The HUMAN POSTURAL DYNAMICS project (1996 – present)

Acronym: POSTURAL DYNAMICS - I
Name: Human Postural Dynamics
Type: Collaborative project
Funds: French Institut Universitaire de France - European Network of Excellence ENACTIVE - French Ministery of Research
UM1 key researchers: Benoît Bardy
Collaborators: Olivier Oullier (Univ. Provence) – Elise Faugloire (univ. Caen), Ludovic Marin, Julien Lagarde, Jérôme Froger, Jacques-Yvon Pelissier, Manuel Varlet (Univ. Montpellier-1), Tom Stoffregen (Univ. Minnesota, USA) - Fabrice Mégrot (Croix Rouge Française) - Déborah Varoqui (RIC Chicago, USA) - Krasimara Tsaneva-Atanasova, Ed Rooke (Univ. Bristol, UK)

POSTURAL DYNAMICS is a theoretical and experimental project demonstrating the existence of typical signatures of self-organization in the human postural system. These hallmarks may be used by the central nervous system to control efficiently and with parsimony the human segments during stance and other supra-postural behaviours.

Theory and past research:

The maintenance of stable upright stance is required in many daily activities in humans and other bipeds. Stable stance requires the body’s center of mass to be kept above the feet. Postural control actions consist mainly of coordinated rotations around the hips and ankles. Many patterns of ankle-hip coordination will maintain the center of mass above the feet, but only a few of these are effective across a broad range of situations.

Functionality depends in part on the task in which the person is engaged. For example, some coordination patterns that prevent falling may be avoided because they hamper the realization of other, simultaneous goals, such as maintaining gaze, or manual contact with an object. Other coordination patterns may both prevent falling and facilitate performance on these supra-postural tasks, and so may be preferred. This fact has implications for pre-existing patterns of postural coordination, but also for the acquisition of new patterns.

In our research on postural dynamics (e.g., Bardy et al., 1999; Bardy et al., 2002; Marin et al., 1999; Oullier et al., 2002, 2004), standing participants have been instructed to maintain a constant distance between their head and a visual target that oscillates along the line of sight.

They have not been given any instructions about how standing posture was to be controlled during the tracking task. We measured rotations at the ankles and hips, and analyzed the relative phase, Φrel, of rotations at these joints. Two coordination modes between ankles and hips have been consistently observed. An in-phase mode, with Φrel of about 20-25°, emerged when the visual tracking target moved at small amplitude (e.g., Bardy et al., 1999) or low frequency (e.g., Bardy et al., 2002). An anti-phase mode, with Φrel close to 180°, has emerged when the visual target moved with large amplitude or high frequency.

The differential emergence of these modes was influenced by intentional constraints (i.e., the instruction to track target motion), by behavioral constraints (i.e., height of the center of mass, length of the feet, body stiffness, expertise in sport), and by environmental constraints (i.e., surface properties, target amplitude or frequency); (see Bardy, 2003 for a review). It was the simultaneous, interacting pressures — cooperative or competitive —imposed by the task, the body, and the environment that determined the selective emergence of the in-phase and anti-phase modes (cf. Newell, 1986).

We also observed that transitions between in-phase and anti-phase ankle-hip modes revealed characteristics of non-equilibrium phase transitions (Bardy et al., 2002). As we increased or decreased the frequency at which the visual target moved, a frequency-induced loss of stability occurred, yielding critical fluctuations in the vicinity of the region of the frequency range in which there was a transition between coordination patterns (see Figure below).

Transitions between in-phase and anti-phase modes were abrupt, and exhibited hysteresis: Transitions from in-phase to anti-phase occurred at a higher frequency of target motion than transitions from anti-phase to in-phase. Finally, we applied an external perturbation (a sudden shift in the direction in which the target was moving). The perturbation was applied either near to or far from the region (frequencies) in which transitions between modes were known to occur.

Each mode was found to be less stable when the perturbation was applied close to the the transition region, and more stable when it was applied far from it, as evidenced by a smaller relaxation time in the latter situation (critical slowing down).

In summary, our research has shown that postural modes (i) emerge out of the coalescence of multiple constraints, (ii) exhibit persistence and change that are characteristic of self-organized systems, and (iii) are modulated by the actor’s intentions. Co-existence of the in-phase and the antiphase patterns during a tracking task (individual subject). Participants were asked to use head motion to track 10-cm (peak-to-peak) oscillations of a visual target while we measured coordination between the ankle and the hip. The grey line (plain) represents a trial during which the frequency of oscillation increased, while the black line (dashed) represents a trial during which the frequency of the target decreased. The shaded region represents the region of bi-stability, that is, the region in which both coordination patterns co-exist (adapted from Bardy et al., 2002).

