The purpose of this study is to analyze the spine stability by comparing muscle activation in various scaffold working conditions. The independent working conditions were designed by two levels of working height, existence and absence of safety handrail, and the two levels of expertise. The corresponding activities of the agonist and antagonist muscles of spine were simultaneously recorded by using EMG. As results, novice worker showed increased muscle activity while doing the task on the 2nd floor, absence of handrail. And expert showed the increase of co-contraction while working on the 2nd floor without handrail. Such co-contraction was found to increase the spine stability when the working condition become risky. On the other hand, the co-contraction was prolonged, the spine muscle fatique and disc pressure could be increased, which would increase the risk of musculo-skeletal disorder. The results of co-contraction in this study indicates that the motor control system responds to maintain the stability of the spine particularly when workers cognitively recognize the danger of falling or imbalance. This study also quantitatively accounted for the biomechanical cause of LBP among workers who has to prevent themselves from falling. Therefore, if can be said that safe environment preventing falling can also prevent workers from MSDs as well. Such knowledge can be applied to design ergonomic workplace environment as well as movable scaffold.