Objective To observe the electromyography (EMG) activities and kinematic characteristics of the back, abdomen and hip muscles under the static symmetric loading and unloading with the flexion and extension posture. Methods Six healthy male subjects stood on a specially designed testing platform, doing flexion/extension under symmetric loading and unloading, respectively. Each trial lasted 4 seconds and was repeated 3 times. The EMG activities of 10 muscles(rectus abdominis，obliquus externus abdominis，erector spinae，multifidus，gluteus Medius), the 3D angular movement, the plantar COP (center of pressure) were recorded during the trial.The normalized EMG, trunk angle and COP displacement were calculated, and statistical analyses of all data were made. Results Muscle activities of the back were greater (10.47~16.94) with flexion both under loading and unloading. Muscle activities of the abdomen were greater with extension under unloading, and those of the back (3.70~17.95) and hip (6.64~11.52) were increasing under loading, with muscle activities of the abdomen decreasing (10.66~4.18). The flashing trunk numbers was increased with the increase of loading, especially 1.55-time increase in the trunk angle. The COP displacement in anterior posterior direction was shifted greater than that in lateral direction, and this shift was more during flexion (14.60) than that during extension (7.65).Conclusions Extension increases the activities of back muscles, especially for multifidus, and it also increases the torso angular displacement and flashing trunk numbers, especially under loading.