Effects of Trunk Rotation and Backrest Activity on Lumbar sEMG Activities, Body Pressure Distribution and Sitting Discomfort during Sitting

The purpose of this study was to examine the effects of trunk rotation and backrest activity on sitting discomfort, body pressure distribution and lumbar surface Electromyographic (sEMG) activities of lumbar erector spinae muscles (LES) and lumbar multifidus muscles (LMF), so as to provide a scientific basis on dynamic chair design and optimization and human-table-chair office system design. Ten healthy male subjects sat on two kinds of office chairs (one fixed dual-back chair and one movable dual-back chair), rotating 60° to the left and the right to simulate the common trunk activities of office work with the seatpan and backrest angle of 110°. Four tests were performed on each subject and sEMG activities of bilateral LES and LMF and body pressure distribution data of seatpan and backrest were statically measured. Meanwhile the Body Part Discomfort (BPD) Scale (Revised) was used to measure the subjective feelings of sitting discomfort. The results showed: (1) backrest activity did not affect marginally on LES and LMF myoelectric activities (P>.05) but trunk rotation had a significant effect on sEMG activities of left LMF (P<.01); (2) trunk rotation did not affect marginally on all the pressure variables (P>.05) but backrest activity affected marginally on average seatpan pressure (ASP), average backrest pressure (ABP) and peak backrest pressure (PBP) (P<.05); (3) low back discomfort (LBD) and total discomfort (OD) subjective ratings were significantly affected by backrest activity which had a large difference between fixed and movable dual-back chair and the latter gained a lower rating of discomfort. This study systematically utilized the combination of objective and subjective measurement to confirm that movable dual-back chairs were significantly better than fixed dual-back chairs which showed the superiority of dynamic chairs.