Authors: Yun Wang Tadayoshi Asaka Vladimir M Zatsiorsky Mark L Latash
Publish Date: 2006/05/19
Volume: 174, Issue: 4, Pages: 679-693
Abstract
We investigated covaried changes in muscle activity during voluntary sway tasks that required a quick shift of the center of pressure COP We hypothesized that multimuscle synergies defined as taskspecific covariation of elemental variables muscle modes stabilize a COP location in the anterior–posterior direction prior to a voluntary COP shift and that during the shift the synergies would weaken Standing subjects performed two tasks a cyclic COP shift over a range corresponding to 80 of the maximal amplitude of voluntary COP shift at 1 Hz and a unidirectional quick COP shift over the same nominal amplitude The cyclic sway task was used to define muscle modes Mmodes leg and trunk muscle groups with parallel scaling of muscle activation level within a group and the relations between small changes in the magnitudes of Mmodes in the principal component analysis PCA the Mmode magnitudes are equivalent to PC scores and COP shifts A novel approach was used involving PCA applied to indices of muscle integrated activity measured both within a trial and across trials The unidirectional sway task was performed in a selfpaced SP manner and under a typical simple reaction time RT instruction Mmodes were also defined along trials at those tasks they have been shown to be similar across tasks Integrated indices of muscle activity in the SPsway and RTsway tasks were transformed into the Mmodes Variance in the Mmode space was partitioned into two components one that did not affect the average value of COP shift V UCM and the other that did V ORT An index ΔV corresponding to the normalized difference between V UCM and V ORT was computed During steadystate posture ΔV was positive corresponding to most Mmode variance lying in a subspace corresponding to a stable COP location across trials Positive ΔV values have been interpreted as reflecting a multiMmode synergy stabilizing the COP location The magnitude of ΔV was larger in SP trials than in RT trials During voluntary COP shifts the ΔV magnitude dropped to zero or even became negative We conclude that Mmode synergies stabilize COP location during quiet standing while these synergies weaken or disappear during fast voluntary COP shifts Under RT conditions the COP stabilizing synergies were weaker supposedly to facilitate a quick COP shift without time for preparation The suggested method of Mmode identification may potentially be applied to analysis of postural synergies in persons with impaired postural control such as elderly persons persons with atypical development or in the course of rehabilitation after an injury
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