Methodological requirement to analyze biomechanical postural control mechanisms with two platforms.

In 1996, Winter and colleagues proposed the existence of two postural control mechanisms in both the anteroposterior and mediolateral axes: a bodyweight (loading/unloading) distribution mechanism and a complementary center of pressure location mechanism. To measure the loading/unloading forces under each foot, the feet had to be placed side by side in the mediolateral axis and one foot ahead of the other in the anteroposterior axis. Our first objective was to reexamine the validity of anteroposterior data published with the feet side by side. In that foot condition, we expected no change in the anteroposterior loading/unloading forces (regardless of the task performed), and consequently no change in the complementary mechanism. Our second objective was to confirm our hypotheses with experimental data. Twelve healthy, young adults performed three types of body oscillation in the anteroposterior axis (at the hips, at the ankles and alternately at the ankles and hips) and a quiet stance condition with the feet side by side. As expected, the bodyweight mechanism did not vary significantly. Although the complementary mechanism was significantly higher in the ankle and alternating conditions, the change was very tiny (<0.3%). Thus, we propose methodological requirements to analyze both mechanisms.