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The electromyographic activity of the paraspinal muscles were examined to investigate the effects of chair sitting on muscle activation patterns and muscle fatigue. Twenty-four subjects were studied on the Balans, Back Up, and standard office chairs during a seated writing task and standing while performing a standardized muscle fatigue assessment test. The results of the study indicate that in 87.5% of the comparisons at the L3 site, the Back Up chair has lower readings of muscular effort compared to the Balans and office chair. During the muscle fatigue testing, the greatest amount of fatigue was noted in those subjects who sat in the Back Up chair last. When the Back Up chair was first or second, the least amount of fatigue was observed. The sEMG results supports a biomechanical model. The ergonomics of the Balans and office chairs give the least amount of support to the low back, while the Back Up chair provides the greatest amount of support. It is suggested that adequate pelvic girdle support during seated tasks will lower the threshold of back fatigue and thus facilitate return to work for low back pain patients. [ABSTRACT FROM AUTHOR]