Postural coordination in elderly subjects on a periodically moving platform.

Objective: To test body segment coordination during dynamic equilibrium on a moving platform, and assess age-related differences in the performance. Design: Subjects had infrared light-emitting diode (LED) markers fixed on their temporomandibular joint, greater trochanter, and lateral malleolus. Subjects stood upright on a sinusoidally translating platform with both feet, either with eyes open (EO) or eyes closed (EC). The platform was displaced horizontally in an anteroposterior direction at frequencies of 0.2 or 0.6Hz. Each trial, for either visual condition, was repeated twice, lasted 30 seconds, with 30-second intervals, for up to 1 hour. Participants: Fifty-three healthy subjects, aged 17 to 89 years. Main Outcome Measures: Kinematics of head and hip were analyzed by measuring periodic shift of LED markers using an optoelectronic device. Standard deviation on traces of the markers were taken as a comprehensive index of segment placement. Cross-correlation between marker traces was determined to obtain measures of degrees of coupling between body segments. Results: At the low translation frequency (0.2Hz), with EO, subjects behaved like a noninverted pendulum, whereby the head tended to be stabilized more than the hip, and the legs taking up most of the imposed displacement. Aging was associated with greater head stabilization, and a looser coupling between head and hip.With EC, the body attitude changed to an inverted pendulum, whereby the head overshot the platform displacement. Aging was associated with reduced head stabilization, and a stronger coupling between head and hip. When the frequency of the platform translation increased to 0.6Hz, with EC, the general picture was similar to the above, but in older subjects vision was no longer able to counteract the imposed head displacement. At this frequency, with both EO and EC, there was a poor coupling between body segments across all ages. Conclusions: Periodic shift of the support base may be a valid protocol to test the ability to control balance in the elderly, and may be a useful tool to assess age-related changes of the sensorimotor mechanisms underlying dynamic equilibrium.