Current research:

The similarities between postural phase transitions in humans and non-biological phenomena (e. g., in physics or economics) suggest the existence of general and common principles governing pattern formation and flexibility in complex systems, and circumscribe the generality of neurophysiologically-based theories of postural coordination. Current research aims at investigating the emergence/disappearance of new postural modes accompanying learning (e.g., Faugloire et al., 2006, Faugloire et al., 2009; Mégrot et al., 2002, 2006), post-stroke rehabilitation (Varoqui, 2006; Varoqui et al., 2010; 2011), auditory tracking (Stoffregen et al., 2009), or in a social context (e.g., Varlet et al., 2009), using virtual reality techniques applied to postural control. The comparison between human and non human (HOAP3 robot, HRP2 robot) postural coordination modes is also under investigation (see the Humanoid postural dynamics project). Last but not least, a new synergetic modelling approach to postural cooordination is under investigation with Krasi Tsaneva and Ed Rooke from the University of Bristol, UK.

Virtual Posture, a customized TM software package to investigate the emergence and stabilization of new postural patterns. Left: Standing participants are instructed to draw with their body the requested figure in the ankle-hip plane (here a 90° phase relation between ankles and hips). Right: Produced postural coordination as a function of requested coordination in normal subjects (adults) showing attractors at 20° and 180° (from Faugloire et al., 2005, cf. Faugloire et al., 2009).

Key references (downloadable version in page Vitae):

1. Bardy, B. G., Oullier, O., Lagarde, J., Stoffregen, T.A. (2007).  On perturbation and pattern co-existence in postural coordination dynamics. Journal of Motor Behavior, 39, 326-334.
2. Bardy, B. G. (2003). Postural coordination dynamics in standing humans. In V.K. Jirsa and J.A.S. Kelso (Eds.), Coordination Dynamics : Issues and Trends, Vol.1 Applied Complex Systems (pp. 103-121). New York: Springer Verlag.
3. Bardy, B.G., Faugloire, E., & Fourcade, P. (2006). Stabilization of old and new postural patterns in standing humans. In M. Latash & F. Lestienne (Eds.), Motor Control and learning over the life span (pp. 77-87). Berling: Springer Verlag.
4. Bardy, B.G., Marin, L., Stoffregen, T.A., & Bootsma, R.J. (1999). Postural coordination modes considered as emergent phenomena. Journal of Experimental Psychology: Human Perception and Performance, 25, 1284-1301.
5. Bardy, B.G., Oullier, O., Bootsma, R.J., & Stoffregen, T.A. (2002). The dynamics of human postural transitions. Journal of Experimental Psychology: Human Perception and Performance, 28, 499-514.
6. Faugloire, E., Bardy, B. G., Merhi, O., & Stoffregen, T.A (2005). Exploring coordination dynamics of the postural system with real-time visual feedback. Neuroscience Letters, 374, 136-141.
7. Faugloire, E., Bardy, B.G., & Stoffregen, T.A. (2006). The dynamics of learning new postural patterns. Influence on pre-existing spontaneous behaviors. Journal of Motor Behavior, 38, 299-312.
8. Faugloire, E., Bardy, B. G., & Stoffregen, T. A. (2009). (De)Stabilization of required and spontaneous postural dynamics with learning. Journal of Experimental Psychology: Human Perception and Performance, 35, 170-187.
9. Mégrot, F., Bardy, B.G., & Dietrich, G. (2002). Dimensionality and the dynamics of human unstable equilibrium. Journal of Motor Behavior, 34, 323-328.
10. Mégrot, F., & Bardy, B. G. (2006). Changes in phase space during learning an unstable balance. Neuroscience Letters, 402, 7-21.
11. Oullier, O., & Bardy, B.G., Stoffregen, T.A., & Bootsma, R.J. (2002). Postural coordination in looking and tracking tasks. Human Movement Science, 21, 147-167.
12. Oullier, O., Bardy, B.G., Stoffregen, T.A., & Bootsma, R.J. (2004). Task-specific stabilization of postural coordination during stance on a beam. Motor Control, 7, 174-187.
13. Stoffregen, T. A., Villard, S., Kim, C., Ito, K. & Bardy, B. G. (2009). Coupling of head and body movement with motion of the audible environment. Journal of Experimental Psychology: Human Perception and Performance, 35, 1221-1231.
14. Varoqui, D., Froger, J., Pélissier, J.-Y., & Bardy, B. G. (2010). Effect of Coordination Biofeedback on (Re)Learning Preferred Postural Patterns in Post-stroke Patients. Motor Control, in press.