Kinematic and kinetic validity of the inverted pendulum model in quiet standing.

Movements of the whole-body center of mass during quiet standing have been estimated from measurements of body segment movements. These whole-body center of mass movements have been compared with movements of the center of mass as predicted from a simple inverted-pendulum model of standing. However, the total body center of mass is a weighted average of the center of mass of all individual body segments. The question arises as to how well the total body center of mass represents the individual segments and lower limb joint angles. This study focuses on the validity of how well the individual segments and lower limb angles temporally and spatially synchronize with the total body center of mass. Eleven healthy university students volunteered to participate. Kinematic data were collected using a 3D optoelectronic camera system; kinetic data were collected using a 3D force plate. Participants stood quietly, with eyes open, for 120 s. Segment and whole body centers of mass were calculated from a 14 segment, 3D bilateral model. Segment and joint angles were calculated for the lower limbs, bilaterally, and the trunk. Segment center of mass root-mean-square displacements were strongly correlated with center of mass height relative to the ankle joint and were synchronized, or temporally locked, to the movement of the whole body center of mass. Sagittal plane ankle angular displacements were highly correlated to sagittal plane center of mass movement; stronger correlations between body center of mass and lower limb angular displacement were observed, the result of compensatory knee joint angular displacements. These data support and extend the use of an inverted pendulum model to represent quiet standing postural control.