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 neuromuscular 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 medio-lateral and antero-posterior axes because these two variables depend on amplitude versus frequency relationship. This relationship was used to evaluate the CG motions based on the CP motions. Seven subjects stood still on a force plate with eyes open and eyes closed. The results showed that the ratio of (CP-CG)/CP trajectory path length was personal for each subject. These results suggest different levels of passive (ligaments, elastic properties) and active (reflex activity) stiffness. For some subjects, this ratio was significantly lower for the eyes open condition than for the eyes closed condition, indicating a decrease of the active stiffness for the eyes open condition. Therefore, a CG-CP comparative analysis appeared helpful in understanding the control of balance and necessary to quantify the subjects' net performance.

Three-dimensional joint moments analysis in a vertical quadrupedal posture associated with a leg release.

This study reports on rock climbers biomechanical adjustments following a leg release from a quadrupedal vertical initial stance. Six climbers of international level have been studied. They were asked to maintain a quadrupedal posture on artificial holds equipped with strain gauges and then to release their right foot. The results show that all the rock climbers adjust differently the vertical forces and the horizontal forces on the holds. They distribute the vertical forces over the three remaining holds and the horizontal forces over the controlateral holds. A quasi-static three-dimensional multi-segment model was used to produce the net reaction forces and joint moments at each joint. Moment results enhance the role of the controlateral limbs, particularly the left leg. The gravity seems to be counteracted at the left knee joint, while body balance seems to be managed at the left hip and ankle joints.

Effect of initial posture on biomechanical adjustments associated with a voluntary leg movement in rock climbers.

This study compares the biomechanical adjustments following a leg displacement in vertical and horizontal position on expert rock climbers. The results show a non-diagonal pattern in the vertical condition and a diagonal one in the horizontal condition. A multi-link analysis shows that the non-diagonal pattern displayed by expert climbers in the vertical condition resulted in a new distribution of body weight to preserve the possibility of moving any limb from the three legged stance. Conversely, in the horizontal condition, the diagonal pattern appears to be strongly dominated by mechanical and anthropometric factors.