The purpose of this study was to investigate the responses of the trunk muscle during expected and unexpected sudden loadings of the hands on the sagittal plane. Twenty, young healthy adults(male 10, female 10) were participated to two different loading conditions : expected and unexpected sudden loadings. Different weights were dropped in hand : 5lb, 6lb, 7lb for male, and 3lb, 4lb, 5lb for female. EMG activity of rectus abdominalis and erector spine muscle were collected. Rectus abdominalis and lumbar erector spine muscle activity significantly increased in unexpected sudden loadings than expected sudden loadings(p<.05). This results indicate that co-contraction of abdominal and back muscle contribute for dynamic spinal stability during expected or unexpected activities of daily living. Preparatory adjustments can be made which reduce the postural perturbation to sudden load and prevent low back injuries.