Role of visual and static vestibular influences on dynamic posture control.

Postural stabilization in altered visual and vestibular conditions was investigated in humans subjected to fast transient disturbances and during sinusoidal movement of the standing support. Visual inputs were varied by applying stroboscopic illumination, stabilizing the visual surround in respect to head movements, inducing apparent body movement in pitch by continuously moving stripe patterns up or down and by eye closure. Static vestibular input was modified by bending the head forwards or backwards, or to the right or left shoulder (eyes closed). Neither biomechanical parameters of standing nor EMG responses of the anterior tibial and triceps surae muscles were modified by the different visual and vestibular conditions during fast transient (80 degrees/s) platform movements 4 degrees toe-up. Continuous regulation of upright stance during sinusoidal movements (1 Hz, 0.3 Hz), however, clearly depended on the different modifications of visual and vestibular inputs. Fast transient disturbances are easily compensated in a reflex-like manner independent of visual and vestibular feedback. Continuous regulation of upright posture during slow disturbances, however, clearly depends on the evaluation of afferent information from the visual, vestibular, and proprioceptive systems.