Relative contributions of postural balance mechanisms reveal studying the CoP displacement alone may be incomplete for analysis of challenging standing postures.

BACKGROUND: The mechanical consequences of the motor actions used to maintain upright standing balance can be discriminated in two mechanisms: i) moving the center of pressure (CoP) within the base of support (M1); and ii) modifying the whole-body angular momentum (M2). Since the contribution of M2 to the whole-body CoM acceleration increases with postural constraints, a postural analysis focusing only on the CoP trajectory (i.e. M1) could ignore the majority of the control actions in challenging postural tasks. The objective of this study was to determine the contributions of the two postural balance mechanisms across postures with different areas of the base of support. METHODS: Forty-one healthy young adults (19 females, 22.9 ± 2.9 years old) stood quietly on a forceplate, maintaining four different postures: bipedal, tandem, unipedal and unipedal on a 4-cm wooden bar; each with eyes open and for 60 s. Relative contributions of the two balance postural mechanisms were computed for each posture, in both directions of the horizontal plane. RESULTS: The posture impacted the mechanisms contributions, where the contribution of M1 decreased between each posture in the mediolateral direction as the area of the base of support was reduced. The contribution of M2 in the mediolateral direction was not negligible (about 1/3) in tandem and unipedal postures and became dominant (nearly 90% on average) in the most challenging unipedal posture. SIGNIFICANCE: This suggests the contribution of M2 should not be neglected for the analysis of postural balance, and particularly in challenging standing postures.

Stepping boundary of external force-controlled perturbations of varying durations: Comparison of experimental data and model simulations.

This study investigated the stepping boundary - the force that can be resisted without stepping - for force-controlled perturbations of different durations. Twenty-two healthy young adults (19-37 years old) were instructed to try not to step in response to 86 different force/time combinations of forward waist-pulls. The forces at which 50% of subjects stepped (F50) were identified for each tested perturbation durations. Results showed that F50 decreased hyperbolically when the perturbation's duration increased and converged toward a constant value (about 10%BW) for longer perturbations (over 1500 ms). The effect of perturbation duration was critical for the shortest perturbations (less than 1 s). In parallel, a simple function was proposed to estimate this stepping boundary. Considering the dynamics of a linear inverted pendulum + foot model and simple balance recovery reactions, we could express the maximum pulling force that can be withstood without stepping as a simple function of the perturbation duration. When used with values of the main model parameters determined experimentally, this function replicated adequately the experimental results. This study demonstrates for the first time that perturbation duration has a major influence on the outcomes of compliant perturbations such as force-controlled pulls. The stepping boundary corresponds to a constant perturbation force-duration product and is largely explained by only two parameters: the reaction time and the displacement of the center of pressure within the functional base of support. Future work should investigate pathological populations and additional parameters characterizing the perturbation time-profile such as the time derivative of the perturbation.

Cognitive-motor dual-task interference modulates mediolateral dynamic stability during gait in post-stroke individuals.

Gait asymmetry and dynamic balance impairments observed in post-stroke individuals increase their risk of fall. Moreover, walking while performing a cognitive task (i.e. dual-task) disturbs the control of balance in post-stroke individuals. Here we investigated the mediolateral dynamic stability in twenty-two community-dwelling participants (12 post-strokes and 10 healthy controls) while walking in single-task (normal gait) and four different dual-tasks (cognitive-motor interference). Positions of the extrapolated center of mass and mediolateral widths of both margin of stability and base of support were extracted from 35 marker trajectories. Post-stroke participants presented larger margin of stability and base of support than controls during single-task (both p < 0.01), with a larger margin of stability on the non-paretic side than on the paretic side at ipsilateral foot-strike (p < 0.05). No significant effect of the dual-task was found between groups. In post-stroke participants, dual-task induced slight modification of the mediolateral stability strategy, as the margin of stability was not different between the two limbs at foot-strike, and significantly reduced the performance in every cognitive task. Post-stroke participants increased their dynamic stability in the frontal plane in single-task by extending their base of support and mainly relying on their non-paretic limb. Under cognitive-motor interference (dual-task), post-stroke participants prioritized dynamic stability over cognitive performance to ensure a safe locomotion. Thus, rehabilitation programs should consider both dynamic balance and dual-task training, even at a chronic delay following stroke, to reduce the risk of fall in post-stroke individuals.

A simplified marker set to define the center of mass for stability analysis in dynamic situations.

The extrapolated center of mass (XCoM), a valuable tool to assess balance stability, involves defining the whole body center of mass (CoMWB). However, accurate three-dimensional estimation of the CoMWB is time consuming, a severe limitation in certain applications. In this study, twenty-four subjects (young and elderly, male and female) performed three different balance tasks: quiet standing, gait and balance recovery. Three different models, based on a segmental method, were used to estimate the three-dimensional CoMWB absolute position during these movements: a reference model based on 38 markers, a simplified 13-marker model and a single marker (sacral) model. CoMWB and XCoM estimations from the proposed simplified model came closer to the reference model than estimations from the sacral marker model. It remained accurate for dynamic tasks, where the sacral marker model proved inappropriate. The simplified model proposed here yields accurate three-dimensional estimation of both the CoMWB and the XCoM with a limited number of markers. Importantly, using this model would reduce the experimental and post-processing times for future balance studies assessing dynamic stability in humans.

Comparison between investigations of induced stepping postural responses and voluntary steps to better detect community-dwelling elderly fallers.

In this paper, we review a physiological task that is predominant in preventing humans from falling, but that simultaneously also challenges balance: taking a step. In particular, two variants of this task are presented and compared: the voluntary step versus a step induced by an external and unpredictable perturbation. We show that, while these contribute different information, it is interesting to compare these. Indeed, they both are relevant in a global balance assessment and should be included within this, at the same level as tests usually dispensed in the clinical environment such as posturography. We choose to focus on the community-dwelling elderly population, to discuss means of early detection of risk of falls, in order to prescribe an appropriate prevention. An overview of posture-movement coordination and balance recovery strategies is also provided. Finally, a working hypothesis is suggested on how "compensatory protective" steps are controlled and how their evaluation could bring additional information to the global balance assessment of risk of fall.