A comparative analysis of the center of gravity and center of pressure trajectory path lengths in standing posture: An estimation of active stiffness

The center of foot pressure (CP) motions, representing the net neuromuscula r control, was compared to the center of gravity (CG) motions, representing the net performance. The comparison focused on the trajectory path length parameter along the mediolateral and antero-posterior axes because these tw o variables depend on amplitude versus frequency relationship. This relatio nship was used to evaluate the CG motions based on the CP motions. Seven su bjects stood still on a force plate with eyes open and eves closed. The res ults showed that the ratio of (CP - CG)/CP trajectory path length was perso nal for each subject. These results suggest different levels of passive (li gaments, elastic properties) and active (reflex activity) stiffness. For so me subjects, this ratio was significantly lower for the eyes open condition than for the eyes closed condition, indicating a decrease of the active st iffness for the eyes open condition. Therefore, a CG - CP comparative analy sis appeared helpful in understanding the control of balance and necessary to quantify the subjects' net performance.