams as well as individuals.