Postural behaviors to combined disturbances of the visual field and base of support

This study examines whether combining a sustained mechanical and visual disturbance will modify the automatic postural responses and alter subsequent recovery to vertical orientation in healthy young and older adults and in patients who are post-stroke. Subjects stood on a platform that tilted 3 deg at 30 deg/sec into dorsiflexion while in the dark, with visual motion matched to motion of the head, or with pitch up visual field motion at 30, 45, and 60 deg/sec. The platform was held stable for 30 sec and then returned to neutral at 0.1 deg/sec for 30 sec. EMG responses of lower limb and neck muscles and angular displacement of body segments were recorded. Latencies of the EMG response and magnitudes of muscle and center of mass (COM) motion were calculated. In older adults, EMG response latencies were delayed compared to young adults. In both young and older adults, muscle RMS values were greater with visual field motion than in the dark. The slope and speed of return to a vertical orientation when the base of support was tilted was affected by the presence of visual field motion in all populations. Older subjects demonstrated more sensitivity to the visual inputs and young adults were more sensitive to motion of the base of support. Increased instability in the patients, particularly while the platform returned to neutral, indicates that they were very sensitive to the visual motion when the base of support motion was not reliable. Our data suggest that alterations in the ability to recover an upright position in the presence of visual field motion might underlie the decreased postural stability observed in patients post-stroke and in elderly individuals that exhibit visual sensitivity.