Multi-muscle synergies in a dual postural task: evidence for the principle of superposition

Abstract

We used the framework of the uncontrolled manifold hypothesis to quantify multi-muscle synergies stabilizing the moment of force about the frontal axis (MY) and the shear force in the anterior–posterior direction (FX) during voluntary body sway performed by standing subjects. We tested a hypothesis whether the controller could stabilize both MY and FX at the same time when the task and the visual feedback was provided only on one of the variables (MY). Healthy young subjects performed voluntary body sway in the anterior–posterior direction while different loads were attached at the ankle level producing horizontal forces acting forward or backwards. Principal component analysis was used to identify three M-modes within the space of integrated indices of muscle activation. Variance in the M-mode space across sway cycles was partitioned into two components, one that did not affect a selected performance variable (MY or FX) and the other that did. Under all loading conditions and for each performance variable, a higher value for the former variance component was found. We interpret these results as reflections of two multi-M-mode synergies stabilizing both FX and MY. The indices of synergies were modulated within the sway cycle; both performance variables were better stabilized when the body moved forward than when it moved backward. The results show that the controller can use a set of three elemental variables (M-modes) to stabilize two performance variables at the same time. No negative interference was seen between the synergy indices computed for the two performance variables supporting the principle of superposition with respect to multi-muscle postural control.

Two aspects of feedforward postural control: anticipatory postural adjustments and anticipatory synergy adjustments

“…A detailed description of the experimental procedures involved in both the control trials and the voluntary body sway trials can be found in Klous et al (2010) and Danna-Dos- Santos et al (2007). The control trials involved holding steadily a standard 5-kg load in front of the body with extended arms; the weight of the load acted downward in one trial and upward (using a set of pulleys) in the other trial.…”

“…At the lower level of the hierarchy, muscles are united into groups such that a single variable can be used to produce parallel changes in the muscle activation levels within each group. Such groups have been documented in many recent studies and referred to as "synergies" Klous et al 2010). The latter terms implies that such muscle groups form the basis (elemental variables) for synergies to be built upon; in that sense, it is similar to the notion of finger modes introduced to study multifinger synergies (Danion et al 2003;Latash et al 2001;Zatsiorsky et al 1998).…”

“…The equipment and setup are described in detail by Klous et al (2010). A force platform (AMTI, OR-6) was used to record the horizontal component of the ground reaction force in the anteriorposterior direction (F X ; resolution 0.677 V/N), the vertical component of the ground reaction force (F Z ; resolution 0.173 V/N), and the moment of force around the frontal axis (M Y ; resolution 1.65 V/ Nm).…”

“…At the lower level of the hierarchy, postural muscles are organized into groups with activation levels scaling in parallel within each group. Such groups have been addressed as muscle synergies (d'Avella et al 2003;Ivanenko et al 2005Ivanenko et al , 2006Ting and Macpherson 2005;Torres-Oviedo and Ting 2007;Torres-Oviedo et al 2006) or muscle modes (M-modes; Danna-Dos- Santos et al 2007;Klous et al 2010;Krishnamoorthy et al 2003aKrishnamoorthy et al , 2003b. At the upper level of the hierarchy, the controller manipulates gains with which the M-modes are recruited (reviewed in Latash et al 2007).…”

“…Until now, there have been no attempts to compare synergy indexes computed for the same data set based on different sets of M-modes. In our study, we used several sets of M-modes defined based on a standard task (voluntary body sway; see Danna-Dos- Santos et al 2007;Klous et al 2010) and on data extracted from different time intervals with respect to the action initiation during the shoulder flexion task. The purpose of using different tasks was to test whether the results were consistent across the methods of M-mode identification.…”