Lateral shift using one hand while sitting on a seat is an important ability in sitting up, for locomotion, to reach the other side. We sometimes control muscle tone of the trunk to change lateral weight bearing on one hand in hemiplegia caused by cerebrovascular diseases in which movements are difficult to perform because of trunk collapse. In this study, we analyzed the activity of the trunk muscles (obliquus abdominis, obliquus externus abdominis, obliquus internus abdominis and lumbar back) with EMG in changing the weight bearing in lateral shift with one hand while sitting on a seat. The results show that the supporting hand is necessary in sitting on the seat. The supported obliquus abdominis muscles, obliquus externus abdominis muscles and lumbar back muscles acted to maintain and fix the trunk as the weight increased. The supported obliquus external abdominis muscles acted to stabilize the upper trunk and rib cage, and the unsupported obliquus abdominis muscles, obliquus internus abdominis muscles and lumbar back muscles elevated pelvis as the weight decreased. Therefore, it is suggested that in the change of weight bearing in lateral shift the supporting hand serves an important purpose in maintaining trunk stability.
