Physiological complexity and system adaptability: evidence from postural control dynamics of older adults

Abstract

The degree of multiscale complexity in human behavioral regulation, such as that required for postural control, appears to decrease with advanced aging or disease. To help delineate causes and functional consequences of complexity loss, we examined the effects of visual and somatosensory impairment on the complexity of postural sway during quiet standing and its relationship to postural adaptation to cognitive dual tasking. Participants of the MOBILIZE Boston Study were classified into mutually exclusive groups: controls [intact vision and foot somatosensation, n = 299, 76 ± 5 (SD) yr old], visual impairment only (<20/40 vision, n = 81, 77 ± 4 yr old), somatosensory impairment only (inability to perceive 5.07 monofilament on plantar halluxes, n = 48, 80 ± 5 yr old), and combined impairments (n = 25, 80 ± 4 yr old). Postural sway (i.e., center-of-pressure) dynamics were assessed during quiet standing and cognitive dual tasking, and a complexity index was quantified using multiscale entropy analysis. Postural sway speed and area, which did not correlate with complexity, were also computed. During quiet standing, the complexity index (mean ± SD) was highest in controls (9.5 ± 1.2) and successively lower in the visual (9.1 ± 1.1), somatosensory (8.6 ± 1.6), and combined (7.8 ± 1.3) impairment groups (P = 0.001). Dual tasking resulted in increased sway speed and area but reduced complexity (P < 0.01). Lower complexity during quiet standing correlated with greater absolute (R = -0.34, P = 0.002) and percent (R = -0.45, P < 0.001) increases in postural sway speed from quiet standing to dual-tasking conditions. Sensory impairments contributed to decreased postural sway complexity, which reflected reduced adaptive capacity of the postural control system. Relatively low baseline complexity may, therefore, indicate control systems that are more vulnerable to cognitive and other stressors.

Fig. 1.

Multiscale entropy (MSE) analysis of postural sway dynamics. MSE curve, created by plotting sample entropy (mean ± SE) of postural sway (i.e., center-of-pressure) displacements as a function of time scale, is shown for each group. Complexity index was determined by computing the area under the MSE curve for each trial.

Fig. 2.

Effects of sensory impairment and cognitive dual tasking on postural sway [center-of-pressure (COP)] dynamics. Postural sway complexity (A), area (B), and speed (C) are shown during quiet-standing and dual-tasking conditions. Complexity index was greatest in controls and successively smaller in visual, somatosensory, and combined impairment groups. In general, somatosensory and/or combined impairment groups exhibited greater sway speed and area than visual impairment and control groups. Dual tasking reduced complexity yet increased postural sway speed and area. Dual tasking led to greater increases in sway speed within combined and somatosensory impairment groups than visual impairment and control groups. Values are group means ± SE.

Fig. 3.

Relationship between postural sway complexity and the impact of cognitive dual tasking. For all participants, lower postural sway complexity index values during quiet standing were associated with greater percent and absolute (not shown) increases in sway speed from quiet-standing to dual-tasking conditions.