Control of frontal plane body motion in human stepping

During a step the body's centre of mass (CoM) typically remains medial to the supporting foot and therefore the body is unstable and falling (sideways) under gravity. This may make it difficult to adjust the frontal-plane body motion appreciably once the step is under way. We have therefore investigated whether this motion could be controlled largely in a ballistic manner, that is by setting the initial (toe-off) position and velocity of the CoM such that the fall develops as required for the particular step without the need for appreciable mid-step adjustment. Subjects stepped in different directions and from different postures, and the resulting motion of their CoM in the frontal plane was compared with that of a single-segment mathematical model of the body which falls freely under the influence of gravity. The lateral position and velocity of subjects' CoM at toe-off varied across the different step types in a manner consistent with a ballistic mode of control. Furthermore the model, given these positions and velocities as initial conditions, closely predicted the subsequent CoM motion. The results suggest that subjects may produce the different body trajectories required for different types of step largely in a ballistic manner. This would imply that the central nervous system must judge in advance the size and direction of the initial "throw" given to the body-mass.