Klous M, Mikulic P, Latash ML. Two aspects of feedforward postural control: anticipatory postural adjustments and anticipatory synergy adjustments. J Neurophysiol 105: 2275-2288, 2011. First published March 9, 2011 doi:10.1152/jn.00665.2010.-We used the framework of the uncontrolled manifold hypothesis to explore the relations between anticipatory synergy adjustments (ASAs) and anticipatory postural adjustments (APAs) during feedforward control of vertical posture. ASAs represent a drop in the index of a multimusclemode synergy stabilizing the coordinate of the center of pressure in preparation to an action. ASAs reflect early changes of an index of covariation among variables reflecting muscle activation, whereas APAs reflect early changes in muscle activation levels averaged across trials. The assumed purpose of ASAs is to modify stability of performance variables, whereas the purpose of APAs is to change magnitudes of those variables. We hypothesized that ASAs would be seen before APAs and that this finding would be consistent with regard to the muscle-mode composition defined on the basis of different tasks and phases of action. Subjects performed a voluntary body sway task and a quick, bilateral shoulder flexion task under self-paced and reaction time conditions. Surface muscle activity of 12 leg and trunk muscles was analyzed to identify sets of 4 muscle modes for each task and for different phases within the shoulder flexion task. Variance components in the muscle-mode space and indexes of multimuscle-mode synergy stabilizing shift of the center of pressure were computed. ASAs were seen ϳ100 -150 ms prior to the task initiation, before APAs. The results were consistent with respect to different sets of muscle modes defined over the two tasks and different shoulder flexion phases. We conclude that the preparation for a self-triggered postural perturbation is associated with two types of anticipatory adjustments, ASAs and APAs. They reflect different feedforward processes within the hypothetical hierarchical control scheme, resulting in changes in patterns of covariation of elemental variables and in their patterns averaged across trials, respectively. The results show that synergies quantified using dissimilar sets of muscle modes show similar feedforward changes in preparation to action. uncontrolled manifold hypothesis DURING STANDING, the human body is inherently unstable because of the small support area, high center of mass, and multijoint design. Voluntary movements performed by a standing person contribute to problems of postural control by producing postural perturbations because of the mechanical coupling of the body segments. When a standing person performs a fast arm movement, changes in the activation of postural muscles are typically seen about 100 ms before movement initiation; these changes have been addressed as anticipatory postural adjustments (APAs; reviewed in Massion 1992). The role of APAs has been assumed to produce joint torques and forces that minimize perturbation of the vertical...

No 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.

For biomechanical research in several sports (e.g. skiing and snowboarding), field experiments are essential because these activities are performed over a great distance and in conditions that could not be reproduced in a controlled laboratory environment. High technical standards in kinematic set-up are necessary to achieve the required accuracy for biomechanical analysis. The purpose of this study was to determine the accuracy of the kinematic data collected in a ski and snowboard field experiment. Eight tests generally used in laboratory settings were adapted to field conditions on a skiing slope to determine the error related to motion capture. The calculated photogrammetric errors in the x-, y-, and z-direction were 11 mm, 9 mm, and 13 mm, respectively. The maximum error caused by soft tissue artifacts was 39 mm. These results indicate that accuracy of kinematic data in the described field experiment was comparable to that found in literature for laboratory experiments. It may be concluded that accurate kinematic data collection for skiing and snowboarding can be performed in a field setting and that these results are accurate enough to serve as input data for further analyses.

Limited data exists on knee biomechanics in alpine ski turns despite the high rate of injuries associated with this maneuver. The purpose of the current study was to compare knee joint loading between a carved and a skidded ski turn and between the inner and outer leg. Kinetic data were collected using Kistler mobile force plates. Kinematic data were collected with five synchronized, panning, tilting, and zooming cameras. Inertial properties of the segments were calculated using an extended version of the Yeadon model. Knee joint forces and moments were calculated using inverse dynamics analysis. The obtained results indicate that knee joint loading in carving is not consistently greater than knee joint loading in skidding. In addition, knee joint loading at the outer leg is not always greater than at the inner leg. Differentiation is required between forces and moments, the direction of the forces and moments, and the phase of the turn that is considered. Even though the authors believe that the analyzed turns are representative, results have to be interpreted with caution due to the small sample size.