Human walking is the most fundamental and widespread motor activity of our daily life.This involves delicate dynamic regulations of our bodies to achieve remarkable stability,agility and energy efficiency.In this study,we examine simple biomechanical rules regulating the dynamic motion of the whole-body centre of mass during level walking.The hypothesis is that the three-dimensional whole-body centre of mass motion with respect to the foot contact point with the ground surface is highly regulated according to some simple biomechanical rules.In order to test this hypothesis,three-dimensional gait measurements were conducted to measure the whole-body motion using a specially design marker cluster system.The time trajectories of the whole-body centre of mass and zero moment point were calculated in each gait cycle.We found that the relative motion pattern of the whole-body center of mass with respect to the zero moment point in a gait cycle can be very well defined using four planes in the three-dimensional space.This would provide new insight into the fundamental principle modulating whole-body dynamics during human walking,and also could offer alternative control scheme to regulate body motions and joint torques for bipedal robots.