The Effects of Chair Design On Back Muscle Fatigue

The electromyographic activity of the paraspinal muscles at four levels were examined to investigate the effects of chair sitting on muscle activation patterns and muscle fatigue. The Balans, Nada-Chair Back Up, and Standard Office Chair were studied. Twenty four subjects were asked to engage in a seated writing task for 10 minutes for each of three chairs. Prior to the first writing period and between each writing period, the subjects were asked to perform a standardized muscle fatigue assessment while standing. The results of the study indicate that the seated writing task differentially effects only the paraspinal muscles at L3 site. During the writing task, the Balans chair required the largest amount of muscular effort. The standard Office chair showed evidence of fatigue. The Back Up chair yielded the best performance. In 87.5% of the comparisons the Nada-Chair Back Up has lower readings of muscular effort compared to the Balans and Office Chair. During the muscle fatigue testing during standing, evidence of increased muscle fatigue was observed as a function of each sit period. 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 IEMG results reflect a biomechanical model. The distribution of the Balans chair is unnatural and gives the least amount of support to the pelvis and lower spine, while the Back Up provides the greatest amount of support. The Balans chair overworks the low and mid back muscles, only to spare the cervical sets, while the Nada-Chair Back Up provides the greatest amount of lumbar support. The energy expenditure of the Office chair and Balans chair is significantly higher in the low and mid back area. This higher energy expenditure leads to fatigue while sitting. In conclusion, the level of muscular effort and accompanying fatigue associated with sitting are directly related to poor or inadequate biomechanical support of the chair.