Standing balance: Age-related differences in postural responses to continuous perturbations

This series of studies used an oscillating platform paradigm to investigate the effects of aging on anticipatory and reactive mechanisms of postural control. We hypothesized that young adults would use anticipatory mechanisms in response to predictable postural perturbations and that aging would be characterized by a decrease in anticipatory postural muscle activity resulting in less effective balance control. Young and old healthy adults were asked to maintain standing balance on a force platform that oscillated continuously 20 cm peak-to-peak in the anterior/posterior (A/P) direction at successively increasing frequencies of 0.1, 0.25, 0.5 to 0.61 Hz. Subjects completed trials of externally- and self-triggered perturbations. The effect of mechanical stimulation of the foot plantar surface boundaries on postural responses of older adults was tested. Postural responses to perturbations were characterized using centre of pressure (COP), centre of mass (COM), muscle activity (EMG) and number of steps. A mathematical model representing the body motion in response to continuous sinusoidal platform perturbations was implemented. Young adults used anticipatory adjustments regardless of the degree of predictability of postural challenges in externally- and self-triggered perturbations. Old adults responded to a predictable externally-triggered postural challenge using reactive postural adjustments independent of the frequency of platform oscillation, the direction of perturbation and without adapting over multiple trials. Old adults used anticipatory adjustments only in self-triggered perturbations or when additional sensory stimulation from foot plantar surface boundaries was available. The present series of experiments demonstrated for the first time that cutaneous stimulation of the foot plantar surface boundaries increases stability and facilitates the use of anticipatory control strategies. These results support the importance of cutaneous mechanoreceptors at the boundaries of the foot plantar surface for the control of postural reactions evoked by continuous perturbations. The results from these experiments clearly show that the ability to compensate for an impending and highly predictable perturbation decreases with aging. The age-related difference in the control of standing balance on a continuous oscillating platform recorded in experimental data was partially explained through increased levels of sensory noise and neural delays in the simulated data of old adults. Our results support the concept of a dynamic stability, according to which, in addition to the horizontal location of the COM with respect to the base of support, the magnitude and direction of its corresponding velocity provide critical information pertaining to one's ability to control balance. Based on model work, we demonstrated that the acceleration parameters of a perturbation must be taken into account when calculating stability limits. We derived for the first time the equations for calculating these stability limits related to continuous translations of the base of support