Age-related changes in multifinger synergies in accurate moment of force production tasks

Abstract

The purpose of this investigation was to document and quantify age-related differences in the coordination of fingers during a task that required production of an accurate time profile of the total moment of force by the four fingers of a hand. We hypothesized that elderly subjects would show a decreased ability to stabilize a time profile of the total moment of force, leading to larger indexes of moment variability compared with young subjects. The subjects followed a trapezoidal template on a computer screen by producing a time profile of the total moment of force while pressing down on force sensors with the four fingers of the right (dominant) hand. To quantify synergies, we used the framework of the uncontrolled manifold hypothesis. The elderly subjects produced larger total force, larger variance of both total force and total moment of force, and larger involvement of fingers that produced moment of force against the required moment direction (antagonist moment). This was particularly prominent during supination efforts. Young subjects showed covariation of commands to fingers across trials that stabilized the moment of total force (moment-stabilizing synergy), while elderly subjects failed to do so. Both subject groups showed similar indexes of covariation of commands to the fingers that stabilized the time profile of the total force. The lack of moment-stabilizing synergies may be causally related to the documented impairment of hand function with age.

Fig. 1

A: schematic illustration of the experimental setup. B: the experimental task shown on the computer screen. C: finger and force sensor configuration.

Fig. 2

The template and the five time intervals. PR, pronation; SU, supination; PR_pre, prepronation; PR_post, postpronation.

Fig. 3

Typical performance of an elderly female subject (A) and a young male subject (B) during the moment production task. Thick solid lines represent average total moment of force (M_tot) (left Y-axis) across trials with SE bars; thin solid lines show average total force (F_tot) (right Y-axis) across trials with SE bars; and the dotted line shows the target template.

Fig. 4

Average performance of elderly (top) and young (bottom) subjects, with SE bars during the moment production task. Thick solid lines represent average M_tot (left Y-axis), and thin solid lines average F_tot (right Y-axis). Dotted line shows the target template.

Fig. 5

A: average variance of the total force (V_F) during the five time intervals with SE bars. The data were averaged over each of the five time intervals and further across subjects. Elderly subjects are represented by solid bars, and young subjects by open bars. B: average variance of the total moment (V_M) during the five time intervals with SE bars. The data were averaged over each of the five time intervals and further across subjects. Elderly subjects are represented by solid bars, and young subjects by open bars.

Fig. 6

Average agonist (M_Ag, open bars) and antagonist moment (M_Ant, solid bars) of elderly (top) and young (bottom) subjects with SE bars. M_Ag and M_Ant were averaged over half-second intervals and further across subjects of each age group.

Fig. 7

Average proportion of the total PR moment produced by the index finger (A and C) and of the total SU moment (B and D) produced by the little finger for elderly (solid bars) and young subjects (open bars) with SE bars. A and B: data averaged over 1-s intervals. C and D: data further averaged over PR and SU intervals.

Fig. 8

Average profiles of change in variance (ΔV) indexes computed for stabilization of the total force (ΔV_F) and total moment (ΔV_M) at each time sample with SE bars. A–D: ΔV_M of elderly and young subjects and ΔV_F of elderly and young subjects, respectively. Dotted lines show the experimental task. Note that ΔV_M of elderly subjects (A) fluctuates around zero, while ΔV_M of young subjects (B) is consistently positive.

Fig. 9

Illustration of a system of two effectors (fingers F₁ and F₂) involved in the task of supporting an object (top). Bottom: two hypothetical distributions of data points over many trials. One of them corresponds to force covariation, stabilizing the total force but not the total moment (ΔV_F > 0, ΔV_M < 0). The other one corresponds to force covariation, stabilizing the total moment but not total force (ΔV_F < 0, ΔV_M > 0).