Emergence of postural patterns as a function of vision and translation frequency

We examined the frequency characteristics of human postural coordination an d the role of visual information in this coordination. Eight healthy adults maintained balance in stance during sinusoidal support surface translation s (12 cm peak to peak) in the anterior-posterior direction at six different frequencies. Changes in kinematic and dynamic measures revealed that both sensory and biomechanical constraints limit postural coordination patterns as a function of translation frequency. At slow frequencies (0.1 and 0.25 H z), subjects ride the platform (with the eyes open or closed). For fast fre quencies (1.0 and 1.25 Hz) with the eyes open, subjects fix their head and upper trunk in space. With the eyes closed, large-amplitude, slow-sway moti on of the head and trunk occurred for fast frequencies above 0.5 Hz. Visual information stabilized posture by reducing the variability of the head's p osition in space and the position of the center of mass (CoM) within the su pport surface defined by the feet for all but the slowest translation frequ encies. When subjects rode the platform, there was Little oscillatory joint motion, with muscle activity limited mostly to the ankles. To support the head fixed in space and slow-sway postural patterns, subjects produced stab le interjoint hip and ankle joint coordination patterns. This increase in j oint motion of the lower body dissipated the energy input by fast translati on frequencies and facilitated the control of upper body motion. CoM amplit ude decreased with increasing translation frequency, whereas the center of pressure amplitude increased with increasing translation frequency. Our res ults suggest that visual information was important to maintaining a fixed p osition of the head and trunk in space, whereas proprioceptive information was sufficient to produce stable coordinative patterns between the support surface and legs. The CNS organizes postural patterns in this balance task as a function of available sensory information, biomechanical constraints, and translation frequency.