Aging-related changes in neuromuscular control strategies and their influence on postural stability

Abstract

Altered neuromuscular strategies are suggested to contribute to age-related decreases in postural stability. Current approaches tend to overlook global (whole body) neuromuscular postural control strategies, potentially due to methodological constraints or residual influence from a longstanding, but outdated, biomechanical view in which postural sway is represented by a single-jointed inverted pendulum. In this study, we investigate age-related differences in postural strategies during upright static balance maintenance by assessing global neuromuscular control. We collected simultaneous posturography and electromyography (EMG) data from young (18-35 years, n = 32) and older (65-85 years, n = 33) participants while they stood upright on a force plate or on foam pads thereon, with eyes open or closed. Postural instability was assessed by the standard deviation and velocity of the center of pressure. EMG sensors recorded the activity of thirty muscles (15 on each hemibody). Co-contraction across all muscle pairs was measured with Falconer's co-contraction index (CCI), and muscle synergy with non-negative matrix factorization. The older group possessed increased global co-contraction intensity, marked by more frequent use of a knee extensor synergy, and was more unstable than the younger group. Notably, advancing age modulated the variability of co-contraction intensity, where the oldest individuals consistently adopted a pure co-contraction strategy marked by the highest CCI values and lowest variability. Age-corrected correlations revealed that knee extensor CCI values were significantly related to postural instability. Taken together, global co-contraction appears to be a signature of elderly postural strategy and age-related instability may be directly related to the extent of knee extensor co-contraction. These results stress the importance of zooming out from classical agonist-antagonist muscle pair investigations in the endeavor to understand elderly postural control strategy.

Fig. 1

Experimental set-up. 15 EMG sensors on each hemibody, and stood on a force plate in 4 experimental conditions: either on a hard surface or on foam pads, and with eyes open or closed.

Fig. 2

EMG placement. EMG electrodes were placed according to SENIAM guidelines (where applicable) over the TA (1), GL (2), SOL (3), VM (4), VL (5), BF (6), ES (7), TRAP (8), LD (9), SCM (10), MD (11), BB (12), TB (13), FDS (14), and EDS (15) of each hemibody. The image was adapted from iStock (credit to elenabs).

Fig. 3

Muscle CCI matrices. (A) and (B)—CCI matrices for the younger (A) and older (B) participants in the 4 conditions. (C) Difference between CCI matrices for the older and younger groups. Brightened red values indicate significant increases in CCI values in the older group (p < 0.05, permutation-based statistics).

Fig. 4

Motor modules for the 8 synergy clusters. ‘Older > young’ and ‘young > older’ indicate which group used the synergy cluster more often. ***, p < 0.001.

Fig. 5

Distribution of older participants’ CCI values across old-age subgroups and conditions. Displayed are the knee extensor (A), lower leg (B) and global (C) CCI values. The darkened central line indicates the mean within each old-age subgroup or condition and dotted lines indicate quartiles. Significant differences between old-age subgroups and between conditions are indicated with horizontal lines (ns, not significant; *p < 0.05; **p < 0.001; ***p < 0.001).

Fig. 6

Relationship between age and knee extensor CCI across postural conditions. Circles indicate individual values and colors indicate old-age subgroups (green, young old-age; yellow, middle old-age; red, old old-age).

Fig. 7

Distribution of postural instability parameters across conditions and age groups. Left depicts violin plots for sdCOP_AP and right for vCOP_AP. All is as in Fig. 5. Significant differences between groups and between conditions are indicated with horizontal lines (ns, not significant; #0.05 < p < 0.1; **p < 0.01; ***p < 0.001).

Fig. 8

Association between global (A) and knee extensor (B) CCI values and instability in the eyes-closed foam condition. Circles indicate individual values and colors indicate age in years, according to the colorbar (bottom).