Multi-muscle synergies in elderly individuals: preparation to a step made under the self-paced and reaction time instructions.

We studied multi-muscle synergies of healthy elderly persons during preparation to making a step (self-paced vs. reaction time). The uncontrolled manifold hypothesis was used to explore the organization of leg and trunk muscles into groups (M-modes) and co-variation of M-mode involvement (M-mode synergies) during stepping tasks. We hypothesized that aging accounts for changes in the structure of M-modes, as well as in M-mode synergies. Subjects performed two tasks: (1) a cyclic COP shift over a range corresponding to the maximal amplitude of voluntary COP shift at 1 Hz, (2) stepping tasks under 3 instructions, "comfortably, self-paced," "very quick, self-paced," and "as fast as possible to a visual signal." Electromyographic signals of 10 postural muscles were recorded and analyzed. Principal component analysis was used to identify M-modes within the space of integrated indices of muscle activity in the cyclic sway task. Variance in the M-mode space across stepping trials was partitioned into two components, one that did not affect the average value of COP shift and the other that did. An index (ΔV) corresponding to the normalized difference between two components of variance was computed. The elderly subjects showed more "co-contraction M-mode" and "mixed M-mode" than that of the young subjects. During stepping tasks, both subject groups showed M-mode synergies stabilizing COP shifts in the stepping and supporting legs. The synergies of elderly subjects showed a smaller and delayed value than that of the young subjects. These results suggest that aging is associated with diminished control in multi-muscle synergies in the anticipatory postural adjustments during gait initiation.