Objective To observe electromyography (EMG) activity and kinematic characteristics of the dorsal and ventral muscles during extension-flexion of the trunk． Methods Ten healthy male subjects stood on a test platform, performing the front and back lifting and lowering. The EMG activity of 10 muscles (rectus abdominus, external oblique, gluteus medius, erector spinae, multifidus), the angular movement, the plantar center of pressure (COP) were recorded at the same time. The average amplitude of EMG, contribution ratio of muscles, and COP displacement were calculated, and statistical analyses of all the data were conducted． Results During front lifting and lowering, the activity of dorsal muscles was greater than that of ventral muscles under both loading and unloading, and the contribution ratio of dorsal muscles was increasing with loading. During back lifting and lowering, the activity of ventral muscles was slightly greater, but the contribution ratio of both dorsal and ventral muscles was very close. The contribution ratio of ventral muscles was increasing with loading, while the activity of dorsal muscles decreased. The activity of gluteus medius was increasing with loading, but the contribution ratio was basically consistent under both loading and unloading. Conclusions During extension-flexion of the trunk, muscle activation varies with the load size and trunk position. Under loading at front, dorsal muscles are working as an agonist, while ventral muscles as antagonist; under loading at back, ventral muscles become an agonist and dorsal muscles as a co-activator. The activity of gluteus medius might play a role of stabilization during front and back lifting and lowering.