Intra- and inter-foot coordination in quiet standing: Footwear and posture effects Melissa C. Kilby a, *, Karl M. Newell b,1 a Department of Training and Movement Science, Johannes Gutenberg University Mainz, Albert Schweitzer Straße 22, 55099 Mainz, Germany b Department of Kinesiology, The Pennsylvania State University, 275 Recreation Building, University Park, PA, 16802, USA 1. Introduction The supportive effects of footwear on balance have been demonstrated in several studies [7,9,12]. However, wearing women’s shoes with elevated heels (high heels) may cause balance control problems [2,11,12], postural alignment problems [16] and higher forefoot loading pressure [8,28]. In addition, increasing heel height increases the risk of falling [12,20,23]. Nevertheless, wearing high heels is a popular fashion trend among the American female population [1]. Center of pressure (COPnet) data from one single force platform have been widely used to assess postural control in quiet standing [4,13]. However, the use of two force platforms provides the capacity to assess the COP separately under each foot [26,27] and determine the coordination dynamics of the feet [14,25]. Wang et al. [25] have shown that there is a negative relation between the standard deviation (SD) of relative phase of foot coupling and the variability of COPnet so that a modest level of enhanced adaptability in coupling between the feet is related to a reduction in the variability of COPnet. The purpose of the present study was to investigate the coordination patterns and the amount and time-dependent structure of postural variability during quiet standing as a function of posture and foot support conditions. Inter-foot coordination between COP of right and left foot during bipedal stance and intra- foot coordination of the ball and heel of the foot in single leg standing were examined as a function of posture (two legs, one leg, and toe postures), footwear (barefoot, different area based high heel shoes) and postural training (ballet group and regular exercising group). The general hypothesis tested was that the negative relation between the variability of foot coupling (both intra- and inter-foot) and SD of COPnet is modulated by the interaction of foot support and postural experience. More specifically, we investigated whether the reduced stability as a function of postural stance, shoe support and posture experience interacted in reducing the complexity in the coupling and increasing the variability of COPnet. This hypothesis is an extension to coordination dynamics of the general finding that the variability of COPnet increases as the dimension of the COPnet decreases [15]. 2. Methods 2.1. Participants Twenty healthy young adult females were recruited. The participants were 10 trained ballet dancers (age 21.5 Æ 3.1 years, height 165 Æ 3.8 cm, weight 58.7 Æ 6.3 kg) and 10 regular exercising students (age 21 Æ 1.8 years, height 162.6 Æ 5.1 cm, weight 58.9 Æ 8.4 kg). The dancers had on average 14.6 years dance experience. All participants indicated that they wear high heels occasionally. They provided written informed consent approved by the Institutional Review Board of the Pennsylvania State University. 2.2. Apparatus The participants stood on one or two adjacent AMTI (American Mechanical Technology, Inc., Watertown, MA) force platforms. Data were collected with a sampling frequency of 100 Hz. The flexion angle of the ankle during toe standing was measured using an electrogoniometer (Biometrics, Ltd, Ladysmith, VA). Gait & Posture 35 (2012) 511–516 A R T I C L E I N F O Article history: Received 2 July 2011 Received in revised form 14 November 2011 Accepted 17 November 2011 Keywords: Center of pressure Foot coordination Regularity High heels Ballet A B S T R A C T Intra-foot coordination between center of pressure (COP) of the ball and heel of the foot in single leg standing and inter-foot coordination of the right and left foot during bipedal stance was examined as a function of postural stance (two legs, one leg, and toe postures), footwear (barefoot, different area based high heel shoes) and postural training (ballet group and regular exercising group). Young adult females performed three 20 s trials in each postural condition. In general, the traditional variability measures of COPnet motion increased under the less stable postural support conditions and ballet dancers had better balance in single leg standing. Regularity analysis revealed a negative relation between the variability of foot coupling (both intra- and inter-foot) and the standard deviation of COPnet that was mediated by the interaction of shoe support and postural stance. The findings show that shoe support and postural stance modulate collective postural motion (COPnet) through the adaptability of the coupling of foot dynamics. ß 2011 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +49 151 17795238. E-mail address: melissak@students.uni-mainz.de (M.C. Kilby). 1 Tel.: +1 814 863 1163. Contents lists available at SciVerse ScienceDirect Gait & Posture jo u rn al h om ep age: ww w.els evier.c o m/lo c ate/g aitp os t 0966-6362/$ – see front matter ß 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.gaitpost.2011.11.018