In order to find effective movements on an office chair for preventing back pain, the paper measures EMG (electromyogram) signal and evaluates cognitive test. Core muscles of subjects on a balance ball are measured when there are forced movement and voluntary movement of the ball. The measurements show the forced movement and discontinuity movement on the ball is more effective in the meaning of muscle activity. The paper also studies the effect of movement with the balance ball on the efficiency of office workers. This is because, even if the movement is good for preventing back pain, it is not desirable to reduce work efficiency. Two cognitive tests, addition test and flanker test, are used to investigate the efficiency of office workers when the movements with the balance ball and no movements with normal chair in the view points of work reduction rate and correct answer rate. The results show the efficiency of office workers is almost the same as the case of movements with the balance ball and no movements with normal chair. Totally the study shows effective movements on the balance ball which activate the core muscle activity efficiently. It also shows the movement does not reduce the office work efficiency.

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