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Background/aim: Kyvelidou et al. (2009) described a change from a more in-phase to a more out-of-phase relationship between the trunk and the pelvis with the development of sitting posture. They proposed that such phase relationship change signals an advancement of sitting postural control. In addition, the centre of pressure (COP) has been considered a reflection of the organization of posture (Massion, 1992). Specifically, measures of its temporal structure of variability as the Largest Lyapunov Exponent (LyE) and the Approximate Entropy (ApEn) are considered valuable tools to reveal postural control strategies (Harbourne & Stergiou, 2003). The aim of this work is then to determine the relationships of the trunk-pelvis phasing with the COP nonlinear measures as sitting postural control develops over time.
Methods: Ten typically developing infants were followed from stages 1 to 3 of sitting (i.e. the abilities to hold-up the head when supported at the trunk and to sit independently). Coordination was evaluated using the mean absolute relative-phase (MARP) between the trunk and the pelvis (Stergiou et al., 2004). LyE and ApEn of the COP data were respectively calculated using the Wolf et al. (1985) and Pincus et al. (1991) algorithms. All dependent measures were obtained in the anterior-posterior (AP) and medio-lateral (ML) planes of motion. Spearman correlations between MARP and LyE/ApEn were conducted.
Results: MARP values were negatively correlated to both LyE and ApEn values in the AP plane of motion (Fig. 1). Accordingly, the more the out-of-phase (MARP values close to 180?) was the coordination between the pelvis and the trunk with the development of sitting posture, the smaller were the values of the LyE and ApEn, reflecting more regular COP oscillations with less divergence in the movement trajectories. No correlations were observed in the ML plane of motion (Fig. 2).
Discussion/Conclusion: The relationships between the coordination and COP variability indicated that more out-of-phase patterns between the trunk and pelvis relate to better controlled and more stable postural behavior. The absence of relationships in the ML plane of motion suggested that the development of early sitting postural control mostly occurs in the backward-forward direction. However, this finding may be related to the testing situation, where the infants were directed to look at activities in front of them. As infants develop greater sitting independence, the degrees of freedom of the pelvis and trunk appear then to be released to allow greater stability with small adjustments of the COP.
