Full Length Article Maintenance of postural stability as a function of tilted base of support Aviroop Dutt-Mazumder a,⇑ , John Challis b , Karl Newell c a College of Health Professions, Medical University of South Carolina, 77, President Street, MSC 700, Charleston, SC, 29425, United States b Department of Kinesiology, The Pennsylvania State University, United States c Department of Kinesiology, University of Georgia, United States article info Article history: Received 29 September 2015 Revised 26 April 2016 Accepted 30 April 2016 Keywords: Multi-joint coordination Postural control Degrees of freedom Spectral analysis Tilted support surface abstract The experiment was set-up to investigate the mechanisms of postural control by manipu- lating the base of support angle, using tilted platform wedges. The primary focus was to analyze the coupling of the motion of the center of mass (CoM) and the center of pressure (CoP), and the motions of the leg joints considered as individual components and synergies. The CoM-CoP coupling (both medio-lateral and anterioposterior) was preserved (0°) across all tilted platform angles (35°, 30°, 20°, 10° Down, 0° Flat and 10°, 20°, 25° Up), reflecting an in-phase pattern. There was high coherence (1) for CoM-CoP in the lower frequency range, whereas contrarily the hip, knee and ankle pair-wise couplings had values ranging between (0.4 and 0.7) across the different platform angle conditions. These find- ings are consistent with the view that the local pair-wise coupled variables of Hip, Knee and Ankle motions adaptively self-organized to preserve the CoM-CoP in-phase coupling at equilibrium over the baseline (0° Flat) platform condition and all other tilted platform angles. The findings support the hypothesis of CoM-CoP coupling acting as a collective vari- able that provides the structural integrity of the system for upright quiet standing across the platform angle conditions. Ó 2016 Elsevier B.V. All rights reserved. Stabilization of quiet upright posture involves multiple joints and muscles and reflects a continuous self-organized coor- dination process (Aruin, 2002; Hsu, Scholz, Schöner, Jeka, & Kiemel, 2007; Massion, 1992; Wang, Ko, Challis, & Newell, 2014). The nature by which the large number of degrees of freedom (dof) are organized in posture is of prime importance in motor control, both theoretically and experimentally (Bernstein, 1967; Horak, Shupert, & Mirka, 1989; Kelso, 1995; Winter, 1995). Several studies have investigated the role of the stabilization of the center of mass (CoM) to maintain postural equilib- rium in upright stance (e.g. Corriveau, Hébert, Raîche, Dubois, & Prince, 2004; Peterka, 2002). It is recognized that upright posture is compromised when the projection of the line of gravity of the postural system falls outside the base of support so that only a certain range of CoM spatial positions are feasible for the maintenance of balance (Winter, 1995). However, the stabilization of the CoM could be a consequence of the control of other variables, meaning that its spatial position may not be directly controlled (Hsu et al., 2007). Joint orientation, in particular of the lower limb, is relevant for the control of upright posture (Krishnamoorthy, Yang, & Scholz, 2005; Scholz et al., 2007). Past studies have also investigated the synergies of body effectors that afford the stabiliza- tion of quiet standing (e.g. Massion, 1994; Nashner & McCollum, 1985). A longstanding hypothesis holds that the stabiliza- http://dx.doi.org/10.1016/j.humov.2016.04.010 0167-9457/Ó 2016 Elsevier B.V. All rights reserved. ⇑ Corresponding author. E-mail address: duttamaz@musc.edu (A. Dutt-Mazumder). Human Movement Science 48 (2016) 91–101 Contents lists available at ScienceDirect Human Movement Science journal homepage: www.elsevier.com/locate/humov