Postural coordination in elderly subjects standing 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 t heir temporomandibular joint, greater trochanter, and lateral malleolus. Su bjects 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 0.6H z. Each trial, for either visual condition, was repeated twice, lasted 30 s econds, 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 measurin g periodic shift of LED markers using an optoelectronic device. Standard de viation on traces of the markers were taken as a comprehensive index of seg ment placement. Cross-correlation between marker traces was determined to o btain measures of degrees of coupling between body segments. Results: At the low translation frequency (0.2Hz), with EO, subjects behave d like a noninverted pendulum, whereby the head tended to be stabilized mor e than the hip, and the legs taking up most of the imposed displacement. Ag ing was associated with greater head stabilization, and a looser coupling b etween head and hip. With EC, the body attitude changed to an inverted pend ulum, whereby the head overshot the platform displacement. Aging was associ ated 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 subjec ts vision was no longer able to counteract the imposed head displacement. A t this frequency, with both EO and EC, there was a poor coupling between bo dy 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 too l to assess age-related changes of the sensorimotor mechanisms underlying d ynamic equilibrium.