Preparation to a quick whole-body action: control with referent body orientation and multi-muscle synergies
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We examined the control of postural stability in preparation to a discrete, quick whole-body sway toward a target and back to the initial position. Several predictions were tested based on the theory of control with referent body orientation and the notion of multi-muscle synergies stabilizing center of pressure (COP) coordinate. Healthy, young adults performed fast, discrete whole-body motion forward-and-back and backward-and-back under visual feedback on the COP. We used two methods to assess COP stability, analysis of inter-trial variance and analysis of motor equivalence in the muscle activation space. Actions were always preceded by COP counter-movements. Backward COP shifts were faster, and the indices of multi-muscle synergies stabilizing COP were higher prior to those actions. Patterns of muscle activation at the motion onset supported the idea of a gradual shift in the referent body orientation. Prior to the backward movements, there was a trend toward higher muscle co-activation, compared to reciprocal activation. We found strong correlations between the sets of indices of motor equivalence and those of inter-trial variance. Overall, the results support the theory of control with referent coordinates and the idea of multi-muscle synergies stabilizing posture by confirming a number of non-trivial predictions based on these concepts. The findings favor using indices of motor equivalence in clinical studies to minimize the number of trials performed by each subject.
KeywordsPostural control Synergy Variance Motor equivalence Referent orientation
The study was supported in part by an NIH grant NS082151. This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior—Brasil (CAPES) to Nardini—Finance Code 001. Freitas is grateful to CNPq/Brazil.
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