Stiffness and damping in postural control increase with age

Abstract

Upright balance is believed to be maintained through active and passive mechanisms, both of which have been shown to be impacted by aging. A compensatory balance response often observed in older adults is increased co-contraction, which is generally assumed to enhance stability by increasing joint stiffness. We investigated the effect of aging on standing balance by fitting body sway data to a previously developed postural control model that includes active and passive stiffness and damping parameters. Ten young (24 +/- 3 years) and seven older (75 +/- 5 years) adults were exposed during eyes-closed stance to perturbations consisting of lateral pseudorandom floor tilts. A least-square fit of the measured body sway data to the postural control model found significantly larger active stiffness and damping model parameters in the older adults. These differences remained significant even after normalizing to account for different body sizes between the young and older adult groups. An age effect was also found for the normalized passive stiffness, but not for the normalized passive damping parameter. This concurrent increase in active stiffness and damping was shown to be more stabilizing than an increase in stiffness alone, as assessed by oscillations in the postural control model impulse response.

Fig. 1

Experimental Setup. A NeuroTest posture platform was used to provide the support surface perturbation (Neurocom, Inc.). A Polhemus Fastrak magnetic-based motion tracking system was used to measure body sway. Subjects stood with eyes closed during experimental trials.

Fig. 2

An example of the experimental Transfer Function (TF) gain and phase curves computed from smoothed (orange) and unsmoothed (blue) spectral estimates from a representative young subject. Smoothing was applied as described in Methods to reduce variability in the transfer function estimates.

Fig. 3

Feedback Model of Postural Control. Sensory channels are limited to vestibular (graviceptive) and proprioceptive sensory channels. The mechanical perturbation provided by the support surface (SS) is indicated in the schematic as a pseudorandom stimulus. Body sway with respect to the feet (i.e. with respect to a sagittal plane perpendicular to the SS) is indicated by BF. Eyes are closed, hence visual sensory feedback is not included in the model. The “Aging” box is hypothesized to have an effect on the “Sensory and neuromuscular system.” (Model schematic adapted from [7], [19].)

Fig. 4

Experimental Transfer Function (TF) estimated from body sway response to SS rotations for young adult (YA) and older adult (OA) age groups. Averages were taken across the three sessions for PRTS-SS trials alone. YA and OA gain curves were similar in the low-frequency range (below 0.3 Hz), while the gain of the older adults was larger in the mid- and high-frequency ranges (above 0.3 Hz) and exhibited a slight peak around 0.7 Hz.

Fig. 5

A. Model parameter values estimated by fitting the postural control model to the experimental transfer functions (also see Table II). Bar plots show average results (mean ± SD) for young adults (YA) and old adults (OA). Significant age differences (p<0.05) are indicated by *. B. Normalized stiffness and damping (also see Table III).

Fig. 6

Plots of the impulse response of the postural control model for nominal values of active and passive stiffness and damping in the young adult (YA) group (solid curve) versus the older adult (OA) group (dash-dot curve). The first peak is similar in terms of timing and amplitude, but the response of the OA group is characterized by larger subsequent peaks, and slightly higher frequency of oscillation, as compared to the YA. Older adults are mainly characterized by larger active stiffness and damping as compared to young adults.

Fig. 7

Plots of the impulse response of the postural control model for nominal values of stiffness and damping in the young adult population (solid curve), versus increased stiffness only (dashed curve), versus increased stiffness and damping (dash-dot curve), and versus increased stiffness, damping, and integral gain (dotted curve). Note that peak-to-peak oscillations are greatest for increased stiffness alone, and that increasing damping concurrent with an increase in stiffness diminishes these oscillations. Increasing the integral gain has little effect compared to increased stiffness and damping.