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An analysis of muscle fatigue due to complex tasks and its relation to the strain index

The 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
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.
Date02 June 2009
CreatorsStephens, John-Paul
ContributorsMoore, John Steven
Source SetsTexas A and M University
Detected LanguageEnglish
TypeBook, Thesis, Electronic Dissertation, text
Formatelectronic, application/pdf, born digital

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