Age-related Changes in Multi-finger Synergy during Constant Force Production with and without Additional Mechanical Constraint

Abstract

Objective: The aim of this study was to investigate age-related changes of multi-finger synergy during a constant force production task with and without an additional mechanical constraint. Method: Fourteen elderly subjects (age: <TEX>$78.50{\pm}4.63yrs$</TEX>, height: <TEX>$157.29{\pm}8.97cm$</TEX>, weight: <TEX>$65.13{\pm}6.93kg$</TEX>) and 14 young subjects (age: <TEX>$21.13{\pm}1.35yrs$</TEX>, height: <TEX>$171.57{\pm}8.43cm$</TEX>, weight: <TEX>$70.29{\pm}16.77kg$</TEX>) participated in this study. The subjects were asked to place their index and middle fingers on two force transducers fixed on a small non-moving teeterboard and produce 10 N by pressing the sensors while watching force feedback on a computer screen under the no additional constraint condition (NAC). The subjects also performed the same task with an additional mechanical constraint (AC) where the subjects were asked to balance a teeterboard that could be rotated by finger forces. An uncontrolled manifold approach was used to calculate within-trial and between-trial multi-finger synergy indices, variance in uncontrolled subspace (<TEX>$V_{UCM}$</TEX>), and variance in subspace orthogonal to UCM subspace (<TEX>$V_{ORT}$</TEX>). Two-way repeated measured ANOVA was performed with the within-factor of task condition (with and without an additional constraint) and the between factor of groups (elderly and young). Results: The elderly group showed significantly increased within-trial <TEX>$V_{ORT}$</TEX> in AC compared with NAC (p < .05) while the young group showed no significant difference between AC and NAC. There was no significant group difference for within-trial <TEX>$V_{UCM}$</TEX>. Between-trial <TEX>$V_{ORT}$</TEX> remained unchanged between groups and conditions. However, between-trial <TEX>$V_{UCM}$</TEX> for the elderly group significantly decreased in AC as compared to NAC, along with no significant difference for the young group. For multi-finger synergy, there was no significant group difference of within-trial synergy. However, between-trial synergy for the elderly group significantly decreased in AC as compared to NAC (p < .05). Conclusion: Our results indicate that aging decreased consistency (i.e., ability to perform the task on a moment-to-moment basis) with an additional mechanical constraint. In addition, aging was associated with decreased multi-finger synergy on a trial-to-trial basis.